5: datalink layer 5a-1 chapter 5 data link layer cmpt 371 data communications and networking
TRANSCRIPT
5: DataLink Layer 5a-1
Chapter 5Data Link Layer
CMPT 371Data Communications
and Networking
5: DataLink Layer 5a-2
Chapter 5: The Data Link LayerOur goals: understand principles behind data link layer
services: error detection, correction sharing a broadcast channel: multiple access link layer addressing reliable data transfer, flow control: done!
instantiation and implementation of various link layer technologies
5: DataLink Layer 5a-3
Chapter 5 outline
5.1 Introduction and services
5.2 Error detection and correction
5.3Multiple access protocols CDMA Aloha, CSMA/CD
5.4 Ethernet
5.5 Hubs and switches 5.6 Wireless links and
LANs 802.11 (WiFi)
5: DataLink Layer 5a-4
Link Layer: IntroductionSome terminology: hosts and routers are nodes (bridges and switches too) communication channels that
connect adjacent nodes along communication path are links wired links wireless links LANs
Data unit is a frame, encapsulates datagram
“ link”
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
5: DataLink Layer 5a-5
Where is the link layer implemented !
link layer implemented in “adaptor” (aka NIC) or on a chip Ethernet card, 802.11
card/chipset
Combining hardware, software, firmware
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
5: DataLink Layer 5a-6
Link layer: context
Datagram transferred by different link protocols over different links: e.g., Ethernet on first
link, frame relay on intermediate links, 802.11 on last link
Each link protocol provides different services e.g., may or may not
provide rdt over link
transportation analogy trip from Princeton to
Lausanne limo: Princeton to JFK plane: JFK to Geneva train: Geneva to Lausanne
tourist = datagram transport segment =
communication link transportation mode =
link layer protocol travel agent = routing
algorithm
5: DataLink Layer 5a-7
Link Layer Services
Reliable delivery between adjacent nodes we learned how to do this already (chapter
3)!• Q: why both link-level and end-end
reliability?
5: DataLink Layer 5a-8
Link Layer Services (more)
Flow Control (Chap 3): pacing between adjacent sending and receiving
nodes
Error Detection: errors caused by signal attenuation, noise. receiver detects presence of errors:
• signals sender for retransmission or drops frame
Error Correction: receiver identifies and corrects bit error(s) without
resorting to retransmission
5: DataLink Layer 5a-9
Chapter 5 outline
5.1 Introduction and services
5.2 Error detection and correction
5.3Multiple access protocols
5.4 Ethernet
5.5 Hubs, bridges, and switches
5.6 Wireless links and LANs
5: DataLink Layer 5a-10
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking, may include header fields
• Error detection not 100% reliable!• protocol may miss some errors, but rarely• larger EDC field yields better detection and correction
5: DataLink Layer 5a-11
Internet checksum
Sender: treat segment contents
as sequence of 16-bit integers
checksum: addition (1’s complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver: compute checksum of
received segment check if computed checksum
equals checksum field value: NO - error detected YES - no error detected.
But maybe errors nonetheless?
Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer only)
5: DataLink Layer 5a-12
Example of checksum - RecallSegments
0110011001100110 0101010101010101+= 1011101110111011
0100010001000100 1’s complement
5: DataLink Layer 5a-13
Parity Checking
Single Bit Parity:Detect single bit errors
Two Dimensional Bit Parity:Detect and correct single bit errors
0 0
5: DataLink Layer 5a-14
Chapter 5 outline
5.1 Introduction and services
5.2 Error detection and correction
5.3Multiple access protocols
5.4 Ethernet
5.5 Hubs, bridges, and switches
5.6 Wireless links and LANs
5: DataLink Layer 5a-15
Multiple Access Links and Protocols
Two types of “links”: point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) traditional Ethernet Satellite 802.11 wireless LAN
Or lecture room
5: DataLink Layer 5a-16
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by
nodes: interference only one node can send successfully at a time
multiple access protocol algorithm that determines how nodes share
channel, i.e., determine when node can transmit
communication about channel sharing must use channel itself!
what to look for in multiple access protocols:
5: DataLink Layer 5a-17
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1. When one node wants to transmit, it can send
at rate R.2. When M nodes want to transmit, each can
send at average rate R/M3. Fully decentralized:
no special node to coordinate transmissions no synchronization of clocks, slots
4. Simple
5: DataLink Layer 5a-18
MAC Protocols: a taxonomy
Three broad classes: Channel Partitioning
divide channel into smaller “pieces” (time slots, frequency, code)
allocate piece to node for exclusive use
Random Access channel not divided, allow collisions “recover” from collisions
“Taking turns” speak only if “token” is at hand tightly coordinate shared access to avoid collisions
5: DataLink Layer 5a-19
Channel Partitioning MAC protocols: TDMA
TDMA: time division multiple access access to channel in "rounds" each station gets fixed length slot (length = pkt
trans time) in each round example: 6-station LAN, 1,3,4 have pkt, slots
2,5,6 idle
Problem unused slots go idle More ?
1 3 4 1 3 4
6-slotframe
6-slotframe
5: DataLink Layer 5a-20
Channel Partitioning MAC protocols: FDMA
FDMA: frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band example: 6-station LAN, 1,3,4 have pkt, frequency bands
2,5,6 idle
Problem unused transmission time in frequency bands go idle More ?
freq
uenc
y ba
nds
time
FDM cable
5: DataLink Layer 5a-21
Channel Partitioning (CDMA)
CDMA (Code Division Multiple Access) Analogy
Public Key Encryption• Only the one holing the key can decrypt
MotivationSender – Mix information encoded with
“codes” of receiversReceiver – Decode mixed information
using its own code, and find that for itself
5: DataLink Layer 5a-22
Channel Partitioning (CDMA)
CDMA (Code Division Multiple Access) used mostly in wireless broadcast channels (cellular,
satellite, etc) unique “code” assigned to each user; i.e., code set
partitioning all users share same frequency, but each user has own
“chipping” sequence (i.e., code) to encode data encoded signal = (original data) X (chipping sequence) decoding: inner-product of encoded signal and chipping
sequence allows multiple users to “coexist” and transmit
simultaneously with minimal interference (if codes are “orthogonal”)
5: DataLink Layer 5a-23
CDMA Encode/Decode
5: DataLink Layer 5a-24
CDMA: two-sender interference
5: DataLink Layer 5a-25
CDMA: two-sender interferenceAssume codes
(1,-1) for recv1
(1,1) for recv2
Data bit 1 for recv 1:
1·(1,-1)
Decoded using code for recv1:
1·(1,-1) ·(1,-1)
=(1,-1) ·(1,-1)=2
Decoded using code for recv 2: 1·(1,-1) ·(1,1)=(1,-1) ·(1,1)=0
Frequency Hopping (origin of CDMA) Frequency Hopping Spread Spectrum: transmitting radio signal by rapidly switching among many frequency channels, using a sequence known to both transmitter and receiver
5: DataLink Layer 5a-26Google “Mother of CDMA”
5: DataLink Layer 5a-27
Random Access Protocols
When node has packet to send transmit at full channel data rate R. no a priori coordination among nodes
two or more transmitting nodes -> “collision”, random access MAC protocol specifies:
how to detect collisions how to recover from collisions (e.g., via delayed
retransmissions)
Examples of random access MAC protocols: slotted ALOHA ALOHA CSMA, CSMA/CD (Ethernet), CSMA/CA (802.11 WLAN or
Wi-Fi)
5: DataLink Layer 5a-28
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots, time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot, all nodes detect collision
Operation when node obtains fresh
frame, it transmits in next slot
no collision, node can send new frame in next slot
if collision, node retransmits frame in each subsequent slot with prob. p until success
5: DataLink Layer 5a-29
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized: only slots in nodes need to be in sync
simple
Cons collisions, wasting slots idle slots nodes may be able to
detect collision in less than time to transmit packet – but sill waste a slot
clock
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
5: DataLink Layer 5a-30
Slotted Aloha efficiency
Suppose a node transmits a frame in a slot with probability p
prob that a node 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: max efficiency 1/e = .37
Efficiency: long-run fraction of successful slots (many nodes, each with many frames to send)
At best: channelused for useful transmissions 37%of time!
5: DataLink Layer 5a-31
CSMA (Carrier Sense Multiple Access)
CSMA: listen before transmit: If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission
No time slot – fully distributed Human analogy: listen and don’t interrupt others
Seems perfect – no collision !
5: DataLink Layer 5a-32
CSMA collisions
collisions can still occur:propagation delay means two nodes may not heareach other’s transmissioncollision:entire packet transmission time wasted
spatial layout of nodes
note:role of distance & propagation delay in determining collision probability
5: DataLink Layer 5a-33
CSMA collisions
collisions can still occur:propagation delay means two nodes may not heareach other’s transmissioncollision:entire packet transmission time wastednote:role of distance & propagation delay in determining collision probability
spatial layout of nodes
5: DataLink Layer 5a-34
CSMA/CD (Collision Detection)CSMA/CD: carrier sensing, deferral as in CSMA
collisions detected within short time colliding transmissions aborted, reducing channel
wastage collision detection:
easy in wired LANs: measure signal strengths, compare transmitted, received signals
difficult in wireless LANs: receiver shut off while transmitting
human analogy: the polite conversationalist
5: DataLink Layer 5a-35
CSMA/CD collision detection
5: DataLink Layer 5a-36
“ Taking Turns” MAC protocolschannel partitioning MAC protocols:
share channel efficiently and fairly at high load
inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node!
Random access MAC protocols efficient at low load: single node can fully
utilize channel high load: collision overhead
“taking turns” protocolslook for best of both worlds! – but practically not
5: DataLink Layer 5a-37
“ Taking Turns” MAC protocolsType 1: Polling: master node
“invites” slave nodes to transmit in turn Ask round-robin: do
you have frame to send ? (e.g., USB polling)
concerns: polling overhead latency single point of failure
(master)
master
slaves
poll
data
data
5: DataLink Layer 5a-38
“ Taking Turns” MAC protocolsType 2: Token passing: control token passed
from one node to next sequentially.
token message concerns:
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5a-39
Case Study: Cable Access Network multiple 40Mbps downstream (broadcast)
channels single CMTS transmits into channels
multiple 30 Mbps upstream channels multiple access: all users contend for certain
upstream channel time slots (others assigned)
cable headend
CMTS
ISP
cable modemtermination system
cablemodem
splitter
…
…
Internet frames,TV channels, control transmitted downstream at different frequencies
upstream Internet frames, TV control, transmitted upstream at different frequencies in time slots
5: DataLink Layer 5a-40
Case Study: Cable Access Network DOCSIS: data over cable service interface spec
FDM over upstream, downstream frequency channels TDM upstream: some slots assigned, some have
contention• downstream MAP frame: assigns upstream slots• request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1, t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
5: DataLink Layer 5a-41
Chapter 5 outline
5.1 Introduction and services
5.2 Error detection and correction
5.3Multiple access protocols
5.4 Ethernet
5.5 Hubs, bridges, and switches
5.6 Wireless links and LANs
5: DataLink Layer 5a-42
Ethernet
“ dominant” LAN technology: cheap $20 for 100Mbs! first widely used LAN technology Simpler, cheaper than competitors Kept up with speed race: 10, 100, 1000 Mbps …
Metcalfe’s Ethernetsketch
5: DataLink Layer 5a-43
Ethernet uses CSMA/CD
adapter doesn’t transmit if it senses that some other adapter is transmitting, that is, carrier sense
transmitting 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 5a-44
Ethernet Technologies: 10Base2 10: 10Mbps; 2: under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces: physical layer device only! has become a legacy technology (mid 90s and earlier)
5: DataLink Layer 5a-45
10BaseT and 100BaseT 10/100 Mbps rate; latter called “fast ethernet”
Now toward gigabit
T stands for Twisted Pair Nodes connect to a switch: “star topology”
Link Layer 5-45
switch
bus: coaxial cablestar
5: DataLink Layer 5a-46
Manchester encoding
Used in 10BaseT, 10Base2 Each bit has a transition Allows clocks in sending and receiving nodes
to synchronize to each other no need for a centralized, global clock among nodes!
5: DataLink Layer 5a-47
Chapter 5 outline
5.1 Introduction and services
5.2 Error detection and correction
5.3Multiple access protocols
5.4 Ethernet
5.5 Hubs and switches 5.6 Wireless links and
LANs
5: DataLink Layer 5a-48
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Can’t interconnect 10BaseT & 100BaseT
hub
hubhub
hub
5: DataLink Layer 5a-49
Switch hosts have dedicated, direct
connection to switch switches buffer packets Ethernet protocol used on
each incoming link, but no collisions; full duplex each link is its own
collision domain switching: A-to-A’ and B-to-B
’ can transmit simultaneously, without collisions
switch with six interfaces(1,2,3,4,5,6)
A
A’
B
B’ C
C’
1 2
345
6
5: DataLink Layer 5a-50
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment, uses
CSMA/CD to access segment transparent
hosts are unaware of presence of switches plug-and-play, self-learning
switches do not need to be configured
5: DataLink Layer 5a-51
Forwarding
Q: how does switch know A’ reachable via interface 4, B’ reachable via interface 5?
switch with six interfaces(1,2,3,4,5,6)
A
A’
B
B’ C
C’
1 2
345
6
Q: how are entries created, maintained in switch table?
something like a routing protocol?
A: each switch has a switch table, each entry: (MAC address of host,
interface to reach host, time stamp)
looks like a routing table!
5: DataLink Layer 5a-52
Self learning
A switch has a switch (forwarding) table
switch learns which hosts can be reached through which interfaces when frame received, switch “learns” location of
sender: incoming LAN segment records sender/location pair in switch table
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5: DataLink Layer 5a-53
Filtering/ForwardingWhen switch receives a frame:
index switch table using MAC dest addressif entry found for destination
then{ if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated } else flood
forward on all but the interface on which the frame arrived
5: DataLink Layer 5a-54
Switch example
Suppose C sends frame to D
Switch receives frame from C notes in switch table that C is on interface 1 because 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 interface
ABEG
1123
12 3
5: DataLink Layer 5a-55
Switch example
Suppose D replies back with frame to C.
Switch receives frame from from D notes in switch table that D is on interface 2 because 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 interface
ABEGC
11231
5: DataLink Layer 5a-56
Switch: traffic isolation switch installation breaks subnet into LAN
segments switch filters packets:
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5: DataLink Layer 5a-57
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5: DataLink Layer 5a-58
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug & play yes no yes
optimal routing
no yes no
cut through
yes no yes
5: DataLink Layer 5a-59
Chapter 5 outline
5.1 Introduction and services
5.2 Error detection and correction
5.3Multiple access protocols
5.4 Ethernet
5.5 Hubs, bridges, and switches
5.6 Wireless links and LANs
5: DataLink Layer 5a-60
IEEE 802.11 Wireless LAN
802.11b 2.4-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
• all hosts use same chipping code
All use CSMA/CA for multiple access All have base-station and ad-hoc network
versions
802.11a 5-6 GHz range up to 54 Mbps
802.11g 2.4-5 GHz range up to 54 Mbps
802.11n (2009) 2.4/5GHz, MIMO
5: DataLink Layer 5a-61
Base station approach Wireless host communicates with a base
station base station = access point (AP)
Basic Service Set (BSS) (a.k.a. “cell”) contains: wireless hosts access point (AP): base station
5: DataLink Layer 5a-62
Ad Hoc Network approach
No AP (i.e., base station) wireless hosts communicate with each other
to get packet from wireless host A to B may need to route through wireless hosts X,Y,Z
Applications: “laptop” meeting in conference room, car interconnection of “personal” devices battlefield
IETF MANET (Mobile Ad hoc Networks) working group
5: DataLink Layer 5a-63
IEEE 802.11: multiple access
Collision if 2 or more nodes transmit at same time
CSMA makes sense: get all the bandwidth if you’re the only one transmitting try to avoid collision if you sense another transmission
5: DataLink Layer 5a-64
Enhancement: CSMA/CD ?
Collision detection doesn’t work: hidden terminal problem
5: DataLink Layer 5a-65
Collision avoidance mechanisms Problem:
two nodes, hidden from each other, transmit complete frames to base station
wasted bandwidth for long duration !
Solution: small reservation packets nodes track reservation interval with internal
“network allocation vector” (NAV)
5: DataLink Layer 5a-66
Collision Avoidance: RTS-CTS exchange sender transmits short
RTS (request to send) packet: indicates duration of transmission
receiver replies with short CTS (clear to send) packet notifying (possibly
hidden) nodes
hidden nodes will not transmit for specified duration: NAV
5: DataLink Layer 5a-67
Collision Avoidance: RTS-CTS exchange
RTS and CTS short: collisions less likely, of
shorter duration end result similar to
collision detection IEEE 802.11 allows:
CSMA CSMA/CA: reservations polling from AP
5: DataLink Layer 5a-68
A word about Bluetooth
Low-power, small radius, wireless networking technology 10-100 meters
omnidirectional not line-of-sight
infrared
Interconnects gadgets 2.4-2.5 GHz
unlicensed radio band up to 721 kbps
Interference from wireless LANs, digital cordless phones, microwave ovens: frequency hopping
helps
MAC protocol supports: error correction ARQ
Each node has a 12-bit address
5: DataLink Layer 5a-69
Chapter 5: Summary
principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access link layer addressing, ARP
link layer technologies: Ethernet, hubs, bridges, switches,IEEE 802.11 LANs, PPP,
journey down the protocol stack now OVER! future stops: multimedia, security,
network management
Link Layer
Finally: a day in the life of a web request journey down protocol stack complete!
application, transport, network, link putting-it-all-together: synthesis!
goal: identify, review, understand protocols (at all layers) involved in seemingly simple scenario: requesting www page
scenario: student attaches laptop to campus network, requests/receives www.google.com
A day in the life: scenario
Comcast network 68.80.0.0/13
Google’s network 64.233.160.0/19 64.233.169.105
web server
DNS server
school network 68.80.2.0/24
web page
browser
router(runs DHCP)
A day in the life… connecting to the Internet
connecting laptop needs to get its own IP address, addr of first-hop router, addr of DNS server: use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in 802.3 Ethernet
Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server Ethernet demuxed to IP demuxed, UDP demuxed to DHCP
router(runs DHCP)
DHCP server formulates DHCP ACK containing client’s IP address, IP address of first-hop router for client, name & IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server, frame forwarded (switch learning) through LAN, demultiplexing at client
Client now has IP address, knows name & addr of DNS server, IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the life… connecting to the Internet
router(runs DHCP)
A day in the life… ARP (before DNS, before HTTP)
before sending HTTP request, need IP address of www.google.com: DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created, encapsulated in UDP, encapsulated in IP, encapsulated in Eth. To send frame to router, need MAC address of router interface: ARP
ARP query broadcast, received by router, which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router, so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network, routed (tables created by RIP, OSPF, IS-IS and/or BGP routing protocols) to DNS server demux’ed to DNS server
DNS server replies to client with IP address of www.google.com
Comcast network 68.80.0.0/13
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the life… using DNS
router(runs DHCP)
A day in the life…TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request, client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established!64.233.169.105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
A day in the life… HTTP request/reply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to www.google.com
IP datagram containing HTTP reply routed back to client64.233.169.105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally (!!!) displayed
5a-78
Data Center Networks (some hot topics in recent years; FYI only)
10’s to 100’s of thousands of hosts, often closely coupled, in close proximity: e-business (e.g. Amazon) content-servers (e.g., YouTube, Akamai, Apple, Microsoft) search engines, data mining (e.g., Google)
Inside a 40-ft Microsoft container, Chicago data center
challenges: multiple applications, each serving
massive numbers of clients managing/balancing load, avoiding
processing, networking, data bottlenecks
5: DataLink Layer
Data Center Networks load balancer: application-layer
routing receives external client requests directs workload within data center returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
5: DataLink Layer
Data Center Networks
rich interconnection among switches, racks: increased throughput between racks
(multiple routing paths possible) increased reliability via redundancy
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8