Byte Stuffing - Computer Network - Lecture Slides, Slides of Computer Networks

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COMPUTER

NETWORKS

Lecture-

Review Lecture 4

 The Concept of Packets

 Why use Packets?

 Packets and Frames

 Byte Stuffing

Byte Stuffing (cont’d)

 Byte stuffing translates each reserved

byte into two unreserved bytes

 For example, can use esc as prefix,

followed by x for soh, y for eot and z for

esc

Byte Stuffing (cont’d)

 The sender replaces each occurance of

special characters with a combination of

two characters (i.e.esc x, esc y, esc z)

 The receiver then replaces each

occurence of esc x, esc y, esc z, by the

corresponding single character

Byte Stuffing (cont’d)

Transmission Errors

 Interference or power surges may

destroy data during transmission

• Bits may be lost
• Bit values may be changed

 Frames include additional information

to detect / correct errors

• Inserted by the sender
• Checked by the receiver

Parity Checking

 Parity refers to the number of bits set

to 1 in the data item

• Even parity - An even number of

bits are 1

• Odd parity - An odd number of bits

are 1

Parity Checking

 A Parity Bit is an extra bit transmitted

with a data item, chose to give the

resulting bits even or odd parity

• Even parity - data: 10010001 ,

parity bit 1

• Odd parity - data: 10010111 ,

parity bit 0

Limitations of Parity Checking

 Parity can only detect errors that

change an odd number of bits

• Original data and parity: 10010001 + 1 (even parity)
• Incorrect data: 10110011 + 1 (even parity!)

 Parity usually used to catch one-bit

errors

Alternative Error Detection Schemes

 Alternative error detection mechanisms have been introduced

 Differences among mechanisms:

• The size of the additional information (transmission overhead)
• Computational complexity of the algorithm (computational overhead)
• The number of bit error that can be detected (how well errors are detected)

Checksum

Checksum

 Can be 8-, 16- or 32-bit integers

 Easy to do - uses only addition

 May not detect all errors

Cyclic Redundancy Check (CRC)

 Consider data in message as coefficients of a polynomial

 Divide that coefficient set by a known polynomial

 Transmit remainder as CRC

• Good error detection properties
• Easy to implement in hardware

Hardware Components

 Uses two hardware components:

• Shift Register
• Exclusive or (xor) unit