CSCI 3335 COMPUTER NETWORKS
CHAPTER 5 LINK LAYER AND LANS
Vamsi ParuchuriUniversity of Central Arkansas
httpfacultyucaeduvparuchuri3335htm
Some of the material is adapted from JF Kurose and KW Ross
Data Link Layer 5-2
Chapter 5 The Data Link LayerOur goals understand principles behind data link layer
services error detection correction sharing a broadcast channel multiple access link layer addressing reliable data transfer flow control done
instantiation and implementation of various link layer technologies
Data Link Layer 5-3
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-4
Link Layer IntroductionTerminology hosts and routers are nodes communication channels
that connect adjacent nodes along communication path are links wired links wireless links LANs
layer-2 packet is a frame encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link
Data Link Layer 5-5
Link layer context datagram transferred
by different link protocols over different links eg Ethernet on first
link frame relay on intermediate links 80211 on last link
each link protocol provides different services eg may or may not
provide rdt over link
transportation analogy trip from Princeton to
Lausanne taxi Princeton to JFK plane JFK to Geneva train Geneva to
Lausanne tourist = datagram transport segment =
communication link transportation mode =
link layer protocol travel agent = routing
algorithm
Data Link Layer 5-6
Link Layer Services framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit-error link (fiber some twisted
pair) wireless links high error rates
bull Q why both link-level and end-end reliability
Data Link Layer 5-7
Link Layer Services (more) flow control
pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame error correction
receiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
Data Link Layer 5-8
Where is the link layer implemented in each and every host link layer implemented
in ldquoadaptorrdquo (aka network interface card NIC) Ethernet card PCMCI
card 80211 card implements link
physical layer attaches into hostrsquos
system buses combination of
hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
Data Link Layer 5-9
Adaptors Communicating
sending side encapsulates datagram
in frame adds error checking
bits rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Data Link Layer 5-10
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-11
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
Data Link Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 5-13
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-14
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-15
CRC Example
R = remainder[ ]D2r
G
r is size of G -1 Steps
Append r 0rsquos to D Divide it by G and
compute reminder Data transmitted is
D followed by R
Message transmitted is101110 011
Data Link Layer 5-16
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access broadcast (shared wire or medium)
old-fashioned Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
timemultiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-19
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-20
MAC Protocols a taxonomyThree broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Data Link Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fre
quen
cy b
ands
time
FDM cable
Data Link Layer 5-23
Random Access Protocols When node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions) Examples of random access MAC protocols
slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-2
Chapter 5 The Data Link LayerOur goals understand principles behind data link layer
services error detection correction sharing a broadcast channel multiple access link layer addressing reliable data transfer flow control done
instantiation and implementation of various link layer technologies
Data Link Layer 5-3
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-4
Link Layer IntroductionTerminology hosts and routers are nodes communication channels
that connect adjacent nodes along communication path are links wired links wireless links LANs
layer-2 packet is a frame encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link
Data Link Layer 5-5
Link layer context datagram transferred
by different link protocols over different links eg Ethernet on first
link frame relay on intermediate links 80211 on last link
each link protocol provides different services eg may or may not
provide rdt over link
transportation analogy trip from Princeton to
Lausanne taxi Princeton to JFK plane JFK to Geneva train Geneva to
Lausanne tourist = datagram transport segment =
communication link transportation mode =
link layer protocol travel agent = routing
algorithm
Data Link Layer 5-6
Link Layer Services framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit-error link (fiber some twisted
pair) wireless links high error rates
bull Q why both link-level and end-end reliability
Data Link Layer 5-7
Link Layer Services (more) flow control
pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame error correction
receiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
Data Link Layer 5-8
Where is the link layer implemented in each and every host link layer implemented
in ldquoadaptorrdquo (aka network interface card NIC) Ethernet card PCMCI
card 80211 card implements link
physical layer attaches into hostrsquos
system buses combination of
hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
Data Link Layer 5-9
Adaptors Communicating
sending side encapsulates datagram
in frame adds error checking
bits rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Data Link Layer 5-10
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-11
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
Data Link Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 5-13
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-14
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-15
CRC Example
R = remainder[ ]D2r
G
r is size of G -1 Steps
Append r 0rsquos to D Divide it by G and
compute reminder Data transmitted is
D followed by R
Message transmitted is101110 011
Data Link Layer 5-16
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access broadcast (shared wire or medium)
old-fashioned Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
timemultiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-19
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-20
MAC Protocols a taxonomyThree broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Data Link Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fre
quen
cy b
ands
time
FDM cable
Data Link Layer 5-23
Random Access Protocols When node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions) Examples of random access MAC protocols
slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-3
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-4
Link Layer IntroductionTerminology hosts and routers are nodes communication channels
that connect adjacent nodes along communication path are links wired links wireless links LANs
layer-2 packet is a frame encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link
Data Link Layer 5-5
Link layer context datagram transferred
by different link protocols over different links eg Ethernet on first
link frame relay on intermediate links 80211 on last link
each link protocol provides different services eg may or may not
provide rdt over link
transportation analogy trip from Princeton to
