Data Link Layer

5/20/2018 Data Link Layer

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Functions of Data Link Layer Providing a well-defined service interface to the network layer.

Dealing with transmission errors.

Regulating the flow of data so that slow receivers are not swamped

by fast senders

DLL takes the packets it gets from the network layer and

encapsulatesthem into frames for transmission

Each frame contains a frame header, a payload field for

holding the packet, and a frame trailer

Data Link Layer


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Design Issues

Reliable Delivery

Flow control

Acknowledged delivery

Error detection and correction

Services provided to the Network Layer Unacknowledged connectionless service

Suitable for applications with low error rate and for real timetraffic such as voice

Acknowledged connectionless service Suitable for applications that needs assurance of delivery

Acknowledged connection-oriented service

Data Link Layer


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Transmission of the data link layer starts with

breaking up the bit stream into discrete frames

Computation of a checksum for each frame, and

Include the checksum into the frame before it is

transmitted.

Receiver computes its checksum error for a receiving

frame and if it is different from the checksum that is

being transmitted will have to deal with the error.

Framing is more difficult than one could think!

DLC functions


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To provide service to the network layer, the datalink layer must use the service provided to it bythe physical layer

Physical layer accept a raw bit stream andattempt to deliver it to the destination.

Bit stream is not guaranteed to be error freeNumber of bits received may be less than, equalto, or more than the number of bits transmitted,and they may have different values.

It is up to the data link layer to detect and, ifnecessary, correct errors

Framing


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1.Byte count

2.Flag bytes with byte stuffing

3.Flag bits with bit stuffing

Framing Methods


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It uses a field in the header to specify the number ofbytes in the frame.

Once the header information is being received it will be

used to determine end of the frame.

Byte/Character Count Framing

Method

A byte stream. (a)Without errors


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DisadvantagesTrouble with this algorithm is that when the count is

incorrectly received the destination will get out of synch

with transmission.

Destination may be able to detect that the frame is in error but it doesnot have a means (in this algorithm) how to correct it

Byte/Character Count Framing

Method

A byte stream (b)With one error.


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Flag Bytes with Byte Stuffing

Framing Method

This methods gets around the boundary detection of theframe by having each appended by the frame start and

frame end special bytes.

If they are the same (beginning and ending byte in the

frame) they are called flag byte.

A frame delimited by flag bytes.


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If the actual data contains a byte that is identicalto the FLAG byte (e.g., picture, data stream, etc.)

the convention that can be used is to have

escape character(ESC) inserted just before the

FLAG character


Framing Method


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It may also appears that escape byte appears inthe data.

To prevent escape byte from being mixed with

the data, each escape byte appearing in data is

stuffed with another escape byte


Framing Method


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Bit Stuffing Framing Method

This methods achieves the same thing as Byte Stuffing

method by using Bits (1) instead of Bytes (8 Bits).

It was developed for High-level Data Link Control (HDLC)

protocol.

Each frames begins and ends with a special bit pattern:

01111110or 0x7E


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Bit Stuffing Framing Method


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During data transmission, one or more bits of

data being transmitted may get corrupted Depending on the number of bits in data that

have been corrupted, there exist two types of

error:1. Single bit error

2. Burst error

Types of Error


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Single-bit error means that only 1 bit of a

given data unit (such as a byte, character, orpacket) is changed from 1 to 0 or from 0 to 1

Singlebit error


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Burst error means that 2 or more bits in the

data unit have changed from 1 to 0 or from 0to 1

Burst error


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During data transmission, errors may occur becauseof certain transmission impairments such as

attenuation and electromagnetic noiseTo detect or correct errors, we need to send someextra bits with our dataError detection uses the concept of redundancy, which

means adding extra bits for detecting errors at thedestination

These extra bits are called Redundant bits

These redundant bits are added by the sender andremoved by the receiver

Their presence allows the receiver to detect orcorrect corrupted bits

The central concept in detecting or correcting errorsis redundancy

Redudancy


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Error Detection:

In error detection, we are only looking to see ifany error has occurred

The answer is a simple yes or no

We are not even interested in the number of

corrupted bits

A single-bit error is the same for us as a bursterror

Some of the commonly error-detecting codes are:1.Simple parity check

2.Two dimensional(2D) parity check

3.Checksum

4.Cyclic Redundancy Check(CRC)

Error detection v/s Correction


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Error Correction:

Emphasis is on not only to detect the errors butalso to correct them

For this, it is important to know the exact

number of bits that are corrupted and, moreimportantly, their locationin the message

The correction of errors is more difficult than

the detection The most commonly used error correcting

code is Hamming code

Error detection v/s Correction


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An extra bit known as parity bit is added to eachdata word that is being transmitted

A k-bit data word is changed to n-bit codewordi.e. n = k+1

Value of parity bit (i.e. 0 or 1) is such thatcharacter has even (even parity) or odd (oddparity) number of onesGenerally, the MSB is parity bit

Example:

Simple Parity check

Even Parity

P Dataword

0 1 0 1 0 0 1 1

1 1 1 1 0 1 1 0

Odd Parity

P Dataword

1 1 0 1 0 0 1 1

0 1 1 1 0 1 1 0


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Simple parity-check code C(5, 4)

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Encoder and decoder for simple parity-check code

Si l P i h k


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What happens if the character 10010101 is

sent and the first two 0s accidentally becometwo 1s?

Thus, the following character is received:

11110101

Will there be a parity error?

Problem: Simple parity only detects odd numbers of

bits in error

Simple Parity check

T di i l i h k


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There are two types of 2D parity check

Longitudinal Redundancy Check (LRC)

Vertical Redundancy Check (VRC)

Longitudinal Redundancy Check (LRC)

Longitudinal parity adds a parity bit to each character

then adds a row of parity bits after a block of characters.

The row of parity bits is actually a parity bit for each

columnof characters

Two dimensional parity check


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T di i l it h k


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LRC and VRC

LRC and VRC are calculated using simple paritycheck method

The binary words being transmitted or received

organized in the form of rows and columns

VRC Parity bits are computed for each column

LRC Parity bits are computed for each row

Parity bits along with the data are sent to thereceiver


T di i l it h k


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If the data to be transmitted are 1100001, 1111001,1011001, 0000101, 0010101, 0010101, 1010001. find the

VRC, LRC and bytes to be transmitted

Coded Bytes: 11000011, 11110011, 10110010, 00001010, 00101011,00101011, 10100011, 00101011


BytesLRC

Bits

BIts

VRC Bits

1 1 1 0 0 0 1

1 1 0 0 0 0 0

0 1 1 0 1 1 1

0 1 1 0 0 0 0

0 0 0 1 1 1 0

0 0 0 0 0 0 01 1 1 1 1 1 1

0

0

1

0

1

01

1 1 0 0 1 1 1 1

T di i l it h k


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Both simple parity and 2D parity do not catchall errors.

Simple parity only catches odd numbers of biterrors (50% of all errors)

2D parity is better at catching errors butrequires too many check bits added to a blockof data.

As such, these methods are not that oftenused. However, a parity bit exists in 1 byte ofdata.


Additi Ch k


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It can be applied to a message of any length

It can be used to detect multiple errors

At the source, message is divided into m-bit units

At the sender, Generator creates an extra m-bit unitcalled checksum by adding the bits of data to betransmitted, which is sent with the message

At the receiver, checksum is again computed byadding the bits of received bytes

If the newly computed checksum is same as onetransmitted, then it implies no error

It is calculated by adding all the bytes bit-by-bit byneglecting the final carry (i.e out of MSB)

It can detect all odd numbers of errors and most ofthe even number of errors.

Additive Checksum

Additi Ch k


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Find the checksum byte for the fallowing

datawords: 10110011, 10101011, 01011010,11010101

Check sum byte: 10001101

Additive Checksum

1 0 1 1 0 0 1 1

1 0 1 0 1 0 1 1

0 1 0 1 1 0 1 0

Neglect Final

Carry1 1 0 1 0 1 0 1

1 1 0 0 0 1 1 0 1

C li R d d Ch k (CRC)


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Most widely and powerful error-detectingtechniques developed by IBM

Also referred to as Polynomial code, as it treatsthe bit stream to be transmitted as a polynomial

The coefficients are the 0 and 1 values in the bit

stringThe transmitter takes the Message polynomialM(x)

Using polynomial arithmetic, divides it by agiven Generator polynomial G(x)

MSB and LSB of the Generator polynomialshould be always 1

Cyclic Redundancy Check (CRC)



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A m-bit frame is regarded as the coefficient list for apolynomial with mterms ranging from xm-1 to x0

The degree of the polynomial is m-1

Polynomial arithmetic is done in modulo-2

In modulo-2, there are no carries for addition or borrowsfor subtraction

Both addition and subtraction are identical to exclusiveOR operation

Sender and receiver must agree upon a generatorpolynomial, G(x), in advance

Degree of M(x) must be greater than degree of G(x)




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mis the number of bits in the frame i.e.in M(x)

kis the degree of G(x)Algorithm for computing the checksum at the

sender

1. Append kbits to the LSB side of the data frame, so

it now contains m+k bits i.e xkM(x)

2. Divide the resultant polynomial xkM(x) by the

generator G(x) in modulo-2 arithmetic

3. After division, the remainder bits (i.e r-bits) calledredundant bits or CRCare added to the dividend to

form the code word (m+r bits)

4. Finally, this code word is transmitted




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At the receivers end, the received codeword

of m+r bits is again divided by the samegenerator as used by the sender

If the remainder is non-zero, the receiver

know that the error has occurred and rejectthe data word

If the remainder is zero, the receiver knows

that there is no error and accept the data

word




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Generate the CRC code for the message x3+1 using the

generator polynomial x3+x+1

M(x) = x3+1 in Binary =1001

G(x) = x3+x+1 in Binary = 1011

x3M(x) = x3(x3+1) = x6+x3= 1001000


1 0 1 1 1 0 0 1

1 0 1 1

0 1 0 0

0 0 0 0

1 0 0 01 0 1 1

0 1 1 0

0 0 0 0

1 1 0

1 0 1 0

DatawordCRC

(Remainder)

1 0 0 1 1 1 0

Codeword

1 0 0 1 1 1 0



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CRC computation at receiver




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Generate the CRC code for the dataword

1101011011 using generator 10011. Also write

both in the polynomial form.

Answer: CRC= 1110 codeword= 11010110111110


Standard polynomials


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Standard polynomials

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Performance of CRC


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Performance of CRC

Error Correcting codes


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Once an error is detected, what is the receiver

going to do? Do nothing (simply toss the frame or packet)

Return an error message for retransmission

Fix the error with no further help from thetransmitter




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Correct the errors

For a receiver to correct the error with nofurther help from the transmitter requires a

large amount of redundant information to

accompany the original data This redundant information allows the receiver to

determine the error and make corrections

This type of error control is often called

forward error correction(FEC) and involves

codes called Hamming codes




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Hamming distance

Hamming distancebetween two codewords of equal length is

the number of positions at which the corresponding

codewords are different

Hamming distance between two codewords X and Y is

denoted as d(X, y)

Example:

It can be calculated by performing the XOR operation on the

codewords

The number of 1 in the result will give you Hamming distance


X= 1 0 0 1 0 0 1 0

Y= 1 1 0 1 1 0 0 1

d X, Y) = 4

X= 1 0 0 1 0 0 1 0

Y= 1 1 0 1 1 0 0 1

XOR

0 1 0 0 1 0 1 1

d X, Y) = 4



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Minimum hamming distance (dmin

)

It is the smallest of the hamming distances

calculated between each possible pair of codes

Possible to detect errors if the total number of

errors in the received codeword is less than dmin,

otherwise errors can not be corrected

Error detection and correction capabilities of any

coding technique largely depend on the dmin




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mis the number of data bits in a frame

ris the number of redudant bitsNumber of bits in Codeword: n=m+r

In most data communication, all 2m possible data

messages are legal, but not all 2n code words are

used

Selection of r

2r>=m+r+1

For example: if m=7, the smallest value of r=424>=7+4+1




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Hamming code

The basic idea is to insert parity bits in between the data bits

to be transmitted

Parity bits are placed at each 2t bit positions where

t=0,1,2,3,,,,,

20= 1 at first position

21= 2 at second position 22= 4 at fourth position and so on




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Number of redundancy bits needed

Let data bits = m

Redundancy bits =r

Total message sent =m+r

The value of r must satisfy the following relation:

2r m+r+1

Example: if m=4, 3 parity bits are required to

satisfy the condition




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Value of r bits set to 1 or 0 to make an even parity

r1= 1, 3, 5, 7, 9, 11, , , , , ,

r2=2,3, 6, 7, 10, 11, , , , , r4 = 4, 5, 6, 7, 12, 13, 14, 15, , , , ,

r8 = 8,9,10,11,12,13,14,15, , , , ,

and so on

Ex-OR the bits to get redundancy bits/ parity bits




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Single bit errors




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Error Detection

The code being received is 10010100101. find the error bit



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Note :Error correcting capability

To detect d errors in the codeword

dmin d+1

To correct d errors in the code word

dmin 2d+1

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Data Link Layer