The document discusses several key functions of link layer protocols: 1) Framing and link access to encapsulate data into frames using physical addresses and implement channel access on shared mediums. 2) Reliable delivery techniques like error detection, error correction, and flow control to ensure reliable transmission. 3) Common error detection techniques used in link layers include parity checks and cyclic redundancy checks (CRCs) which use checksums to detect errors. CRCs can detect all single and double bit errors and most burst errors.

1. Link Layer Protocols
04/29/12 1

2. Link Layer Services
• Framing and link access:
– encapsulate datagram into frame implement channel
access if shared medium,
– ‘physical addresses’ are used in frame headers to
identify source and destination of frames on broadcast
links
• Reliable Delivery:
– seldom used on fiber optic, co-axial cable and some
twisted pairs too due to low bit error rate.
– Used on wireless links, where the goal is to reduce
errors thus avoiding end-to-end retransmissions
04/29/12 2

3. Link Layer Services (more)
• Flow Control:
– pacing between senders and receivers
• Error Detection:
– errors are caused by signal attenuation and noise.
– Receiver detects presence of errors:
– it signals the sender for retransmission or just drops the
corrupted frame
• Error Correction:
– mechanism for the receiver to locate and correct the
error without resorting to retransmission
04/29/12 3

4. Link Layer Protocol
Implementation
• Link layer protocol entirely implemented in
the adapter (eg,PCMCIA card). Adapter
typically includes: RAM, DSP chips, host
bus interface, and link interface
04/29/12 4

5. Network Adapters
• Network adapters, sometimes called
Network Interface Cards (NIC), plug into
the data bus on a computer and into a
network cable
• The NIC performs all of the functions
required to communicate on a network
• Some computers have a NIC built in
04/29/12 5

6. Selecting a NIC
• Before selecting a NIC for your computer,
you need to determine three things:
– What type of network are you connecting to?
– What type of media (cable) are you using?
– What type of bus does your computer have?
• If you have a choice of bus, you will want
to consider the performance of each bus and
how will affect the NIC
04/29/12 6

7. Error Detection
04/29/12 7

8. Errors
• Transmission errors are a way of life.
• In the digital world an error means that a bit
value is flipped.
• An error can be isolated to a single bit.
• Errors on some media come in bursts
– Harder to detect and correct than isolated
errors.
04/29/12 8

9. Dealing with Errors
• Error detecting codes
– provide enough redundant information to enable the
receiver to deduce that an error occurred
• Error correcting codes
– provide enough redundant information to enable the
receiver to deduce that an error occurred AND how to
fix it
• So a message consists of m data bits and r
redundant or check bits.
04/29/12 9

10. Hamming Distance
• Hamming distance:
– the number of bit positions in which two codewords
differ
• Simple to calculate find the XOR
• If two codewords are a Hamming distance d apart,
it will require d single-bit errors to convert one
into the other.
• The Hamming distance of a code is the minimum
Hamming distance between any two codewords.
04/29/12 10

11. Hamming Distance 2 Code
000
011
101
110
• Note that not all of the 8 different bit
patterns are included in the code
•Any single error will not convert a valid
codeword into another valid codeword.
04/29/12 11

12. How Error Detection Works
2e
Valid Valid
codeword codeword
Invalid Code Words
04/29/12 12

13. Parity
• A simple single error detecting code could be
constructed by counting bits.
– Any codeword with an even number of bits is consider
valid (you could also make it the other way around).
• The minimum distance of this code is 2, so it is
capable of detecting single errors.
• This code can be created by adding a parity bit:
– chose the parity bit so that the number of ones in the
codeword is even (or odd).
04/29/12 13

14. Parity in Action
Data With Parity Bit
00 000
01 101
10 110
11 011
Want to send: 10
Data Link Sends: 110 Receive: 111 (ERROR)
04/29/12 14

15. Protecting Blocks
• The probability of detecting a burst error on a
block using a single parity bit is 50%.
• This can be improved by viewing the block as a n
by k bit matrix.
• A parity bit is then computed for each column.
• The check bits are placed in a k-bit row and
affixed to the matrix as the last row.
• Bursts of length n can be detected.
04/29/12 15

