chapter 5 link layer and lans - oregon state...
TRANSCRIPT
5 DataLink Layer 5-1
Chapter 5Link Layer and LANs
5 DataLink Layer 5-2
Chapter 5 The Data Link LayerOur goals
understand principles behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressingreliable data transfer flow control done
instantiation and implementation of various link layer technologies
5 DataLink Layer 5-3
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-4
Link Layer IntroductionSome terminology
hosts and routers are nodescommunication channels that connect adjacent nodes along communication path are links
wired linkswireless linksLANs
layer-2 packet is a frameencapsulates datagram
ldquolinkrdquo
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
5 DataLink Layer 5-5
Link layer contextDatagram 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 analogytrip from Princeton to Lausanne
limo Princeton to JFKplane JFK to Genevatrain Geneva to Lausanne
tourist = datagramintermediate trips=communication linktransportation mode = link layer protocoltravel agent = routing algorithm
5 DataLink Layer 5-6
Link Layer ServicesFraming link access
encapsulate datagram into frame adding header trailerchannel access if shared mediumldquoMACrdquo addresses used in frame headers to identify source dest
bull different from IP addressReliable 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
5 DataLink Layer 5-7
Link Layer Services (more)
Flow Controlpacing between adjacent sending and receiving nodes
Error Detectionerrors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
Error Correctionreceiver identifies and corrects bit error(s) without resorting to retransmission
Half-duplex and full-duplexwith half duplex nodes at both ends of link can transmit but not at same time
5 DataLink Layer 5-8
Adaptors Communicating
link layer implemented in ldquoadaptorrdquo (aka NIC)
Ethernet card PCMCI card 80211 card
sending sideencapsulates datagram in a frameadds error checking bits rdt flow control etc
receiving sidelooks for errors rdt flow control etcextracts datagram passes to rcving node
adapter is semi-autonomouslink amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
5 DataLink Layer 5-9
Link Layer
51 Introduction and services52 Error detection and correction53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-10
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
5 DataLink Layer 5-11
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-12
Internet checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal 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 detectedcan detect all burst errors less than r+1 bits
widely used in practice (ATM HDCL)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC80211 wireless LAN
5 DataLink Layer 5-17
Multiple Access protocolssingle 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 transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-18
Ideal Multiple Access Protocol
Broadcast 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 transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-19
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess 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
5 DataLink Layer 5-21
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-22
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-23
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotno collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-2
Chapter 5 The Data Link LayerOur goals
understand principles behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressingreliable data transfer flow control done
instantiation and implementation of various link layer technologies
5 DataLink Layer 5-3
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-4
Link Layer IntroductionSome terminology
hosts and routers are nodescommunication channels that connect adjacent nodes along communication path are links
wired linkswireless linksLANs
layer-2 packet is a frameencapsulates datagram
ldquolinkrdquo
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
5 DataLink Layer 5-5
Link layer contextDatagram 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 analogytrip from Princeton to Lausanne
limo Princeton to JFKplane JFK to Genevatrain Geneva to Lausanne
tourist = datagramintermediate trips=communication linktransportation mode = link layer protocoltravel agent = routing algorithm
5 DataLink Layer 5-6
Link Layer ServicesFraming link access
encapsulate datagram into frame adding header trailerchannel access if shared mediumldquoMACrdquo addresses used in frame headers to identify source dest
bull different from IP addressReliable 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
5 DataLink Layer 5-7
Link Layer Services (more)
Flow Controlpacing between adjacent sending and receiving nodes
Error Detectionerrors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
Error Correctionreceiver identifies and corrects bit error(s) without resorting to retransmission
Half-duplex and full-duplexwith half duplex nodes at both ends of link can transmit but not at same time
5 DataLink Layer 5-8
Adaptors Communicating
link layer implemented in ldquoadaptorrdquo (aka NIC)
Ethernet card PCMCI card 80211 card
sending sideencapsulates datagram in a frameadds error checking bits rdt flow control etc
receiving sidelooks for errors rdt flow control etcextracts datagram passes to rcving node
adapter is semi-autonomouslink amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
5 DataLink Layer 5-9
Link Layer
51 Introduction and services52 Error detection and correction53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-10
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
5 DataLink Layer 5-11
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-12
Internet checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal 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 detectedcan detect all burst errors less than r+1 bits
widely used in practice (ATM HDCL)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC80211 wireless LAN
5 DataLink Layer 5-17
Multiple Access protocolssingle 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 transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-18
Ideal Multiple Access Protocol
Broadcast 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 transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-19
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess 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
5 DataLink Layer 5-21
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-22
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-23
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotno collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-3
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-4
Link Layer IntroductionSome terminology
hosts and routers are nodescommunication channels that connect adjacent nodes along communication path are links
wired linkswireless linksLANs
layer-2 packet is a frameencapsulates datagram
ldquolinkrdquo
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
5 DataLink Layer 5-5
Link layer contextDatagram 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 analogytrip from Princeton to Lausanne
limo Princeton to JFKplane JFK to Genevatrain Geneva to Lausanne
tourist = datagramintermediate trips=communication linktransportation mode = link layer protocoltravel agent = routing algorithm
5 DataLink Layer 5-6
Link Layer ServicesFraming link access
encapsulate datagram into frame adding header trailerchannel access if shared mediumldquoMACrdquo addresses used in frame headers to identify source dest
bull different from IP addressReliable 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
5 DataLink Layer 5-7
Link Layer Services (more)
Flow Controlpacing between adjacent sending and receiving nodes
Error Detectionerrors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
Error Correctionreceiver identifies and corrects bit error(s) without resorting to retransmission
Half-duplex and full-duplexwith half duplex nodes at both ends of link can transmit but not at same time
5 DataLink Layer 5-8
Adaptors Communicating
link layer implemented in ldquoadaptorrdquo (aka NIC)
Ethernet card PCMCI card 80211 card
sending sideencapsulates datagram in a frameadds error checking bits rdt flow control etc
receiving sidelooks for errors rdt flow control etcextracts datagram passes to rcving node
adapter is semi-autonomouslink amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
5 DataLink Layer 5-9
Link Layer
51 Introduction and services52 Error detection and correction53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-10
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
5 DataLink Layer 5-11
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-12
Internet checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal 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 detectedcan detect all burst errors less than r+1 bits
