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Computer Networks: Bit and Byte Stuffing 1

Bit and Byte Stuffing

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Computer Networks: Bit and Byte Stuffing 2

Synchronous versus AsynchronousTransmissions

There exists a hierarchy of synchronization

tasks:

Bit level: recognizing the start and end of each

bit

Character or byte level: recognizing the start

and end of each character (or small unit of data)

Block or message level: recognize the start andend of each large unit of data (in networks this

is a frame).

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Computer Networks: Bit and Byte Stuffing 3

Synchronous versus AsynchronousTransmissions [Halsall]

A fundamental requirement of digital data communicationsis that the receiver knows the starting time and theduration of each bit.

Asynchronous transmission :: each character (or byte) istreated independently for clock (bit) and character (byte)synchronization purposes and the receiverresynchronizes at the start of each character received.

Synchronous transmission :: the complete frame istransmitted as a contiguous string of bits and thereceiver endeavors to keep in synchronism with theincoming bit stream for the duration of the frame.

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Computer Networks: Bit and Byte Stuffing 4

Start

bit

Stop

bit1 2 3 4 5 6 7 8

Data bits

Line

idle

3T/2 T T T T T T T

Receiver samples the bits

Figure 3.68

Transmitted at random intervals (e.g., keyboard)

Direction of transmission

Copyright 2000 The McGraw Hill Companies Leon-Garcia & Widjaja: Communication Networks

Synchronizationin Asynchronous Transmissions

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Computer Networks: Bit and Byte Stuffing 5

More efficient, i.e., less overhead

Blocks of characters transmitted without

start and stop codes The transmitted stream is suitably encoded

so the receiver can stay in synchby:

Using a separate clock lineEmbedding clocking information into data (e.g.

biphase coding).

Synchronous Transmissions

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Computer Networks: Bit and Byte Stuffing 6

1. Byte counts

2. Starting/ending bytes [byte stuffing]

3. Starting/ending flags [bit stuffing]

4. Using physical layer coding

violations (i.e., invalid physicalcodes)

Frame Identification Methods

[Tanenbaum]

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Preamble

Bit Pattern

Postamble

Bit Pattern

frame

The contents of each frame are encapsulatedbetween a

pair of reserved characters or bytes for frame synchronization.

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Computer Networks: Bit and Byte Stuffing 8

Header Body

8 8 8 8 168

CRC

BISYNC Frame Format

Figure 2.9

P&D slide

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Computer Networks: Bit and Byte Stuffing 9

ProtocolControlAddressFlag Pay load

88 816168

FlagChecksum

PPP Frame Format

Figure 2.10P&D slide

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Computer Networks: Bit and Byte Stuffing 10

Figure 2.11

Header Body

8 8 4214 168

CR CCount

DDCMP Frame Format

P&D slide

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Computer Networks: Bit and Byte Stuffing 11

Figure 2.12

Header Body

8 16 16 8

CRCBeginningsequence

Endingsequence

HDLC Frame Format

P&D slide

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Computer Networks: Bit and Byte Stuffing 12

Byte Stuffing[HDLC Example]

Also referred to as character stuffing. ASCII characters are used as framing delimiters

(e.g. DLE STX and DLE ETX)

The problem occurs when these character patternsoccur within the transparent data.

Solution: sender stuffs an extra DLEinto the datastream just before each occurrence of anaccidentalDLE in the data stream.

The data link layer on the receiving end unstuffs theDLEbefore giving the data to the network layer.

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Computer Networks: Bit and Byte Stuffing 13

DLE STX DLE ETXTransparent Data

DLE STX DLE ETXA B DLE H W

DLE STX DLE ETXA B DLE H WDLE

DLE STX DLE ETXA B DLE H W

Stuffed

Unstuffed

Before

Byte Stuffing[HDLC Example]

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Computer Networks: Bit and Byte Stuffing 14

Bit Stuffing

Each frame begins and ends with a special bitpattern called a flag byte [01111110].{Note this is7E in hex}

Whenever sender data link layer encountersfiveconsecutive ones in the data stream, itautomatically stuffs a 0 bit into the outgoingstream.

When the receiver seesfive consecutive incomingones followed by a 0 bit, it automatically destuffsthe 0 bit before sending the data to the networklayer.

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Computer Networks: Bit and Byte Stuffing 15

Input Stream

Stuffed Stream

Unstuffed Stream

0110111111100111110111111111100000

01101111101100111110011111011111000000

0110111111100111110111111111100000

Stuffed bits

Bit Stuffing

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Flag flagAddress Control Information CRCProtocol01111110 011111101111111 00000011

Unnumbered

frameSpecifies what kind of packet is contained in the

payload, e.g., LCP, NCP, IP, OSI CLNP, IPX

All stations are to

accept the frame

Figure 5.40Copyright 2000 The McGraw Hill Companies Leon-Garcia & Widjaja: Communication Networks

PPP (Point-to-Point Protocol)Frame Format

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Flag flagAddress Control Information CRCProtocol01111110 1111111 00000011 01111110

7E7E 7E7D

5D

33 42

7E7E

7E7E

7D

7D 5D 33

33

7D 5E 42

427E

Input

Stuffed Stream

Unstuffed Stream

Flag01111110

Flag01111110

PPP Byte Stuffing