data link layer and error control week4

7/30/2019 Data Link Layer and Error Control Week4

1/25

The Data Link Layer


2/25

Data Link Layer Design Issues

Services Provided to the Network Layer Framing

Error Control

Flow Control


3/25

Functions of the Data Link Layer

Provide service interface to the network layer

Dealing with transmission errors

Regulating data flow Slow receivers not swamped by fast senders


4/25

Functions of the Data Link Layer (2)

Relationship between packets and frames.


5/25

Services Provided to Network Layer

(a) Virtual communication.

(b) Actual communication.


6/25

Services Provided to Network Layer (2)

Placement of the data link protocol.


7/25

Framing

A character stream. (a) Without errors. (b)

With one error.


8/25

Framing (2)

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


9/25

Framing (3)

Bit stuffing(a) The original data.

(b) The data as they appear on the line.

(c)The data as they are stored in receivers memory afterdestuffing.


10/25

Error Control

Error-Correcting Codes

Error-Detecting Codes

Check for error at the receiver

Error: Request a retransmission

No error: Either continue or send an ACK


11/25

Error control codes

Adding redundancy to overcome errors

m message bits

r check bits

2m messages

2n possible codewords


12/25

Hamming distance

100000100000101101011010111101

The two codewords have a

Hamming distance of two (differ intwo bits)Hamming distance of the code:

Minimum Hamming distancebetween any two codewords


13/25

Error detection/correction capacity

Detect d errors: distance d+1 code

Correct d errors: distance 2d+1 code

Parity bit: Simple error detecting bit

Added so that # of 1 bits is odd/even.

d=1


14/25

Error-Correcting Codes

Use of a Hamming code to correct burst errors.


15/25

Error detecting codes

Parity codes

Odd parity (tek elik)

0/1 added to make the sum of 1s equal to an odd

number

Even parity (ift elik)

0/1 added to make the sum of 1s equal to an even

number


16/25

Error detecting codes Cyclic

Redundancy Check (CRC )

Polynomial codes

Polynomial arithmetic is modulo 2.

110100111257

xxxx

101110001

11010110

110100111

456

2467

257

xxx

xxxxx

xxxx


17/25

Error-Detecting

Codes

Calculation of the polynomial

code checksum.


18/25

Cyclic Redundancy Check

(evrimli Fazlalk Snamas)

M(x): message polynomial

G(x): generator polynomial available at

transmitter and receiver of degree r

Transmitter operation:

R(x): remainder

T(x): transmitted

Channel effects

Receiver: check

...)( r

x

)(

)()(

xG

xMxxR

r

)()()( xRxMxxTr

)()()()( xExTxVxTchannel

0)(

)( ?

xG

xV


19/25

Cyclic Redundancy Check

(evrimli Fazlalk Snamas)

Note that by definition

Can catch errors if

Design G(x) to catch single bit errors:

catch double bit errors:

catch odd number of errors:

catch burst errors of

length r:

0)(

)( xGxT

0)(when0)(

)( xE

xG

xE

.........)( ji

xxxG

0)(

1

xG

xk

)(~)1()( xGxxG

rkx

xx

xG

xEr

ki

if0)1...(

)1...(

)(

)(1


20/25

Elementary data link layer protocols

An Unrestricted Simplex Protocol Sender sends frames as long as they come from the

network layer

A Simplex Stop-and-Wait Protocol After sending each frame sender holds until an

acknowledgement arrives back from the receiver

A Simplex Protocol for a Noisy Channel If an ack is not sent by receiver sender times out and

retransmits In case the ack gets lost to prevent duplicate frames at

the receiver the frame contains a seq. no.


21/25

Sliding Window Protocols

A One-Bit Sliding Window Protocol

A Protocol Using Go Back N

A Protocol Using Selective Repeat


22/25

Sliding Window Protocols (2)

A sliding window of size 1, with a 3-bit sequence number.

(a) Initially.(b) After the first frame has been sent.

(c) After the first frame has been received.

(d) After the first acknowledgement has been received.


23/25


Pipelining and error recovery. Effect on an error when

(a)Receivers window size is 1.

(b)Receivers window size is large.


24/25

A One-Bit Sliding Window Protocol (2)

Two scenarios for protocol 4. (a) Normal case. (b)Abnormal case. The notation is (seq, ack, packetnumber). An asterisk indicates where a network layer

accepts a packet.


25/25


Pipelining and error recovery. Effect on an error when

(a)Receivers window size is 1.

(b) Receivers window size is large

data link layer and error control week4

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