widely used in practice (ATM HDCL)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC80211 wireless LAN
5 DataLink Layer 5-17
Multiple Access protocolssingle 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 transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-18
Ideal Multiple Access Protocol
Broadcast 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 transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-19
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess 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
5 DataLink Layer 5-21
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-22
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-23
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotno collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-4
Link Layer IntroductionSome terminology
hosts and routers are nodescommunication channels that connect adjacent nodes along communication path are links
wired linkswireless linksLANs
layer-2 packet is a frameencapsulates datagram
ldquolinkrdquo
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
5 DataLink Layer 5-5
Link layer contextDatagram 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 analogytrip from Princeton to Lausanne
limo Princeton to JFKplane JFK to Genevatrain Geneva to Lausanne
tourist = datagramintermediate trips=communication linktransportation mode = link layer protocoltravel agent = routing algorithm
5 DataLink Layer 5-6
Link Layer ServicesFraming link access
encapsulate datagram into frame adding header trailerchannel access if shared mediumldquoMACrdquo addresses used in frame headers to identify source dest
bull different from IP addressReliable 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
5 DataLink Layer 5-7
Link Layer Services (more)
Flow Controlpacing between adjacent sending and receiving nodes
Error Detectionerrors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
Error Correctionreceiver identifies and corrects bit error(s) without resorting to retransmission
Half-duplex and full-duplexwith half duplex nodes at both ends of link can transmit but not at same time
5 DataLink Layer 5-8
Adaptors Communicating
link layer implemented in ldquoadaptorrdquo (aka NIC)
Ethernet card PCMCI card 80211 card
sending sideencapsulates datagram in a frameadds error checking bits rdt flow control etc
receiving sidelooks for errors rdt flow control etcextracts datagram passes to rcving node
adapter is semi-autonomouslink amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
5 DataLink Layer 5-9
Link Layer
51 Introduction and services52 Error detection and correction53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-10
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
5 DataLink Layer 5-11
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-12
Internet checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal 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 detectedcan detect all burst errors less than r+1 bits
widely used in practice (ATM HDCL)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC80211 wireless LAN
5 DataLink Layer 5-17
Multiple Access protocolssingle 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 transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-18
Ideal Multiple Access Protocol
Broadcast 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 transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-19
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess 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
5 DataLink Layer 5-21
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-22
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-23
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotno collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-5
Link layer contextDatagram 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 analogytrip from Princeton to Lausanne
limo Princeton to JFKplane JFK to Genevatrain Geneva to Lausanne
tourist = datagramintermediate trips=communication linktransportation mode = link layer protocoltravel agent = routing algorithm
5 DataLink Layer 5-6
Link Layer ServicesFraming link access
encapsulate datagram into frame adding header trailerchannel access if shared mediumldquoMACrdquo addresses used in frame headers to identify source dest
bull different from IP addressReliable 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
5 DataLink Layer 5-7
Link Layer Services (more)
Flow Controlpacing between adjacent sending and receiving nodes
Error Detectionerrors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
Error Correctionreceiver identifies and corrects bit error(s) without resorting to retransmission
Half-duplex and full-duplexwith half duplex nodes at both ends of link can transmit but not at same time
5 DataLink Layer 5-8
Adaptors Communicating
link layer implemented in ldquoadaptorrdquo (aka NIC)
Ethernet card PCMCI card 80211 card
sending sideencapsulates datagram in a frameadds error checking bits rdt flow control etc
receiving sidelooks for errors rdt flow control etcextracts datagram passes to rcving node
adapter is semi-autonomouslink amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
5 DataLink Layer 5-9
Link Layer
51 Introduction and services52 Error detection and correction53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-10
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
5 DataLink Layer 5-11
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-12
Internet checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal 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 detectedcan detect all burst errors less than r+1 bits
widely used in practice (ATM HDCL)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC80211 wireless LAN
5 DataLink Layer 5-17
Multiple Access protocolssingle 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 transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-18
Ideal Multiple Access Protocol
Broadcast 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 transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-19
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess 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
5 DataLink Layer 5-21
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-22
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-23
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotno collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-6
Link Layer ServicesFraming link access
encapsulate datagram into frame adding header trailerchannel access if shared mediumldquoMACrdquo addresses used in frame headers to identify source dest
bull different from IP addressReliable 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
5 DataLink Layer 5-7
Link Layer Services (more)
Flow Controlpacing between adjacent sending and receiving nodes
Error Detectionerrors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
Error Correctionreceiver identifies and corrects bit error(s) without resorting to retransmission
Half-duplex and full-duplexwith half duplex nodes at both ends of link can transmit but not at same time
5 DataLink Layer 5-8
Adaptors Communicating
link layer implemented in ldquoadaptorrdquo (aka NIC)
Ethernet card PCMCI card 80211 card
sending sideencapsulates datagram in a frameadds error checking bits rdt flow control etc
receiving sidelooks for errors rdt flow control etcextracts datagram passes to rcving node
adapter is semi-autonomouslink amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
5 DataLink Layer 5-9
Link Layer
51 Introduction and services52 Error detection and correction53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-10
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
5 DataLink Layer 5-11
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-12
Internet checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal 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 detectedcan detect all burst errors less than r+1 bits
widely used in practice (ATM HDCL)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC80211 wireless LAN
5 DataLink Layer 5-17
Multiple Access protocolssingle 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 transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-18
Ideal Multiple Access Protocol
Broadcast 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 transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-19
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess 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
5 DataLink Layer 5-21
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-22
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-23
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotno collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-7
Link Layer Services (more)