Lausanne taxi Princeton to JFK plane JFK to Geneva train Geneva to
Lausanne tourist = datagram transport segment =
communication link transportation mode =
link layer protocol travel agent = routing
algorithm
Data Link Layer 5-6
Link Layer Services framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit-error link (fiber some twisted
pair) wireless links high error rates
bull Q why both link-level and end-end reliability
Data Link Layer 5-7
Link Layer Services (more) flow control
pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame error correction
receiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
Data Link Layer 5-8
Where is the link layer implemented in each and every host link layer implemented
in ldquoadaptorrdquo (aka network interface card NIC) Ethernet card PCMCI
card 80211 card implements link
physical layer attaches into hostrsquos
system buses combination of
hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
Data Link Layer 5-9
Adaptors Communicating
sending side encapsulates datagram
in frame adds error checking
bits rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Data Link Layer 5-10
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-11
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
Data Link Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 5-13
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-14
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-15
CRC Example
R = remainder[ ]D2r
G
r is size of G -1 Steps
Append r 0rsquos to D Divide it by G and
compute reminder Data transmitted is
D followed by R
Message transmitted is101110 011
Data Link Layer 5-16
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access broadcast (shared wire or medium)
old-fashioned Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
timemultiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-19
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-20
MAC Protocols a taxonomyThree broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Data Link Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fre
quen
cy b
ands
time
FDM cable
Data Link Layer 5-23
Random Access Protocols When node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions) Examples of random access MAC protocols
slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-4
Link Layer IntroductionTerminology hosts and routers are nodes communication channels
that connect adjacent nodes along communication path are links wired links wireless links LANs
layer-2 packet is a frame encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link
Data Link Layer 5-5
Link layer context datagram transferred
by different link protocols over different links eg Ethernet on first
link frame relay on intermediate links 80211 on last link
each link protocol provides different services eg may or may not
provide rdt over link
transportation analogy trip from Princeton to
Lausanne taxi Princeton to JFK plane JFK to Geneva train Geneva to
Lausanne tourist = datagram transport segment =
communication link transportation mode =
link layer protocol travel agent = routing
algorithm
Data Link Layer 5-6
Link Layer Services framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit-error link (fiber some twisted
pair) wireless links high error rates
bull Q why both link-level and end-end reliability
Data Link Layer 5-7
Link Layer Services (more) flow control
pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame error correction
receiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
Data Link Layer 5-8
Where is the link layer implemented in each and every host link layer implemented
in ldquoadaptorrdquo (aka network interface card NIC) Ethernet card PCMCI
card 80211 card implements link
physical layer attaches into hostrsquos
system buses combination of
hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
Data Link Layer 5-9
Adaptors Communicating
sending side encapsulates datagram
in frame adds error checking
bits rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Data Link Layer 5-10
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-11
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
Data Link Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 5-13
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-14
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-15
CRC Example
R = remainder[ ]D2r
G
r is size of G -1 Steps
Append r 0rsquos to D Divide it by G and
compute reminder Data transmitted is
D followed by R
Message transmitted is101110 011
Data Link Layer 5-16
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access broadcast (shared wire or medium)
old-fashioned Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
timemultiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-19
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-20
MAC Protocols a taxonomyThree broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Data Link Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fre
quen
cy b
ands
time
FDM cable
Data Link Layer 5-23
Random Access Protocols When node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions) Examples of random access MAC protocols
slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-5
Link layer context datagram transferred
by different link protocols over different links eg Ethernet on first
link frame relay on intermediate links 80211 on last link
each link protocol provides different services eg may or may not
provide rdt over link
transportation analogy trip from Princeton to
Lausanne taxi Princeton to JFK plane JFK to Geneva train Geneva to
Lausanne tourist = datagram transport segment =
communication link transportation mode =
link layer protocol travel agent = routing
algorithm
Data Link Layer 5-6
Link Layer Services framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit-error link (fiber some twisted
pair) wireless links high error rates
bull Q why both link-level and end-end reliability
Data Link Layer 5-7
Link Layer Services (more) flow control
pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame error correction
receiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
Data Link Layer 5-8
Where is the link layer implemented in each and every host link layer implemented
in ldquoadaptorrdquo (aka network interface card NIC) Ethernet card PCMCI
card 80211 card implements link
physical layer attaches into hostrsquos
system buses combination of
hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
Data Link Layer 5-9
Adaptors Communicating
sending side encapsulates datagram
in frame adds error checking
bits rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Data Link Layer 5-10
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-11
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
Data Link Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 5-13
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-14
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-15
CRC Example
R = remainder[ ]D2r
G
r is size of G -1 Steps
Append r 0rsquos to D Divide it by G and
compute reminder Data transmitted is
D followed by R
Message transmitted is101110 011
Data Link Layer 5-16
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access broadcast (shared wire or medium)
old-fashioned Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
timemultiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-19
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-20
MAC Protocols a taxonomyThree broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Data Link Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fre
quen
cy b
ands
time
FDM cable
Data Link Layer 5-23
Random Access Protocols When node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions) Examples of random access MAC protocols
slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-6
Link Layer Services framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit-error link (fiber some twisted
pair) wireless links high error rates
bull Q why both link-level and end-end reliability
Data Link Layer 5-7
Link Layer Services (more) flow control
pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame error correction
receiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
Data Link Layer 5-8
Where is the link layer implemented in each and every host link layer implemented
in ldquoadaptorrdquo (aka network interface card NIC) Ethernet card PCMCI
card 80211 card implements link
physical layer attaches into hostrsquos
system buses combination of
hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
Data Link Layer 5-9
Adaptors Communicating
sending side encapsulates datagram
in frame adds error checking
bits rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Data Link Layer 5-10
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-11
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
Data Link Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 5-13
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-14
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-15
CRC Example
R = remainder[ ]D2r
G
r is size of G -1 Steps
Append r 0rsquos to D Divide it by G and
compute reminder Data transmitted is
D followed by R
Message transmitted is101110 011
Data Link Layer 5-16
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access broadcast (shared wire or medium)
old-fashioned Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
timemultiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-19
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-20
MAC Protocols a taxonomyThree broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Data Link Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fre
quen
cy b
ands
time
FDM cable
Data Link Layer 5-23
Random Access Protocols When node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions) Examples of random access MAC protocols
slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-7
Link Layer Services (more) flow control
pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame error correction
receiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
Data Link Layer 5-8
Where is the link layer implemented in each and every host link layer implemented
in ldquoadaptorrdquo (aka network interface card NIC) Ethernet card PCMCI
card 80211 card implements link
physical layer attaches into hostrsquos
system buses combination of
hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
Data Link Layer 5-9
Adaptors Communicating
sending side encapsulates datagram
in frame adds error checking
bits rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Data Link Layer 5-10
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-11
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
Data Link Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 5-13
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-14
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-15
CRC Example
R = remainder[ ]D2r
G
r is size of G -1 Steps
Append r 0rsquos to D Divide it by G and
compute reminder Data transmitted is
D followed by R
Message transmitted is101110 011
Data Link Layer 5-16
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access broadcast (shared wire or medium)
old-fashioned Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
timemultiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-19
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-20
MAC Protocols a taxonomyThree broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Data Link Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fre
quen
cy b
ands
time
FDM cable
Data Link Layer 5-23
Random Access Protocols When node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions) Examples of random access MAC protocols
slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-8
Where is the link layer implemented in each and every host link layer implemented
in ldquoadaptorrdquo (aka network interface card NIC) Ethernet card PCMCI
card 80211 card implements link
physical layer attaches into hostrsquos
system buses combination of
hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
Data Link Layer 5-9
Adaptors Communicating
sending side encapsulates datagram
in frame adds error checking
bits rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Data Link Layer 5-10
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-11
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
Data Link Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 5-13
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-14
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-15
CRC Example
R = remainder[ ]D2r
G
r is size of G -1 Steps
Append r 0rsquos to D Divide it by G and
compute reminder Data transmitted is
D followed by R
Message transmitted is101110 011
Data Link Layer 5-16
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access broadcast (shared wire or medium)
old-fashioned Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
timemultiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-19
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-20
MAC Protocols a taxonomyThree broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Data Link Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fre
quen
cy b
ands
time
FDM cable
Data Link Layer 5-23
Random Access Protocols When node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions) Examples of random access MAC protocols
slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-9
Adaptors Communicating
sending side encapsulates datagram
in frame adds error checking
bits rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Data Link Layer 5-10
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-11
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
Data Link Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 5-13
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-14
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-15
CRC Example
R = remainder[ ]D2r
G
r is size of G -1 Steps
Append r 0rsquos to D Divide it by G and
compute reminder Data transmitted is
D followed by R
Message transmitted is101110 011
Data Link Layer 5-16
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access broadcast (shared wire or medium)
old-fashioned Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
timemultiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-19
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-20
MAC Protocols a taxonomyThree broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Data Link Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fre
quen
cy b
ands
time
FDM cable
Data Link Layer 5-23
Random Access Protocols When node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions) Examples of random access MAC protocols
slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-10
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-11
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
Data Link Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 5-13
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-14
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-15
CRC Example
R = remainder[ ]D2r
G
r is size of G -1 Steps
Append r 0rsquos to D Divide it by G and
compute reminder Data transmitted is
D followed by R
Message transmitted is101110 011
Data Link Layer 5-16
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access broadcast (shared wire or medium)
old-fashioned Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
timemultiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-19
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-20
MAC Protocols a taxonomyThree broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Data Link Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fre
quen
cy b
ands
time
FDM cable
Data Link Layer 5-23
Random Access Protocols When node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions) Examples of random access MAC protocols
slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-11
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
Data Link Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 5-13
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-14
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-15
CRC Example
R = remainder[ ]D2r
G
r is size of G -1 Steps
Append r 0rsquos to D Divide it by G and
compute reminder Data transmitted is
D followed by R
Message transmitted is101110 011
Data Link Layer 5-16
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access broadcast (shared wire or medium)
old-fashioned Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
timemultiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-19
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-20
MAC Protocols a taxonomyThree broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Data Link Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fre
quen
cy b
ands
time
FDM cable
Data Link Layer 5-23
Random Access Protocols When node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions) Examples of random access MAC protocols
slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 5-13
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-14
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-15
CRC Example
R = remainder[ ]D2r
G
r is size of G -1 Steps
Append r 0rsquos to D Divide it by G and
compute reminder Data transmitted is
D followed by R
Message transmitted is101110 011
Data Link Layer 5-16
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access broadcast (shared wire or medium)
old-fashioned Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
timemultiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-19
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-20
MAC Protocols a taxonomyThree broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Data Link Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fre
quen
cy b
ands
time
FDM cable
Data Link Layer 5-23
Random Access Protocols When node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions) Examples of random access MAC protocols
slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-13
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-14
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-15
CRC Example
R = remainder[ ]D2r
G
r is size of G -1 Steps
Append r 0rsquos to D Divide it by G and
compute reminder Data transmitted is
D followed by R
Message transmitted is101110 011
Data Link Layer 5-16
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access broadcast (shared wire or medium)
old-fashioned Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
timemultiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-19
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-20
MAC Protocols a taxonomyThree broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Data Link Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fre
quen
cy b
ands
time
FDM cable
Data Link Layer 5-23
Random Access Protocols When node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions) Examples of random access MAC protocols
slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-14
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-15
CRC Example
R = remainder[ ]D2r
G
r is size of G -1 Steps
Append r 0rsquos to D Divide it by G and
compute reminder Data transmitted is
D followed by R
Message transmitted is101110 011
Data Link Layer 5-16
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access broadcast (shared wire or medium)
old-fashioned Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
timemultiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-19
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-20
MAC Protocols a taxonomyThree broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Data Link Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fre
quen
cy b
ands
time
FDM cable
Data Link Layer 5-23
Random Access Protocols When node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions) Examples of random access MAC protocols
slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-15
CRC Example
R = remainder[ ]D2r
G
r is size of G -1 Steps
Append r 0rsquos to D Divide it by G and
compute reminder Data transmitted is
D followed by R
Message transmitted is101110 011
Data Link Layer 5-16
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access broadcast (shared wire or medium)
old-fashioned Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
timemultiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-19
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-20
MAC Protocols a taxonomyThree broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Data Link Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fre
quen
cy b
ands
time
FDM cable
Data Link Layer 5-23
Random Access Protocols When node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions) Examples of random access MAC protocols
slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-16
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access broadcast (shared wire or medium)
old-fashioned Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
timemultiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-19
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-20
MAC Protocols a taxonomyThree broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Data Link Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fre
quen
cy b
ands
time
FDM cable
Data Link Layer 5-23
Random Access Protocols When node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions) Examples of random access MAC protocols
slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access broadcast (shared wire or medium)
old-fashioned Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
timemultiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-19
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-20
MAC Protocols a taxonomyThree broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Data Link Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fre
quen
cy b
ands
time
FDM cable
Data Link Layer 5-23
Random Access Protocols When node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions) Examples of random access MAC protocols
slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
timemultiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-19
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-20
