Hamming Code

Redundancy Bits (r)

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

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 knowing if the intended receiver is ready and able to receive it
- No way of keeping a second device from TX at the same time
Communication requires at least 2 devices working together:
- Sender
- Receiver
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

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

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

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:
- able to receive
- 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

Line Discipline is done in 2 ways:

Section 9.7, 10.1 “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan