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Computer and Data Networks, 3. Date Link Layer 1 © Dr.Z.Sun

3. The Data link layer




Outline

n Link layer services

n Framing

n Error correction and detection codes

n Error and flow control: Slide window protocols

n HDLC

n SLIP and PPP




Functions of the Data Link Layer

Relationship between packets and frames.




Link layer services

n Unacknowledged connectionless

n Acknowledged connectionless n Acknowledged connection-oriented

n Framing

n Error control n Flow control




Framing methods:

Four methods:

n Character count

n Flag bytes with bytes stuffing

n Starting & ending flags with bit stuffing

n Physical layer coding violations




Framing – Character count

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




Framing – Flag bytes and byte stuffing

(a) A frame delimited by flag bytes.

(b) Four examples of byte sequences before and after stuffing.




Framing – 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 receiver’s memory after destuffing.




Error correction codes

n Codeword = message + redundant

n If two codewords are a Hamming distance d apart, it will require d single- bit error to convert one into the other

n To detect d errors, you need a distance

of d+1 code n To correct d errors, you need a distance

2d+1 code

n Example: parity bit (even or odd)

Hamming method: (m+r+1)<=2 r

n power of 2 (1,2,4, ...) check bits

n the rest (3,5,6,7,9, ...) data bits

n example bit 11 checked by (1,2,8)




Assume Xi are data bits and Ri data bits

n X3 corresponds to R1 and R2

n

X5 corresponds to R1 and R4 n X6 corresponds to R2 and R4

n X7 corresponds to R1, R2 and R4



n X11 corresponds to R1, R2 and R8

n R1 corresponds to {X3, X5, X7,

X9, X11} n R2 corresponds to {X3, X6, X7,

X10, X11}

n R4 corresponds to {X5, X6, X7}

n R8 corresponds to {X9, X10,X11}

n Example : m = 1101101 => ?? 1 ? 101 ? 101 => 11 1 0 101 0 101

n If the receiving node received: 11 1 0 1 11 1 0 101

=> R1 = 1, R2 = 0, R4= 1, R8 =0 => 0 (1) + 1(2) + 1(4) + 0 (8) = 6 6

This implies that bit 6 is incorrect, as it is checked by R2 and R4




Error detection codes

Polynomial code (cyclic redundancy code or CRC code)

n Let r be the degree of generator polynomial G(x). Append r zeros to the low-order end of the frame M(x) => x r M(x).

n Divide the bit string corresponding to G(x) into the bit string corresponding to x r M(x) using modulo 2 division

n Subtract the remainder from the bit

string corresponding to x r M(x) using modulo 2 subtraction. The results is the check-summed frame called polynomial T(x).




Stop-and-wait and slide window protocol

n Stop-and-wait ARQ : the sender sends a frame and

waits for acknowledgement before sending any further frame. There are two sorts of errors: sending and acknowledgement.

n How about efficiency (stop and wait or window size is 1)?

Satellite link is 50 kbit/s, round trip time 500 ms, frame size 1000 bits, frame transmission time 20 ms.

Efficiency = 20/(20+500) =1/26 n Slide window protocol allows the sender sends more

frames up to a window size before stopping and waiting for acknowledgment




Go Back N and Selective repeat

n Pipe line technique: channel capacity is B bit/s, frame size L,round-trip time R.

Channel Efficiency = L/(L+BR)

If L< BR the efficiency will be less

than 50%.n Two techniques trade-off

bandwidth and buffer space

Go back to N (receive window = 1)

only receives frame in order and retransmit all frames after errors.

Selective repeat buffers frames and retransmit only the errored frame.




Window size and Negative acknowledgment

n NAK acknowledges when frame damaged or unexpected (potential lost frame)

n The maximum window size should be at most half of the range of the sequence number (Max_Seq+1)/2

n Trade-off risk of unnecessary retransmission, or idle for a long period after errors

n Standard deviation of acknowledgment is small, the timer can be set “tight”. Otherwise “loose”.




High-level Data link Control (HDLC)

n Bit oriented and bit

stuffing n 2x8 bits for framing

n 8 bit address field identify multiple terminals

n Control field can be used for three types of frames: information, supervision and unnumbered.

n Data field

n 16 bits for CRC error check






PPP frame format (character oriented)

n Closely resemble HDLC frame format

n Character oriented

Default for unnumbered frame

0: Net layer protocol 1: other

Default is 2. Default is 1500.

Normally 2 but can be negotiated

for 4.

Default




Summary

n Link layer services

n Framing n Error control

• Error correction and Error detection

n Flow control • Stop-and-wait, slide window

• Go back N, Selective repeat, Negative Acknowledgement

n Dial-up protocols for Internet access

• HDLC and PPP

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