MAC Protocols a taxonomyThree broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Data Link Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fre
quen
cy b
ands
time
FDM cable
Data Link Layer 5-23
Random Access Protocols When node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions) Examples of random access MAC protocols
slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-19
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-20
MAC Protocols a taxonomyThree broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Data Link Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fre
quen
cy b
ands
time
FDM cable
Data Link Layer 5-23
Random Access Protocols When node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions) Examples of random access MAC protocols
slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-20
MAC Protocols a taxonomyThree broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Data Link Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fre
quen
cy b
ands
time
FDM cable
Data Link Layer 5-23
Random Access Protocols When node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions) Examples of random access MAC protocols
slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fre
quen
cy b
ands
time
FDM cable
Data Link Layer 5-23
Random Access Protocols When node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions) Examples of random access MAC protocols
slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fre
quen
cy b
ands
time
FDM cable
Data Link Layer 5-23
Random Access Protocols When node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions) Examples of random access MAC protocols
slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-23
Random Access Protocols When node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions) Examples of random access MAC protocols
slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-24
Slotted ALOHAAssumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-25
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able
to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-
1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage human analogy the polite
conversationalist collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-32
CSMACD collision detection
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-36
Summary of MAC protocols channel partitioning by time frequency or
code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-37
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-38
MAC Addresses and ARP 32-bit IP address
network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-40
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-41
ARP Address Resolution Protocol Each IP node (host
router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live)
time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53 LAN
137196723
137196778
137196714
137196788
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-42
ARP protocol Same LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address =
FF-FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that times out (goes away)
unless refreshed ARP is ldquoplug-and-playrdquo
nodes create their ARP tables without intervention from net administrator
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-43
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-44
Ethernetldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-45
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-46
Ethernet Frame StructureSending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011 used to synchronize receiver sender clock
rates
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-47
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame Type indicates higher layer protocol (mostly IP
but others possible eg Novell IPX AppleTalk) CRC checked at receiver if error is detected
frame is dropped
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-48
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-49
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-50
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended Another here
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-51
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-52
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-53
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-54
Link Layer51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-55
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-56
Switch link-layer device smarter than hubs take
active role store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-57
Switch allows multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-58
Switch Table Q how does switch
know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing table
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-59
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-60
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo frame destination
unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-61
Interconnecting switches switches can be connected together
AB
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C DE
FS2
S4
S3
HI
G
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-62
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
AB
S1
C DE
FS2
S4
S3
HI
G
12
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-63
Switches vs Routers both store-and-
forward devices routers network-
layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
application
transportnetwork
linkphysical
frameframe
framedatagram
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-64
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-65
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based
on MAC addresses of endpoints rather than switch port dynamic membership
ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell
combined switches plus routers
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-66
Link Layer51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization
MPLS59 A day in the life of a
web request
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-67
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack)
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-68
PPP Data Frame Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-69
PPP Data Frame info upper layer data being carried check cyclic redundancy check for error
detection
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-
Data Link Layer 5-70
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs PPP virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
- Slide 1
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Link Layer (2)
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Link Layer (3)
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slotted ALOHA (2)
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- ldquoTaking Turnsrdquo MAC protocols (2)
- ldquoTaking Turnsrdquo MAC protocols (3)
- Summary of MAC protocols
- Link Layer (4)
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Link Layer (5)
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Ethernet Unreliable connectionless
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Manchester encoding
- Link Layer (6)
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Switches vs Routers
- VLANs
- Port-based VLAN
- Link Layer (7)
- Point to Point Data Link Control
- PPP Data Frame
- PPP Data Frame (2)
- Chapter 5 Summary
-