Flow Controlpacing between adjacent sending and receiving nodes
Error Detectionerrors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
Error Correctionreceiver identifies and corrects bit error(s) without resorting to retransmission
Half-duplex and full-duplexwith half duplex nodes at both ends of link can transmit but not at same time
5 DataLink Layer 5-8
Adaptors Communicating
link layer implemented in ldquoadaptorrdquo (aka NIC)
Ethernet card PCMCI card 80211 card
sending sideencapsulates datagram in a frameadds error checking bits rdt flow control etc
receiving sidelooks for errors rdt flow control etcextracts datagram passes to rcving node
adapter is semi-autonomouslink amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
5 DataLink Layer 5-9
Link Layer
51 Introduction and services52 Error detection and correction53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-10
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
5 DataLink Layer 5-11
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-12
Internet checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal 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 detectedcan detect all burst errors less than r+1 bits
widely used in practice (ATM HDCL)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC80211 wireless LAN
5 DataLink Layer 5-17
Multiple Access protocolssingle 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 transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-18
Ideal Multiple Access Protocol
Broadcast 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 transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-19
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess 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
5 DataLink Layer 5-21
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-22
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-23
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotno collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-8
Adaptors Communicating
link layer implemented in ldquoadaptorrdquo (aka NIC)
Ethernet card PCMCI card 80211 card
sending sideencapsulates datagram in a frameadds error checking bits rdt flow control etc
receiving sidelooks for errors rdt flow control etcextracts datagram passes to rcving node
adapter is semi-autonomouslink amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
5 DataLink Layer 5-9
Link Layer
51 Introduction and services52 Error detection and correction53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-10
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
5 DataLink Layer 5-11
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-12
Internet checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal 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 detectedcan detect all burst errors less than r+1 bits
widely used in practice (ATM HDCL)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC80211 wireless LAN
5 DataLink Layer 5-17
Multiple Access protocolssingle 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 transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-18
Ideal Multiple Access Protocol
Broadcast 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 transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-19
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess 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
5 DataLink Layer 5-21
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-22
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-23
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotno collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-9
Link Layer
51 Introduction and services52 Error detection and correction53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-10
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
5 DataLink Layer 5-11
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-12
Internet checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal 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 detectedcan detect all burst errors less than r+1 bits
widely used in practice (ATM HDCL)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC80211 wireless LAN
5 DataLink Layer 5-17
Multiple Access protocolssingle 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 transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-18
Ideal Multiple Access Protocol
Broadcast 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 transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-19
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess 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
5 DataLink Layer 5-21
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-22
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-23
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotno collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-10
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
5 DataLink Layer 5-11
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-12
Internet checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal 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 detectedcan detect all burst errors less than r+1 bits
widely used in practice (ATM HDCL)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC80211 wireless LAN
5 DataLink Layer 5-17
Multiple Access protocolssingle 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 transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-18
Ideal Multiple Access Protocol
Broadcast 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 transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-19
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess 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
5 DataLink Layer 5-21
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-22
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-23
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotno collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-11
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-12
Internet checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal 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 detectedcan detect all burst errors less than r+1 bits
widely used in practice (ATM HDCL)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC80211 wireless LAN
5 DataLink Layer 5-17
Multiple Access protocolssingle 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 transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-18
Ideal Multiple Access Protocol
Broadcast 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 transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-19
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess 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
5 DataLink Layer 5-21
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-22
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-23
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotno collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-12
Internet checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal 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 detectedcan detect all burst errors less than r+1 bits
widely used in practice (ATM HDCL)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC80211 wireless LAN
5 DataLink Layer 5-17
Multiple Access protocolssingle 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 transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-18
Ideal Multiple Access Protocol
Broadcast 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 transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-19
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess 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
5 DataLink Layer 5-21
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-22
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-23
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotno collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal 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 detectedcan detect all burst errors less than r+1 bits
widely used in practice (ATM HDCL)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC80211 wireless LAN
5 DataLink Layer 5-17
Multiple Access protocolssingle 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 transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-18
Ideal Multiple Access Protocol
Broadcast 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 transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-19
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess 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
5 DataLink Layer 5-21
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-22
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-23
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotno collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC80211 wireless LAN
