Data Link Layer

5-1

Data Link Layer

Objective: to achieve reliable and efficient communication between 2 adjacent machines

Data link layer design issues services provided to the network layer

– unacknowledged connectionless service

– acknowledged connectionless service

– acknowledged connection-oriented service call set-up transmission call release

5-2

Data Link Layer (cont’d)

Framing character count

– use a field in the header to indicate the number of characters in the frame

– e.g. 51234 56789 80123456

– unreliable and rarely used

starting & ending characters, with character stuffing– STX and ETX are used for frame synchronization

– DLE and DLE insertion are used for data transparency

5-3

Data Link Layer (cont’d) Framing

starting & ending characters/flags, with bit stuffing– 01111110 as flag byte (flag pattern) for frame and character sync. (for

beginning and end)

– zero bit insertion (bit stuffing) to achieve data transparency -- to insert a 0 after 5 contiguous 1’s

– e.g. original data 011011111111111111110010

data sent 011011111011111011111010010

data stored 011011111111111111110010

physical layer (bit) encoding violation hybrid methods (char. count with one of the others)

5-4


Error control error detection ACK’s and NACK’s time-out mechanism sequence numbers

Flow control for speed mismatch (sender faster than receiver), finite

receiver buffer or occasional unavailability of the receiver feedback mechanism is typically required

5-5


Error detection and correction causes of errors random vs. bursty errors error-correcting codes vs. error-detecting codes

Error-correcting codes codeword (m data-bits + r check bits; let n=m+r) Hamming distance (H.D.) between 2 codewords Hamming distance of the complete code: the min. Hamming

distance between any 2 distinct codewords

5-6


Error-correcting codes (cont’d) to detect d errors required a distance d+1 code (e.g. d =1

for parity check where H.D.=2) to correct d errors requires a distance 2d +1 code

e.g. 0000000000 0000011111

1111100000 1111111111 a lower bound on r to correct any single-bit error:

Each of the 2m legal messages has n illegal codewords at

a distance of 1 from it. Then 2m(n+1) 2n, and

2m(m+r+1) 2m+r. Consequently, m+r+1 2r.

5-7


Error-correcting codes (cont’d) Hamming code (corrects only single-bit errors)

e.g. m = 7. Then 7 + r + 1 2r, and consequently r 4.

Consider an ASCII code 1001000, where even

parity is adopted.

Then, arrange the data and error-correcting bits

as follows.

20 21 22 23

X X 1 X 0 0 1 X 0 0 0

r1 r2 m1 r3 m2 m3 m4 r4 m5 m6 m7

5-8

Data Link Layer (cont’d) Error-correcting codes (cont’d)

Hamming code (cont’d) 20 21 22 23 (position)

r1 r2 r3 r4 (check bits)

(3) m1 1 1

(5) m2 0 0

(6) m3 0 0

(7) m4 1 1 1

(9) m5 0 0

(10) m6 0 0

(11) m7 0 0 0

Consequently, r1= 0, r2 = 0, r3 = 1 and r4 = 0.

5-9


Error-correcting codes (cont’d) Hamming code (cont’d)

– to correct at most k consecutive bit errors

n

k

order of transmission

If each row contains at most 1 bit error, then the error(s) can be corrected.

5-10


Error-detecting codes parity check block sum check cyclic redundancy code (CRC)

– using methods based upon polynomial codes which is more efficient to detect error bursts

a set of check digits is generated and appended to the end of each frame to be transmitted

the receiver performs a similar computation on the message and the check bits

if an error is found, then claim error(s); otherwise, claim correctness (true with a high probability)

5-11


Error-detecting codes (cont’d) cyclic redundancy code (cont’d)

– theory behindM(x) = an m-bit number (message)

G(x) = an (r+1)-bit number (generator/divisor)

R(x) = an r-bit number (remainder based on modulo-2 arithmetic)

Q(x) = quotient

Property: If M(x)*xr/G(x) = Q(x) + R(x)/G(x), then

[M(x)*xr + R(x)]/G(x) = Q(x).

Proof: [M(x)*xr + R(x)]/G(x) = Q(x) + R(x)/G(x) + R(x)/G(x)

= Q(x)

5-12



– algorithm sender

append r zeros to M(x) (M(x) is multiplied by xr) divide M(x)*xr (using modulo-2 arithmetic/bit-wise XOR) by

G(x) to calculate R(x) append R(x) to M(x) and transmit

receiver divide the received info. (M(x)*xr+R(x)) by G(x) if the remainder is 0, OK; otherwise, report error(s)

5-13

Data Link Layer (cont’d) Error-detecting codes (cont’d)

cyclic redundancy code (cont’d)– example

(Fig. 3-7, p. 189)

5-14



– G(x) of K bits should detect all single-bit errors all double-bit errors all odd number of bit errors all error bursts with length < K most error bursts with length K

– example CRC CRC-16 CRC-CCITT CRC-32

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Data Link Layer