# CS601 - Data Communication - Lecture Handout 36

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# Hamming Code

## Redundancy Bits (r)

• r must be able to indicate at least m+r+1 states
• m+r+1 states must be discoverable by r bits
• Therefore, 2r ≥ m+r+1
• If m=7, r=4 as 24 ≥ 7+4+1

## Hamming Code

• Each r bit is the VRC bit for one combination of data bits
• r1(r2) bit is calculated using all bit positions whose binary representation includes a 1 in the first(second) position, and so on

### Introduction

• Unless accurately received by a 2nd device, a signal TX over a wire is a waste of electricity
• With TX alone, we can put a signal to the line, but we have
• no way of controlling which of several devices attached to that line will receive it
• No way of keeping a second device from TX at the same time
• Communication requires at least 2 devices working together:
• Sender
• Even such a basic arrangement requires great deal of coordination
• For Example, in Half Duplex TX, it is essential that only one device TX at a time
• If both device TX, the signals will collide leaving nothing on the line but Noise
• The coordination of half-duplex TX is a part of a procedure called Line Discipline, which is one of the functions included in the second layer of OSI Model, the data link layer
• In addition to Line Discipline, the most important functions in the data link layer are Flow Control and Error Control
• Collectively these functions are called Data Link Control

## Data Link Layer & its Functions

### Line Discipline

• Coordinates the link systems, which device can send and when it can send?

### Flow Control:

• The amount of data that can be sent before the receiving acknowledgement
• It also provides the receiver’s acknowledgement for frames received intact and so is linked to error control

### Error Control:

• Means Error detection and Correction
• It allows the receiver to inform the sender of any frames lost or damaged in TX and coordinates Retransmission of those frames by the sender

### Line Discipline

• How efficient the system is, no device in it should be allowed to transmit until that device has the evidence that the intended receiver is:
• is prepared to accept the TX
• What if the Rx device does not expect a transmission or is busy
• With no way of determining the status of the intended receiver, the transmitting device may waste its time sending data to a non-functioning receiver
• The Line Discipline functions of the data link layer oversee:
• the establishment of links and
• The right of a particular device to transmit at a given time

### Ways to do Line Discipline

Line Discipline is done in 2 ways:

• Enquiry / Acknowledgement (ENQ/ACK)
• Used in Peer-to-Peer Communication
• Poll / Select
• Primary-Secondary communication

• Hamming Code