16. Detecting Burst Errors
Data
1001000 10010000
1100001 11000011 VRC (Vertical Redundancy Check)
1101101 11011011
1101101 11011011
1101001 11010010
1101110 11011101
n 1100111 11001111
0100000 01000001
1100011 11000110
1101111 11011110 LRC (Longitudinal Redundancy Check)
1100100 11001001
1100101 11001010
11001001
04/29/12 16

17. What About Error Correction?
• How do we get error correction?
– Must increase the minimum distance of the
code
• The key to error correction is that it must be
possible to detect and locate the error.
• The minimum distance must be at least
2e+1
04/29/12 17

18. Error Correction
The +1 ensures the circles will not overlap
Valid Valid
codeword codeword
Invalid codewords
04/29/12 18

19. A Simple Single Error Correcting
Code
Data Send Received
0 000 000
001
010
100
1 111 011
101
110
111
04/29/12 19

20. Hamming Codes
• Hamming codes are n-bit codes that can
correct single errors.
• The basic idea is to split the codeword into
two portions
– information or message bits (m)
– parity bits (k)
• The result are codewords that consist of
m+k bits
04/29/12 20

21. Choosing m and k
• Selecting m is easy, you are usually told what it is.
• How do you pick k?
• The parity bits are used to generate a k-bit word
that identifies where in the codeword the error, if
any, occurred.
• Consequently, k must satisfy the following:
2k ≥ m + k + 1
04/29/12 21

22. Constructing the Codeword
• The codeword consists of m+k bits.
• The location of each of the m+k bits is
assigned a decimal value, 1 is assigned to
the MSB, and m+k to the LSB.
• Parity bits go in positions 1, 2, 4, …, 2k-1
p0 p1 m0 p2 m1 m2 m3 p3 m4 ... mm+k
1 2 3 4 5 6 7 8 9 ... m+k
04/29/12 22

23. Parity Checks
• The parity checks must be specified so that when
an error occurs, the position number will take on
the the value assigned to to location of the error
Error Position Position Number
0 (no error) 000
1 001
2 010
3 011
4 100
5 101
6 110
7 111
04/29/12 23

24. Putting It Together
Position 1 2 3 4 5 6 7
p0 p1 m0 p2 m1 m2 m3
Original Message 0 1 0 0
Parity check in positions 1 0 1 0 0
1,3,5,7 requires p0=1
Parity check in positions 1 0 0 1 0 0
2,3,6,7 requires p1=0
Parity check in positions 1 0 0 1 1 0 0
4,5,6,7 requires p2=1
04/29/12 24

25. Example
Send the message 0010 using a hamming code
Step 1: Find k. Here k=3
Step 2: Determine where things go
Step 3: Figure out the parity bits
p1 will cover 1,3,5,7,9,11,…
p2 will cover 2,3,6,7,10,11,…
p3 will cover 4,5,6,7,12,13,14,15,...
04/29/12 25

26. Correcting Burst Errors
• Hamming codes can be used to correct burst errors
• A sequence of s consecutive codewords are
arranged as a matrix, one codeword per row.
• Transmit data one column (s bits) at a time.
• The matrix is reconstructed by the receiver one
column at a time.
• If a burst error of size s occurs, only a single
column will be affected.
04/29/12 26

27. Correcting Burst Errors
Character ASCII Check Bits
H 1001000 00110010000
a 1100001 10111001001
m 1101101 11101010101
m 1101101 11101010101
I 1101001 01101011001
n 1101110 01101010110
s
g 1100111 01111001111
0100000 10011000000
c 1100011 11111000011
o 1101111 10101011111
d 1100100 11111001100
e 1100101 00111000101
04/29/12 27

28. Correcting vs. Detecting
• Most often error detection followed by
retransmission is more efficient.
• Consider a channel with an error rate is 10-6 per bit
(one error per million bits sent)
– Block size 1000 == 10 check bits ( k == 10 )
– For parity one check bit will suffice
• Overhead for sending 1MB
– Hamming == 10,000 bits
– Parity == 2001 bits (since 1 block will be
retransmitted)
04/29/12 28

29. Cyclic Redundancy Check
• The transmitter generates a bit pattern called a
Frame Check Sequence (FCS), based on the
contents of the frame.
• The combined contents of the frame and the FCS
are exactly divisible by some predetermined
polynomial.
• If the contents of the frame are damaged during
transmission, the receiver’s division will yield a
non-zero remainder.
04/29/12 29

30. Cyclic Redundancy Check
• CRC detects all of the following errors:
– All single bit errors
– All double bit errors if the divisor is at least
three terms
– Any odd number of errors, if the divisor
contains a factor (x+1)
– Any error in which the length of the error (an
error burst) is less than the length of the FCS
– Most errors with larger bursts
04/29/12 30

31. Polynomials
• CRC is based upon treating bit strings as
representations of polynomials with
coefficients of 0 and 1 only.
– 11001 == x5+ x4+ x0
• A k-bit frame is regarded as the coefficient
list for a polynomial with k terms, ranging
from xk-1 to x0
04/29/12 31

32. The Generator Polynomial
• Both the sender and the receiver must agree
upon a generator polynomial, G(x).
• Both the high and low order bits of the
generator must be 1.
• The length of the generator is one bit longer
than the FCS.
• Finally the frame must be longer than the
generator.
04/29/12 32

33. The Algorithm
• The frame contents are appended by a set of
0s equal in number to the length of the FCS.
• The resulting value is divided modulo 2 by
the generator polynomial
• Each division step is carried out in the
conventional manner, except that the
subtraction is done modulo 2.
04/29/12 33

34. Modulo 2 Subtraction
• Subtraction and addition are identical to
XOR (no borrows or carries).
1 1 0 0
-1 -0 -1 -0
-- -- -- --
0 1 1 0
1 1 0 0
+1 +0 +1 +0
-- -- -- --
0 1 1 0
04/29/12 34

35. The Algorithm (continued)
• The division produces a quotient which is
discarded.
• The remainder replaces the 0s appended to the
frame (subtracted from the frame modulo 2).
• The resulting frame is now evenly divisible by the
generator polynomial.
• The receiver performs the same division, a non-
zero remainder indicates that an error occurred.
04/29/12 35

36. CRC Example (transmit)
Frame contents: 111011
Polynomial: 11101 (x4+ x3+x2 + x0)
Frame with 0s: 1110110000
100001
11101 1110110000
11101
-----
10000
11101
-----
1101
Frame to send: 1110111101
04/29/12 36

37. CRC Example (receive)
Frame contents: 1110111101
Polynomial: 11101 (x4+ x3+x2 + x0)
100001
11101 1110111101
11101
-----
11101
11101
-----
0
04/29/12 37

38. Standard Polynomials
• CRC-12 (x12+x11+x3+x2+x1+1)
– used when the character length is 6
• CRC-16 (x16+x15+x2+1)
• CRC-CCITT (x16+x12+x5+1)
– used for 8 bit characters
– catches all single and double errors
– all errors of an odd length
– all bursts of 16-bits or less, 99.997% of 17-bits, and
99.998% of 18-bits and longer.
04/29/12 38

39. Doing the Calculation Quickly
• The CRC calculations are actually performed in a
cyclic shift register that uses XOR gates.
x5 x12 x16
MSB LSB
CRC-CCITT Input
04/29/12 39

40. Warning
• All the analysis that has been done on CRC
techniques assume the frames contain
random bits.
• Real data, however, is not all that random.
• As a consequence, under some
circumstances, undetected errors are much
more common than had been previously
thought.
04/29/12 40

41. Multiple Access Links and
Protocols
Three types of links:
(a) Point-to-point (single wire)
(b) Broadcast (shared medium)
(c) Switched
04/29/12 41

42. Multiple Access Control (MAC)
Protocols
• MAC protocol: coordinates transmissions
from different stations in order to minimize/
avoid collisions
– Channel Partitioning MAC protocols
– Random Access MAC protocols
– “Taking turns” MAC protocols
• Goal: efficient, fair, simple, decentralized
04/29/12 42

43. Channel Partitioning MAC
protocols
• TDM (Time Division Multiplexing): channel
divided into N time slots, one per user; inefficient
with low duty cycle users and at light load.
• FDM (Frequency Division Multiplexing)
04/29/12 43

44. Cocktail Party Analogy
• TDM
– Everyone stands in the middle of the room,
taking turns talking
• FDM
– People group into widely separated clumps,
each clumping holding a conversation at the
same time, but still independent of the others.
04/29/12 44

45. Code Division Multiple Access
• CDMA
– Used widely in wireless technologies
– Allows each station to transmit using the same
frequency
• Cocktail party example
– Everyone is talking at the same time, but in
different languages
04/29/12 45

46. Spread Spectrum
• The bandwidth of a signal is increased by
artificially increasing the bit data rate
• This is done by breaking each bit into a number of
sub-bits called "chips“
– Assuming this number is 10, each bit of the original
signal would be divided up into 10 separate bits, or
"chips." This results in an increase in the data rate by
10. By increasing the data rate by 10, we also increase
the bandwidth by 10.
04/29/12 46

47. Chipping Sequence
• All users share the same frequency
– A Unique N-bit “code” assigned to each user
– Each bit is multiplied by the code and
transmitted
04/29/12 47

48. Channel Partitioning (CDMA)
• Chipping sequence like a mask: used to encode
the signal
– encoded signal = (original signal) X (chipping
sequence)
– decoding: innerproduct of encoded signal and chipping
sequence (note, the innerproduct is the sum of the
component-by-component products)
• To make CDMA work, chipping sequences must
be chosen orthogonal to each other (i.e.,
innerproduct = 0)
04/29/12 48

49. 04/29/12 49

50. 04/29/12 50

51. 04/29/12 51

52. CDMA (cont’d)
CDMA Properties:
• protects users from interference and jamming
(used in WW II)
• protects users from radio multipath fading
• allows multiple users to “coexist” and transmit
simultaneously with minimal interference (if
codes are “orthogonal”)
04/29/12 52

53. Random Access protocols
• A node transmits at random at full channel data
rate R.
• If two or more nodes “collide”, they retransmit
• The random access MAC protocol specifies how
to detect collisions and how to recover from them
• Examples of random access MAC protocols:
– SLOTTED ALOHA
– ALOHA
– CSMA and CSMA/CD
04/29/12 53

54. Slotted Aloha
• Time is divided into equal size slots (= full
packet size)
• a newly arriving station transmits a the
beginning of the next slot
• if collision occurs the source retransmits the
packet at each slot with probability P
• S-ALOHA is channel utilization efficient; it
is fully decentralized.
04/29/12 54

55. Slotted Aloha
04/29/12 55

56. Pure (unslotted) ALOHA
• Slotted ALOHA requires slot synchronization
• A simpler version, pure ALOHA, does not require
slots
• A node transmits without waiting for the
beginning of a slot
• Collision probability increases (packet can collide
with other packets which are transmitted within a
window twice as large as in S-Aloha)
• Throughput is reduced by one half
04/29/12 56

57. Pure Aloha
04/29/12 57

58. Carrier Sense Multiple Access
• CSMA: listen before transmit. If channel is sensed
busy, defer transmission
– Persistent CSMA: retry immediately when channel
becomes idle (this may cause instability)
– Non persistent CSMA: retry after random interval
• Collisions may still exist, since two stations may
sense the channel idle at the same time
• In case of collision, the entire packet transmission
time is wasted
04/29/12 58

59. “Taking Turns” MAC protocols
• So far we have seen that channel partitioning
MAC protocols (TDM, FDM and CDMA) can
share the channel fairly; but a single station cannot
use it all
• Random access MAC protocols allow a single
user full channel rate; but cannot share the channel
fairly (in fact, capture is often observed)
• Also there are “taking turns” protocols...
04/29/12 59

60. “Taking Turns” MAC protocols
• Taking Turns MAC protocols achieve both
fairness and full rate, at the expense of some extra
control overhead
– Polling: a Master station on a LAN in turn “invites”
the slave stations to transmit their packets. Problems:
Request to Send/Clear to Send overhead, latency, single
point of failure (Master)
– Token passing: the control token is passed from one
node to the next sequentially. Can alleviate the latency
and improve fault tolerance Still, elaborate procedures
to recover from lost token, etc.
04/29/12 60

61. IEEE Standard 802
• A collection of standards for LANs
Description
802.1 Architecture & Overview
802.2 Logical Link Control
802.3 CSMA/CD
802.4 Token Bus
802.5 Token Ring
802.6 DQDB
802.7 Broadband LANs
802.8 Fiber Optic LANs & MANs
802.9 Integrated Services LAN Interface for MAC & PHY Layers
802.10 Secure Data Exchange
802.11 Wireless LAN MAC & PHY specifications
802.12 Demand Priority Access Method
04/29/12 61

62. 802.3 - CSMA/CD
• The 802.3 standard encompasses both the MAC
layer and the physical layer.
• This standard is for a 1-persistent CSMA/CD
LAN.
• Xerox developed a CSMA/CD system to connect
personal workstations. This system was called
Ethernet.
• Ethernet is often used to refer to all CSMA/CD
protocols.
04/29/12 62

63. 802.3 Architecture
LLC - Logical Link Control
Data Link DTE
MAC - Media Access Control
PLS - Physical Signaling
Physical
Physical Medium Attachment
Medium
04/29/12 63

64. Cabling
Name/Rate Topology Media Maximum Rate
Segment (Mbps)
Ethernet Bus 50 ohm thick 500 10
coax
10Base5 Bus 50 ohm thin coax 500 10
10Base2 Bus 50 ohm thin coax 200 10
10BaseT Star UTP 100 10
10Broad36 Bus 75 ohm coax 1800 10
1Base5 Star UTP 250 1
10BaseF Star Fiber 2000 10
04/29/12 64

65. 802.3 Addresses
• Every machine on a 802.3 network is
assigned a unique physical address
– the address is typically defined in hardware and
cannot be changed
• This address is often referred to as a MAC
address.
• MAC addresses can vary in size
– 48 bits is a common length for a MAC address
04/29/12 65

66. Performance
• As more and more stations are added to an
802.3 LAN, the traffic will go up, and
eventually the LAN will saturate.
• What do you do?
– Faster LAN?
– Segment your LAN
04/29/12 66

67. Switched 802.3 LANs
• The main component of this system is a box
containing a high-speed backplane and room for
plug in adapter cards.
• A station does what it always does.
• When the switch sees the packet
– if destined for a machine on the same card send it to the
appropriate station
– else ship it using the high-speed backplane to the card
that handles that station
04/29/12 67

68. Switch
04/29/12 68

69. Collisions
• How are collisions handled?
– One possibility is to wire each card so that it
forms a local on-card LAN.
• Only one transmission per card is possible, but all
the cards can transmit in parallel.
– Buffer each input port, so incoming frames are
stored in RAM as they arrive.
• Collisions do not occur.
04/29/12 69

70. Topology Issues
• The original Ethernet specification was for
a bus topology
– only one channel
– if the cable breaks the whole network is down
• Today most organizations install 10BaseT
– star topology
– machines connect to hubs
04/29/12 70

71. Concerns About 802.3
• During the development of the 802 standard,
people doing networking on factory floors had
serious reservations about it.
• Due to the probabilistic MAC protocol, a station
may have to wait arbitrarily long to send a frame.
• In a real time system time, timing is critical.
• A simple solution is to form a ring, in which
stations take turns sending frames. No frame will
have to wait more than nT seconds.
04/29/12 71

72. 802.4 Token Bus
• Physically, the token bus is a linear or tree-shaped
cable onto which stations are attached.
• Logically, the stations are arranged in a ring.
• Each station has an address and knows the address
of the machine to its “left” and “right”.
• When the ring starts, the highest numbered
machine sends.
• Permission to send is then sent around the ring.
04/29/12 72

73. Tokens
• A control frame known as a token regulates the
right of access.
• The token frame contains a destination address.
• The station receiving the token is granted control
of the medium for up to some maximum time.
• The token must be passed when:
– The station has no (more) data to send
– The station’s time expires
04/29/12 73

74. FDDI
• FDDI was developed to support high-capacity
LANs (operates at 100Mbps).
• Is typically used as a backbone to connect copper
LANs.
• Uses multimode optical fibers and LEDs (as
opposed to laser).
• The cabling consists of two fiber rings, one
transmitting clockwise and the other counter-
clockwise.
04/29/12 74

75. FDDI Cabling
04/29/12 75

76. Hubs
• Hubs provide a single connection between each
workstation and the hub
• The connection is typically made over twisted pair
cabling
• Hubs typically have a fixed number of ports.
Common sizes are 8 and 16 port hubs.
• Hubs can be daisy chained (up to a limit) to
provide more ports for connections
04/29/12 76

77. Hubs
04/29/12 77

78. Working with Hubs
• There is a limit to the number of hubs that
can be connected together to extend a
network (a typical maximum is 4)
• When possible, connect each hub directly to
the backbone or a server
• Label the connections on the hub
• The more hubs data passes through, the
slower the connection
04/29/12 78

79. Passive Hubs
• A passive hub simply combines the signals
of network segments
• The passive hub absorbs some of the signal
and reduces in half the maximum cabling
distance permitted
• Passive hubs pass received signals onto to
all the computers connected to the hub
04/29/12 79

80. Active Hubs
• Active hubs have electronic components that
regenerate or amplify signals
• Distances supported by active hubs are greater
than those of passive hubs
• The main drawback is that they amplify noise as
well as the signal
• Because active hubs function as repeaters, they are
sometimes called multiport repeaters
04/29/12 80

81. Intelligent Hubs
• In addition to signal regeneration, intelligent hubs
perform some network management and
intelligent path selection
• A switching hub chooses only the port of the
device where the signal needs to go, rather than
sending the signal along all paths
• Many switching hubs can choose which
alternative path will be the quickest and send the
signal that way
04/29/12 81

82. Making LANs Bigger
• Usually it does not take too long until an
organization needs to expand a LAN.
• This can be done in a number of ways
– physically make the cable longer
– use repeaters
– use bridges
• The primary difference between a repeater and a
bridge is the OSI level that they operate at.
04/29/12 82

83. Repeaters
• All transmission media attenuate (weaken) the
signals that travel through them
• Attenuation is one of the things that limit the
distance any medium can carry data
• Adding a device that amplifies the signal can
allow it to travel further, increasing the size of the
network
• Devices that amplify signals are called repeaters
04/29/12 83

84. Repeaters
• A repeater is an electronic device that
receives an electronic signal from a cable
and sends it out on another cable segment.
• Repeaters are simple devices because they
do not need to understand what they
receive. Signals are simply retransmitted.
• A repeater is used when the physical length
of the cable has reached its maximum.
04/29/12 84

85. Repeaters
• Repeaters fall into two categories
– amplifiers simply amplify the entire incoming signal
(which includes noise)
– signal regenerating repeaters create an exact duplicate
of incoming data by identifying the signal amidst the
noise, reconstructs it, and retransmits only the signal
• Since repeaters simply deal with the actual signals
on a network, they operate at the physical layer of
the OSI model
04/29/12 85

86. Bridges
• Bridges connect network segments
• Unlike a repeater a bridge understands the data
link protocols and using addressing information to
determine the appropriate segment to send a frame
to
• Like repeaters, bridges regenerate signals. Since
they actually understand frame formats, the are
considered to operate at the data link layer of the
OSI model
04/29/12 86

87. Bridges
Hubs Bridge
04/29/12 87

88. Bridges
• How a bridge handles traffic
– receives all signals from segments A and B
– the bridge filters all packets from segment A addressed
to segment A (and likewise for segment B)
– signals from A to B are retransmitted to segment B
– signals form B to A are retransmitted to segment A
• Through address filtering, bridges can divide busy
networks into segments and reduce traffic
04/29/12 88

89. Problems that Bridges Deal With
• Each LAN uses a different frame format
– any copying between different LANs requires
reformatting
• All LANs do not necessarily run at the same rate.
• All three 802 LANs have a different maximum
frame length.
– Frames that are too large to be forwarded must be
discarded.
• Many problems going from 802.x to 802.y
04/29/12 89

90. Routers
• Routers are devices that connect two or more
networks
• They consist of hardware and software
– the hardware can be a network server, a PC, or a special
black box
– The software components consist of an operating
system and routing software
• Routers operate at the network layer of the OSI
reference model
04/29/12 90

91. Routing
04/29/12 91