dlc slides1

Chapter 3

Data Link Layer & Data Link Layer & Error Control CodingError Control Coding

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DATA LINK CONTROL (DLC)

DLC

FRAMEHEADERSEQUENCENUMBERS

TRAILERCRC

OBJECTIVE: PROVIDE ANERROR FREE BIT PIPEPACKET

PACKET

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DATA LINK CONTROL (2)

DLC

MAC

PHYSICAL

DLC

MAC

PHYSICAL

UNRELIABLEBIT PIPE

RELIABLEBIT PIPE

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Functions of the Data Link Layer (DLL)

n Provide service interface to the network layer

n Dealing with transmission errors error-control coding & retransmission

n Regulating data flowslow receivers not swamped by fast sendersdone by ARQ..

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Data Link Layer Design Issues

n Service provided to the Network Layern Framingn Error Controln Flow Control

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Packets vs. Frames

Relationship between packets and frames.

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Services Provided to Network Layer

(a) Virtual communication.(b) Actual communication.

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Service types

n Unacknowledged connectionless servicen Acknowledged connectionless servicen Acknowledged connection-oriented service

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Services

n Unacknowledged connectionless service Source machine sends independent frames without having the

destination machine acknowledge them No logical connection is established If a frame is lost due to noise on the link, no detection or

recovery Appropriate for reliable links and leave the recovery to higher

layers. (LANs)

n Acknowledged connectionless service No logical connection is established Each frame sent is individually acknowledged Retransmit if time out Useful for unreliable channels (wireless systems!) Delay of a packet is long if many frames are retransmitted

Regular bulk postal mail

certified postal mail

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Services (2)

n Acknowledged connection-oriented service Connection is established before any data are transferred Each frame is numbered and guaranteed to be received Each frame is received only once and all frames are

received in the right order. Provides the network layer processes with the equivalent of

a reliable bit stream

Reliable pipe !

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Framing

n How to decide the successive frames start and stop ?

n Inserting time gaps between frames will not work. Why?

n Three types of framing used in practiceCharacter-based countFlag bytes with byte stuffingStarting and ending flags, with bit stuffing

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Framing: Character-based count

A character stream. (a) Without errors. (b) With one error.

A header to specify the no. of characters in the frame

Even the destination knows that the frame is bad, it still has no way of telling where the next frame starts. Resending a frame back to the source

asking for retransmission does not help either. Rarely use any more

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Framing: Flag bytes with byte stuffing

(a) A frame delimited by flag bytes.(b) Four examples of byte sequences before and after stuffing.

Use flag bytes to separate the frames

What if the data has the same byte pattern as flag bytes? Insert ESC as byte stuffing, but limited to 8-bit characters.

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Framing: Starting/ending flags with bit stuffing

Bit stuffing

(a) The original data.

(b) The data as they appear on the line.

(c) The data as they are stored in receivers memory after de-stuffing.

a flag byte = 01111110

Delete 0 at the receiver

Many DLL protocols use combined method!

Error Control CodingError Control Coding

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Error Detection and Correction

n The primary function of the DLC (Data Link Control) is to convert the unreliable bit pipe into a reliable bit pipe.

n This can be done by using FEC (Forward Error Correction) or ARQ (Automatic Repeat reQuest) for retransmission.

n The FEC relies upon some form of Error Control Coding and the ARQ relies on Error Control Coding but also on the introduction of Sequence Number(SN) and Request Numbers (RN).

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Error Detection and Correction

n Error-Correcting CodesHamming Code

n Error-Detecting CodesParity CheckCyclic Redundancy Check (CRC)

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Properties n Errors on a link tend to be dependent and occur in

burstsn For any reasonable code, the frequency of

undetectable errors is very small and thus difficult to measure and difficult to model analytically.

n The minimum distance of the coden The burst detecting capabilityn The probability that a completely random string will

be accepted as error free

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Definitions n Codeword : n-bit unit (n = m + r ) containing m data and

r redundant, or check bits, is referred to as an n-bit codeword

n The minimum distance of the code : the smallest number of errors that can convert one codeword into another

n The length of a burst of errors : the number of bits from the first error to the last error (inclusive). The burst error detecting capability of a code is defined as the largest integer B such that a code can detect all bursts of length B or less.

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Definitions (2)

n Hamming Distance d: the number of bit positions in which two codewords differGiven any two codewords, exclusive OR (XOR)

100010011011000100111000

n It requires d single-bit errors to convert one codeword to another.

0 for same values1 for different values

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n The probability that all k bits in a message are received correctly is equal to ( )1- pe

k

0

1

0

11- pe

1- pe

pepe

Binary Symmetric Channel

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Bit Error Raten A simple error correction coding method is to send every

packet three times.n Three values are then received for each bit and the receiver

uses a majority vote on each bit.n Evaluate the effectiveness of this method assuming that all

the bits errors are independent. The probability that a given bit is incorrectly received two or three

times is equal to e = 3 Pe2 (1-Pe) + Pe3

Therefore, the probability that at least one of the N bits of the packet is received incorrectly two or three times is equal to

PER (packet error rate) = 1 (1 e)N Ne 3N Pe2

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Packet Error Rate (Contd)

n Compare the PERFor N = 105 and BER =Pe= 10-7, the PER of one time

transmission is 1- (1-BER)N N BER = 10-2.The PER of three times transmission is

PER Ne 3N Pe2 = 3 10-9

n Price? The useful transmission rate has been reduce by a factor

of 3.

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Single Parity Coder

n Parity check append a single bit to the data. If the number of 1 bits in the codeword is even (or odd), a bit 0 (1) is appended. E.g., 1011010, a bit 0 will be added to the end and it makes

10110100 It can be used to detect single errors. Minimum distance of the code ?

1 2 ks s s 1 2 ks s s cCODER

where c = s1 s2 L sk , the modulo-2 sum

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njjjjj

ikiiii

ssssrssssc

L

L

==

321

321

Horizontal and Vertical Parity Checksl A two-dimensional array with one parity check

for each row and one for each column.

s11 s12 s13 L s1k c1s21 s22 s23 L s2k c2s31 s31 s33 L s3k c3M M M M M M

sn1 sn2 sn3 L snk cnr1 r2 r3 L rk cn +1

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njjjjj

ikiiii

ssssr

ssssc

L

L

=

=

321

321

Detects all single, double and triple errors and most quadruple but not all!

Horizontal and Vertical Parity Checks (2)

1 0 0 1 0 1 0 10 1 1 1 0 1 0 0 1 1 1 0 0 0 1 01 0 0 0 1 1 1 00 0 1 1 0 0 1 11 0 1 1 1 1 1 0

Horizontal checks

Vertical checks

1 0 0 1 0 1 0 10 1 1 1 0 1 0 0 1 1 1 0 0 0 1 01 0 0 0 1 1 1 00 0 1 1 0 0 1 11 0 1 1 1 1 1 0

If the bit in each circle is changed, all parity checks are still satisfied

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Observationsn To detect d errors, we need a distance d+1 code so

that d single-bit errors cannot change a codeword to another.

n To correct d errors, we need a distance 2d+1 code so that even d-bit errors, the original codeword is still closer than any other codewords. E.g., there are 4 valid codewords: 0000000000, 0000011111,

1111100000, 1111111111. This code has distance of 5 (2d +1= 5). If a codeword 0000000111 arrives, the closest valid codeword is

0000011111 (2-bit error, d = 2). Then the receiver can determine the original codeword. If there are 3-bit errors, this code is unable to correct.

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Hamming Code (Error Correcting Code)n The bits of the codeword are numbered consecutively,

starting from bit 1 at the left end. 1 2 3 4 5 6 7

Original code 1 0 0 1 0 0 0n The bits that are powers of 2 (1,2,4,8) are check bits, so

the co