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
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
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
data sent
data stored
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
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 , where even
parity is adopted.
Then, arrange the data and error-correcting bits
as follows.
X X X X
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)
(position)
r r r r4 (check bits)
(3) m
(5) m
(6) m
(7) m
(9) m
(10) m
(11) m
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)*2r/G(x) = Q(x) + R(x)/G(x), then
[M(x)*2r + R(x)]/G(x) = Q(x).
Proof: [M(x)*2r + 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 by2r)
divide M(x)*2r (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)*2r+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