5 DataLink Layer 5-17
Multiple Access protocolssingle 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 transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-18
Ideal Multiple Access Protocol
Broadcast 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 transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-19
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess 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
5 DataLink Layer 5-21
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-22
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-23
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotno collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-15
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches57 PPP
5 DataLink Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC80211 wireless LAN
5 DataLink Layer 5-17
Multiple Access protocolssingle 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 transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-18
Ideal Multiple Access Protocol
Broadcast 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 transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-19
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess 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
5 DataLink Layer 5-21
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-22
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-23
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotno collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC80211 wireless LAN
5 DataLink Layer 5-17
Multiple Access protocolssingle 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 transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-18
Ideal Multiple Access Protocol
Broadcast 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 transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-19
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess 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
5 DataLink Layer 5-21
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-22
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-23
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotno collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-17
Multiple Access protocolssingle 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 transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-18
Ideal Multiple Access Protocol
Broadcast 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 transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-19
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess 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
5 DataLink Layer 5-21
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-22
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-23
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotno collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-18
Ideal Multiple Access Protocol
Broadcast 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 transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-19
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess 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
5 DataLink Layer 5-21
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-22
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-23
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotno collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-19
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess 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
5 DataLink Layer 5-21
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-22
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-23
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotno collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess 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
5 DataLink Layer 5-21
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-22
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-23
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotno collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-21
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-22
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-23
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotno collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-22
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-23
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotno collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-23
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotno collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-24
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-25
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes 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 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-26
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-27
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= 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
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-28
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
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-29
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-30
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-31
CSMACD collision detection
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-32
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-34
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-35
LAN technologiesData link layer so far
services error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-36
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-38
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)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer 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 portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (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
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-41
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall 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-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-44
Dynamic Host Configuration Protocol (DHCP) Src IP 0000
Dest IP 255255255255
MAC FF-FF-FF-FF-FF-FF
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-45
DHCP
Use transaction ID to match the query request with the response
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-46
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-47
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-48
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-49
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-50
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-51
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-52
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 Adaptor receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-54
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 BackoffGoal 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 timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
transprop tt 511efficiency
+=
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-56
10BaseT and 100BaseT10100 Mbps rate latter called ldquofast ethernetrdquoT stands for Twisted PairNodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-57
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-58
Manchester encoding
Used in 10BaseTEach bit has a transitionAllows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-59
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyuses hubs called here ldquoBuffered DistributorsrdquoFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-60
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Interconnections Hubs and switches
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-61
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-62
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-63
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-64
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-65
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-66
Switch exampleSuppose C sends frame to D
Switch receives frame from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-67
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
A
B CD
EF G H
I
address interfaceABEGC
11231
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-68
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-69
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-70
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frame
slight reduction in latencycombinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-71
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-72
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS
5 DataLink Layer 5-73
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-74
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS