This document discusses the link layer and local area networks. It begins with an introduction to link layer services including framing, link access, reliable delivery, flow control, and error detection and correction. It then covers topics like multiple access protocols, including random access protocols like ALOHA and CSMA, and controlled access protocols. Local area network technologies are discussed next, focusing on Ethernet, switches, and addressing protocols like ARP. The document concludes with sections on link virtualization using MPLS and data center networking.

1. UNIT -5 The Link layer and Local
Area Networks

2. Link Layer 5-2
Link layer, LANs: outline
5.1 introduction, services
5.2 error detection,
correction
5.3 multiple access
protocols
5.4 LANs
 addressing, ARP
 Ethernet
 switches

3. Link Layer 5-3
Link layer: introduction
terminology:
 hosts and routers: nodes
 communication channels that
connect adjacent nodes along
communication path: links
 wired links
 wireless links
 LANs
 layer-2 packet: frame,
encapsulates datagram
data-link layer has responsibility of
transferring datagram from one node
to physically adjacent node over a link
global ISP

4. Link Layer 5-4
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. Link Layer 5-5
Link layer services
 framing, link access:
 encapsulate datagram into frame, adding header, trailer
 channel access if shared medium
 “MAC” addresses used in frame headers to identify
source, dest
• different from IP address!
 reliable delivery between adjacent nodes
 seldom used on low bit-error link (fiber, some twisted
pair)
 wireless links: high error rates

6. Link Layer 5-6
 flow control:
 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
 half-duplex and full-duplex
 with half duplex, nodes at both ends of link can transmit, but not
at same time
Link layer services (more)

7. Link Layer 5-7
Where is the link layer implemented?
 in each and every host
 link layer implemented in
“adaptor” (aka network
interface card NIC) or on a
chip
 Ethernet card, 802.11
card; Ethernet chipset
 implements link, physical
layer
 attaches into host’s system
buses
 combination of hardware,
software, firmware

8. Link Layer 5-8
Link layer, LANs: outline
5.1 introduction, services
5.2 error detection,
correction
5.3 multiple access
protocols
5.4 LANs
 addressing, ARP
 Ethernet
 switches
 VLANS
5.5 link virtualization:
MPLS
5.6 data center
networking
5.7 a day in the life of a
web request

9. Figure 9-1

10. Single-bit error
Figure 9-2

11. Multiple-bit error
Figure 9-3

12. Burst error
Figure 9-4

13. 13
Error Detection
 Data transmission can contain errors
 Single-bit
 Burst errors of length n
(n: distance between the first and last errors in data
block)
 How to detect errors
 If only data is transmitted, errors cannot be detected
 Send more information with data that satisfies a
special relationship
 Add redundancy

14. Redundancy
Figure 9-5

15. Figure 9-6

16. 16
Error Detection Methods
 Vertical Redundancy Check (VRC)
 Append a single bit at the end of data block such that
the number of ones is even
 Even Parity (odd parity is similar)
0110011  01100110
0110001  01100011
 VRC is also known as Parity Check
 Performance:
• Detects all odd-number errors in a data block

17. Winter 2005
ECE
ECE 766 17
Error Detection Methods
 Longitudinal Redundancy Check (LRC)
 Organize data into a table and create a parity for each
column
11100111 11011101 00111001 10101001
11100111
11011101
00111001
10101001
10101010
11100111 11011101 00111001 10101001 10101010
Original Data LRC

18. Winter 2005
ECE
ECE 766 18
Error Detection Methods
 Performance:
• Detects all burst errors up to length n
(number of columns)
• Misses burst errors of length n+1 if there are n-1 uninverted
bits between the first and last bit
• If the block is badly garbled, the probability of acceptance is
( )n
2
1

19. CRC
Figure 9-10

20. Binary Division
Figure 9-11

21. Polynomial
Figure 9-12
WCB/McGraw-Hill © The McGraw-Hill Companies, Inc., 1998

22. Figure 9-13
WCB/McGraw-Hill © The McGraw-Hill Companies, Inc., 1998
Polynomial and Divisor

23. Standard Polynomials
Figure 9-14
WCB/McGraw-Hill © The McGraw-Hill Companies, Inc., 1998

24. Example-CRC
Data Link Layer 5-24

25. Checksum
Figure 9-15
WCB/McGraw-Hill © The McGraw-Hill Companies, Inc., 1998
• Used by upper layer protocols
• Similar to LRC, uses one’s complement arithmetic

26. Figure 9-16
WCB/McGraw-Hill © The McGraw-Hill Companies, Inc., 1998
Data Unit and Checksum

27. Error Correction
Figure 9-17
WCB/McGraw-Hill © The McGraw-Hill Companies, Inc., 1998

28. Hamming CodeFigure 9-18

29. Figure 9-19
WCB/McGraw-Hill © The McGraw-Hill Companies, Inc., 1998
Hamming Code

30. Figure 9-19-continued
WCB/McGraw-Hill © The McGraw-Hill Companies, Inc., 1998
Hamming Code

31. Figure 9-20
WCB/McGraw-Hill © The McGraw-Hill Companies, Inc., 1998
Example of Hamming Code

32. Single-bit error
Figure 9-21
WCB/McGraw-Hill © The McGraw-Hill Companies, Inc., 1998

33. Figure 9-22
WCB/McGraw-Hill © The McGraw-Hill Companies, Inc., 1998
Error
Detection

34. Link Layer 5-34
Link layer, LANs: outline
5.1 introduction, services
5.2 error detection,
correction
5.3 multiple access
protocols
5.4 LANs
 addressing, ARP
 Ethernet
 switches
 VLANS

35. Link Layer 5-35
Multiple access protocols
 It avoids two or more nodes from transmitting at
the same time over a broadcast channel.
 If different nodes send data at the same time
collisions occurs and the receiver will not be able
to understand the signal.
 Classes of MAP
 Partitioning the channel eg. FDM & TDM
 Taking turns eg. Token based, polling based &
reservation based
 Contention based protocol e.g. ALOHA and Ethernet

36. Link Layer 5-36
Multiple access protocols
Different multiple access protocols..
Random access protocols-
 channel not divided, allow collisions
 “recover” from collisions
Controlled access protocols-
 Nodes take turns, but nodes with more to send can
take longer turns.
Channelization protocols-
 divide channel into smaller “pieces” (time slots,
frequency, code)
 allocate piece to node for exclusive use

37. Link Layer 5-37
Multiple access protocols
 Different multiple access protocols..
 Random access protocols
 ALOHA
 CSMA
 CSMA/CD
 Controlled access protocols
 Reservation
 Polling
 Token passing
 Channelization protocols
 FDMA
 TDMA

38. Link Layer 5-38
Multiple access links,
two types of “links”:
 point-to-point
 PPP for dial-up access
 point-to-point link between Ethernet switch, host
 broadcast (shared wire or medium)
 old-fashioned Ethernet
 upstream HFC
 802.11 wireless LAN
shared wire (e.g.,
cabled Ethernet) (e.g., 802.11 WiFi)

39. Link Layer 5-39
An ideal multiple access protocol
given: broadcast channel of rate R bps
desiderata:
1. 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/M
3. fully decentralized:
• no special node to coordinate transmissions
• no synchronization of clocks, slots
4. simple

40. Link Layer 5-40
Channel partitioning MAC protocols: TDMA
TDMA: time division multiple access
 access to channel in "rounds"
 each station gets fixed length slot (length = pkt
trans time) in each round
 unused slots go idle
 example: 6-station LAN, 1,3,4 have pkt, slots
2,5,6 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame

41. Link Layer 5-41
FDMA: frequency division multiple access
 channel spectrum divided into frequency bands
 each station assigned fixed frequency band
 unused transmission time in frequency bands go idle
 example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6
idle
frequencybands
time
FDM cable
Channel partitioning MAC protocols: FDMA

42. Link Layer 5-42
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:
 Pure (Unslotted )ALOHA
 slotted ALOHA
 CSMA, CSMA/CD, CSMA/CA

43. Link Layer 5-43
ALOHA
 It signifies a simple communications schemes in
which each transmitter within a network sends
data whenever there is a frame to send.
 Next frame is sent only if the previous frame
successfully reaches the destination.
 If frame fails to reach at the destination , it is sent
again.
 Types of ALOHA
 Pure ALOHA
 Slotted ALOHA

44. Link Layer 5-44
Pure (unslotted) ALOHA
 unslotted Aloha: simpler, no synchronization
 when frame first arrives
 transmit immediately
 collision probability increases:
 frame sent at t0 collides with other frames sent in [t0-1,t0+1]

45. Link Layer 5-45
Pure (unslotted) ALOHA
 unslotted Aloha: simpler, no synchronization
 when frame first arrives
 transmit immediately
 collision probability increases:
 frame sent at t0 collides with other frames sent in [t0-1,t0+1]

46. Link Layer 5-46
Pure ALOHA efficiency
P(success by given node) = P(node transmits) .
P(no other node transmits in [t0-1,t0] .
P(no other node transmits in [t0-1,t0]
= p . (1-p)N-1 . (1-p)N-1
= p . (1-p)2(N-1)
… choosing optimum p and then letting n
= 1/(2e) = .18

47. Link Layer 5-47
Slotted ALOHA
assumptions:
 all frames same size
 time divided into equal size
slots (time to transmit 1
frame)
 nodes start to transmit
only slot beginning
 nodes are synchronized
 if 2 or more nodes transmit
in slot, all nodes detect
collision
operation:
 when node obtains fresh
frame, transmits in next slot
 if no collision: node can send
new frame in next slot
 if collision: node retransmits
frame in each subsequent
slot with prob. p until
success

48. Link Layer 5-48
Pros:
 single active node can
continuously transmit at
full rate of channel
 highly decentralized: only
slots in nodes need to be
in sync
 simple
Cons:
 collisions, wasting slots
 idle slots
 nodes may be able to
detect collision in less
than time to transmit
packet
 clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E

49. Link Layer 5-49
 suppose: N nodes with
many frames to send,
each transmits in slot with
probability p
 prob that given node has
success in a slot = p(1-p)N-
1
 prob that any node has a
success = Np(1-p)N-1
 max efficiency: find p* that
maximizes
Np(1-p)N-1
 for many nodes, take limit
of Np*(1-p*)N-1
as N goes
to infinity, gives:
max efficiency = 1/e = .37
efficiency: long-run
fraction of successful slots
(many nodes, all with many
frames to send)
at best: channel
used for useful
transmissions 37%
of time! !
Slotted ALOHA: efficiency

50. Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA: listen before transmit:
if channel sensed idle: transmit entire frame
 if channel sensed busy, defer transmission
 human analogy: don’t interrupt others!

51. Link Layer 5-51
CSMA (carrier sense multiple access)
• Non-persistent CSMA
• 1-persistent CSMA
• P-persistent CSMA

52. Link Layer 5-52
CSMA (carrier sense multiple access)
• Non-persistent CSMA
• In this , if a station wants to transmit a frame and it
finds that the channel is busy then it will wait for fixed
interval of time.
• After some time ,it again checks the status of the
channel
• If channel is free it will transmit
• 1-persistent CSMA
• P-persistent CSMA

53. Link Layer 5-53
CSMA (carrier sense multiple access)
• Non-persistent CSMA
• 1-persistent CSMA
• The station which wants to transmit , continuously
monitors the channel until it is idle and then transmits
immediately.
• If two are waiting then they will transmit
simultaneously and collision take place then require
retransmission
• P-persistent CSMA

54. Link Layer 5-54
CSMA (carrier sense multiple access)
• Non-persistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• All the waiting stations are not allowed to transmit
simultaneously as soon as the channel become idle
• Possibility of collision and retransmission is reduced
• A station is assumed to be transmitted with a
probability p

