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Data Link Layer

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

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Data Link Layer

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  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

  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. 512345678980123456 • 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

  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)

  4. Data Link Layer (cont’d) • 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. Data Link Layer (cont’d) • 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

  6. Data Link Layer (cont’d) • 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.

  7. Data Link Layer (cont’d) • 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

  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.

  9. Data Link Layer (cont’d) • 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.

  10. Data Link Layer (cont’d) • 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)

  11. Data Link Layer (cont’d) • Error-detecting codes (cont’d) • cyclic redundancy code (cont’d) • theory behind M(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)

  12. Data Link Layer (cont’d) • Error-detecting codes (cont’d) • cyclic redundancy code (cont’d) • 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)

  13. Data Link Layer (cont’d) • Error-detecting codes (cont’d) • cyclic redundancy code (cont’d) • example (Fig. 3-7, p. 189)

  14. Data Link Layer (cont’d) • Error-detecting codes (cont’d) • cyclic redundancy code (cont’d) • 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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