[PDF] 3 The Data link layer

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Computer and Data Networks, 3. Date Link Layer

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3. The Data link layer

Computer and Data Networks, 3. Date Link Layer

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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

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Functions of the Data Link Layer

Relationship between packets and frames.

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Link layer services

n

Unacknowledged connectionless

n

Acknowledged connectionless

n

Acknowledged connection-oriented

n

Framing

n

Error control

n

Flow control

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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

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Framing - Character count

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

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Framing - Flag bytes and byte stuffing

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

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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.

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Error correction codes

n

Codeword = message + redundant

n

If two codewords are a Hamming

distanced 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)

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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

X9 corresponds to R1 and R8

n

X10 corresponds to R2 and R8

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 10 101 0101 n

If the receiving node received: 11 10 1

11

1 0101

=> R1 = 1, R2 = 0 , R4= 1 , R8 =0 => 0(1) + 1(2) + 1(4) + 0(8) = 66
This implies that bit 6 is incorrect, as it is checked by R2 and R4

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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).

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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 protocolallows the sender sends more frames up to a window size before stopping and waiting for acknowledgment

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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 repeatbuffers frames and

retransmit only the errored frame.

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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".

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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

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Internet SLIP and PPP protocols

n Serial Line IP (SLIP):no error detection and correction, support only IP, need to know IP address in advance, no authentication, not a proved Internet standard. n Point-to-Point protocol (PPP):handles no error detection, support multiple protocol, address can be negotiated at connection time, permit authentication, many improvement over SLIP

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PPP frame format (character oriented)

n

Closely resemble HDLC frame format

n

Character oriented

Default for

unnumbered frame

0: Net layer protocol

1: otherDefault is 2.

Default is 1500.

Normally 2 but

can be negotiated for 4.

Default

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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