55. Link Layer 5-55
CSMA/CD (collision detection)

56. Link Layer 5-56
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: received signal strength
overwhelmed by local transmission strength
 human analogy: the polite conversationalist

57. Link Layer 5-57
CSMA/CD (procedure)

58. Link Layer 5-58
CSMA/CD efficiency
 Tprop = max prop delay between 2 nodes in LAN
 ttrans = time to transmit max-size frame
so ttrans >= 2Tprop
iL/R>=2*D/S
L>=2DR/S so of less than frame length then
collision detection mechanism not
possible
 efficiency goes to 1
 as tprop goes to 0
 as ttrans goes to infinity
transprop /tt
efficiency
51
1
+
=

59.  Station ready to transmit, senses the the line by
using one of persistent technique.
 As soon as it find the line to be idle , the station
waits for a time equal to an IFG(Interframe Gap).
 It then wait for some random time and sends the
frame.
 After sending the frame . It sets a timer and waits
for the ack from the receiver.
 If the ack is received before expiry of the timer ,
then the transmitter is successful.
 But if the transmitting station does not receive
the expected ack before the timer expiry then
the increments the back off parameter , waits for
the back off time and senses the line again.
Data Link Layer 5-59
CSMA/CA (Collision Avoidance)

60. Data Link Layer 5-60
CSMA/CA (Collision Avoidance)

61. Link Layer 5-61
Multiple access protocols
 Different multiple access protocols..
 Random access protocols
 ALOHA
 CSMA
 CSMA/CD
 Controlled access protocols
 Polling
 Token passing
 Channelization protocols
 FDMA
 TDMA

62. Link Layer 5-62
• It requires one of the nodes to be designated as
a master node.
• The master node polls each of the nodes in a
round-robin fashion.
• the master node first sends a message to node
1, saying that it (node 1) can transmit up to
some maximum number of frames.
• After node 1 transmits some frames, the master
node tells node 2 it (node 2) can transmit up to
the maximum number of frames.
Controlled access protocols-Polling

63. Link Layer 5-63
• The master node can determine when a node has
finished sending its frames by observing the lack
of a signal on the channel.
 The procedure continues in this manner, with the
master node polling each of the nodes in a cyclic
manner.
 It eliminates the collisions and allows polling to
achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data

64. Link Layer 5-64
Disadvantage -
•Polling delay—the amount of time required to notify
a node that it can transmit.
•if the master node fails, the entire channel becomes
inoperative.
Controlled access protocols-Polling

65. 5-65
 In this protocol there is
no master node.
 A small, special-purpose
frame known as a token
is exchanged among the
nodes in some fixed
order.
 When a node receives a
token, it holds onto the
token only if it has some
frames to transmit
otherwise, it immediately
forwards the token to
the next node.
Controlled access protocols-Token Passing
T
data
(nothing
to send)
T

66. Link Layer 5-66
• If a node does have frames to transmit when it
receives the token, it sends up to a maximum
number of frames and then forwards the token
to the next node.
• It is decentralized and highly efficient.
Disadvantage-
 The failure of one node can crash the entire
channel.
 if a node accidentally neglects to release the
token, then some recovery procedure must be
invoked to get the token back in circulation.
Controlled access protocols-Token Passing

67. Link Layer 5-67
Link layer, LANs: outline
5.1 introduction, services
5.2 error detection,
correction
5.3 multiple access
protocols
5.4 LANs
 addressing, ARP
 Ethernet
 switches
 VLANS

68. Link Layer 5-68
MAC addresses and ARP
 32-bit IP address:
 network-layer address for interface
 used for layer 3 (network layer) forwarding
 MAC (or LAN or physical or Ethernet) address:
 function: used ‘locally” to get frame from one interface to
another physically-connected interface (same network, in IP-
addressing sense)
 48 bit MAC address (for most LANs) burned in NIC
ROM, also sometimes software settable
 e.g.: 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each “number” represents 4 bits)

69. Link Layer 5-69
LAN addresses and ARP
each adapter on LAN has unique LAN address
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 or
wireless)

70. Link Layer 5-70
LAN addresses (more)
 MAC address allocation administered by IEEE
 manufacturer buys portion of MAC address space
(to assure uniqueness)
 analogy:
 MAC address: like Social Security Number
 IP address: like postal address
 MAC flat address ➜ portability
 can move LAN card from one LAN to another
 IP hierarchical address not portable
 address depends on IP subnet to which node is
attached

71. Link Layer 5-71
ARP: address resolution protocol
ARP table: each IP node (host,
router) on LAN has table
 IP/MAC address
mappings for some LAN
nodes:
< IP address; MAC address; TTL>
 TTL (Time To Live):
time after which address
mapping will be
forgotten (typically 20
min)
Question: how to determine
interface’s MAC address,
knowing its 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
137.196.7.23
137.196.7.78
137.196.7.14
137.196.7.88

72. Link Layer 5-72
ARP protocol: same LAN
 A wants to send datagram
to B
 B’s MAC address not in A’s
ARP table.
 A broadcasts ARP query
packet, containing B's IP
address
 dest MAC address = FF-FF-
FF-FF-FF-FF
 all nodes on LAN receive
ARP query
 B receives ARP packet,
replies to A with its (B's)
MAC address
 frame sent to A’s 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 “plug-and-play”:
 nodes create their ARP
tables without intervention
from net administrator

73. Link Layer 5-73
walkthrough: send datagram from A to B via R
 focus on addressing – at IP (datagram) and MAC layer (frame)
 assume A knows B’s IP address
 assume A knows IP address of first hop router, R
 assume A knows R’s MAC address
Addressing: routing to another LAN
R
1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
74-29-9C-E8-FF-55
A
222.222.222.222
49-BD-D2-C7-56-2A
222.222.222.221
88-B2-2F-54-1A-0F
B

74. R
1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
74-29-9C-E8-FF-55
A
222.222.222.222
49-BD-D2-C7-56-2A
222.222.222.221
88-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing: routing to another LAN
IP
Eth
Phy
IP src: 111.111.111.111
IP dest: 222.222.222.222
 A creates IP datagram with IP source A, destination B
 A creates link-layer frame with R's MAC address as dest, frame
contains A-to-B IP datagram
MAC src: 74-29-9C-E8-FF-55
MAC dest: E6-E9-00-17-BB-4B

75. R
1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
74-29-9C-E8-FF-55
A
222.222.222.222
49-BD-D2-C7-56-2A
222.222.222.221
88-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing: routing to another LAN
IP
Eth
Phy
 frame sent from A to R
IP
Eth
Phy
 frame received at R, datagram removed, passed up to IP
MAC src: 74-29-9C-E8-FF-55
MAC dest: E6-E9-00-17-BB-4B
IP src: 111.111.111.111
IP dest: 222.222.222.222
IP src: 111.111.111.111
IP dest: 222.222.222.222

76. R
1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
74-29-9C-E8-FF-55
A
222.222.222.222
49-BD-D2-C7-56-2A
222.222.222.221
88-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing: routing to another LAN
IP src: 111.111.111.111
IP dest: 222.222.222.222
 R forwards datagram with IP source A, destination B
 R creates link-layer frame with B's MAC address as dest, frame
contains A-to-B IP datagram
MAC src: 1A-23-F9-CD-06-9B
MAC dest: 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy

77. R
1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
74-29-9C-E8-FF-55
A
222.222.222.222
49-BD-D2-C7-56-2A
222.222.222.221
88-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing: routing to another LAN
 R forwards datagram with IP source A, destination B
 R creates link-layer frame with B's MAC address as dest, frame
contains A-to-B IP datagram
IP src: 111.111.111.111
IP dest: 222.222.222.222
MAC src: 1A-23-F9-CD-06-9B
MAC dest: 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy

78. R
1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
74-29-9C-E8-FF-55
A
222.222.222.222
49-BD-D2-C7-56-2A
222.222.222.221
88-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing: routing to another LAN
 R forwards datagram with IP source A, destination B
 R creates link-layer frame with B's MAC address as dest, frame
contains A-to-B IP datagram
IP src: 111.111.111.111
IP dest: 222.222.222.222
MAC src: 1A-23-F9-CD-06-9B
MAC dest: 49-BD-D2-C7-56-2A
IP
Eth
Phy

79. Link Layer 5-79
Link layer, LANs: outline
5.1 introduction, services
5.2 error detection,
correction
5.3 multiple access
protocols
5.4 LANs
 addressing, ARP
 Ethernet
 switches

80. Link Layer 5-80
Ethernet 802.3
• IEEE 802.3 (CSMA/CD - Ethernet) standard –
originally 2Mbps
• IEEE 802.3u standard for 100Mbps Ethernet
• IEEE 802.3z standard for 1,000Mbps Ethernet
 Most popular packet-switched LAN technology
 Bandwidths: 10Mbps, 100Mbps, 1Gbps
 Max bus length: 2500m
 500m segments with 4 repeaters
 Bus and Star topologies are used to connect host.
 For transmission 802.3 LAN , it make use of 1-
persistant CSMA/CD protocol.
 When collision they wait for random time, to
select random time Ethernet use Binary
Exponential Backoff Algo

81. Link Layer 5-81
Ethernet CSMA/CD algorithm
1. NIC receives datagram
from network layer,
creates frame
2. If NIC senses channel
idle, starts frame
transmission. If NIC
senses channel busy,
waits until channel idle,
then transmits.
3. If NIC transmits entire
frame without detecting
another transmission,
NIC is done with frame !
4. If NIC detects another
transmission while
transmitting, aborts and
sends jam signal
5. After aborting, NIC
enters binary (exponential)
backoff:
 after mth collision, NIC
chooses K at random
from {0,1,2, …, 2m
-1}.
NIC waits K·512 bit
times, returns to Step 2
 longer backoff interval
with more collisions

82. Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
preamble:
 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011
 used to synchronize receiver, sender clock rates
dest.
address
source
address
data
(payload) CRCpreamble
type

83. CS 640 83
Ethernet Frames
 Preamble is a sequence of 7 bytes, each set to “10101010”
 Used to synchronize receiver before actual data is sent
 Addresses
 unique, 48-bit unicast address assigned to each adapter
• example: 8:0:e4:b1:2
• Each manufacturer gets their own address range
 broadcast: all 1s
 multicast: first bit is 1
 Type field is a demultiplexing key used to determine which
higher level protocol the frame should be delivered to
 Body can contain up to 1500 bytes of data

84. Link Layer 5-84
Ethernet frame structure (more)
 addresses: 6 byte source, destination MAC addresses
 if adapter receives frame with matching destination
address, or with broadcast address (e.g. ARP packet), it
passes data in frame to network layer protocol
 otherwise, adapter discards frame
 type: indicates higher layer protocol key (mostly IP
but others possible)
 CRC: cyclic redundancy check at receiver
 error detected: frame is dropped
dest.
address
source
address
data
(payload) CRCpreamble
type

85. Link Layer 5-85
Ethernet: physical topology
 bus:
 all nodes in same collision domain (can collide with each
other)
 star: prevails today
 active switch in center
 each “spoke” runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus: coaxial cable
star

86. CS 640 86
Ethernet Technologies: 10Base2
 10: 10Mbps; 2: under 185 (~200) meters cable length
 Thin coaxial cable in a bus topology

87. CS 640 87
10BaseT and 100BaseT
 10/100 Mbps rate
 T stands for Twisted Pair
 Hub(s) connected by twisted pair facilitate “star topology”
 Distance of any node to hub must be < 100M

88. CS 640 88
Physical Layer Configurations for 802.3
 Physical layer configurations are specified in three parts
 Data rate (10, 100, 1,000)
 10, 100, 1,000Mbps
 Signaling method (base, broad)
 Baseband
• Digital signaling
 Broadband
• Analog signaling
 Cabling (2, 5, T, F, S, L)
 5 - Thick coax (original Ethernet cabling)
 F – Optical fiber
 S – Short wave laser over multimode fiber
 L – Long wave laser over single mode fiber

89. CS 640 89
Fast and Gigabit Ethernet
 Fast Ethernet (100Mbps) has technology very similar to
10Mbps Ethernet
 Uses different physical layer encoding (4B5B)
 Many NIC’s are 10/100 capable
• Can be used at either speed
 Gigabit Ethernet (1,000Mbps)
 Compatible with lower speeds
 Uses standard framing and CSMA/CD algorithm
 Distances are severely limited
 Typically used for backbones and inter-router connectivity
 Becoming cost competitive
 How much of this bandwidth is realizable?

90. CS 640 90
Experiences with Ethernet
 Ethernets work best under light loads
 Utilization over 30% is considered heavy
• Network capacity is wasted by collisions
 Most networks are limited to about 200 hosts
 Specification allows for up to 1024
 Most networks are much shorter
 5 to 10 microsecond RTT

91. Link Layer 5-91
Link layer, LANs: outline
5.1 introduction, services
5.2 error detection,
correction
5.3 multiple access
protocols
5.4 LANs
 addressing, ARP
 Ethernet
 switches

92. Switch
 Device that channels incoming data from any of
multiple input ports to the specific output port.
Example-traditional telephone network.
 In Internet, a switch determines from the IP
address in each packet which output port to use
for the next part of its trip to the intended
destination.

93. Link Layer 5-93
Ethernet switch
 link-layer device: takes an active role
 store, forward Ethernet frames
 examine incoming frame’s MAC address,
selectively forward frame to one-or-more
outgoing links 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

94. Link Layer 5-94
Switch: multiple simultaneous transmissions
 hosts have dedicated, direct
connection to switch
 switches buffer packets
 Ethernet protocol used on each
incoming link, but no collisions;
full duplex
 each link is its own collision
domain
 switching: A-to-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

95. Link Layer 5-95
Switch forwarding table
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 A: each switch has a switch
table, each entry:
 (MAC address of host, interface to
reach host, time stamp)
 looks like a routing table!
Q: how are entries created,
maintained in switch table?
 something like a routing protocol?

96. A
A’
B
B’ C
C’
1 2
345
6
Link Layer 5-96
Switch: self-learning
 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
A A’
Source: A
Dest: A’
MAC addr interface TTL
Switch table
(initially empty)
A 1 60

97. Link Layer 5-97
Switch: frame filtering/forwarding
when frame received at switch:
1. record incoming link, MAC address of sending host
2. index switch table using MAC destination address
3. if entry found for destination
then {
if destination on segment from which frame arrived
then drop frame
else forward frame on interface indicated by entry
}
else flood /* forward on all interfaces except arriving
interface */

98. A
A’
B
B’ C
C’
1 2
345
6
Link Layer 5-98
Self-learning, forwarding: example
A A’
Source: A
Dest: A’
MAC addr interface TTL
switch table
(initially empty)
A 1 60
A A’A A’A A’A A’A A’
 frame destination, A’,
locaton unknown: flood
A’ A
 destination A location
known:
A’ 4 60
selectively send
on just one link

99. Link Layer 5-99
Interconnecting switches
 switches can be connected together
Q: sending from A to G - how does S1 know to
forward frame destined to F via S4 and S3?
A: self learning! (works exactly the same as in single-
switch case!)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G

100. Link Layer5-100
Self-learning multi-switch example
Suppose C sends frame to I, I responds to C
 Q: show switch tables and packet forwarding in S1, S2, S3,
S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G

101. Link Layer5-101
Institutional network
to external
network
router
IP subnet
mail server
web server

102. Link Layer5-102
Switches vs. routers
both are store-and-forward:
routers: network-layer
devices (examine network-
layer headers)
switches: link-layer devices
(examine link-layer headers)
both have forwarding tables:
routers: compute tables using
routing algorithms, IP
addresses
switches: learn forwarding
table using flooding, learning,
MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram