Redundancy check in computer network

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Redundancy:-

Instead of repeating the entire data stream, a shorter group of bits may be appended to the end of each unit.

This technique is called Redundancy because the extra bit are redundant to the information.

They are discarded as soon as the accuracy of the transmission has been determined.

These are the basically four types of redundancy check.
1. VRC (vertical redundancy check)
2. LRC (longitudinal redundancy check)
3. CRC ( cyclical redundancy check)

Vertical redundancy check:-

It is also known as parity check.
It is least expensive mechanism for error detection.
In this technique, the redundant bit called parity bit is appended to every data unit so that the total number 1s in the unit becomes even.

Features:-
  • VRC can detect all single bit errors .
  • It can detect burst errors if the total number of  errors in each data unit is odd.
  • VRC can not detect erros where the total number of bits changed is even.




Drawback:-

 This method is unrealible because if there are even number of errors in data the error will go undetected.

Longitudinal redundancy check :-

In telecommunications, LRC is a form of redundancy check that is applied independently to each of a parallel group of bit streams. The data must be devided into transmission blocks to each the additional check data is added.

In this method, a block of bits is organised into table(rows and columns) calculate the parity bit for each column and the set of this parity bit is also sending with original data .
From the block of parity we can check the redundancy.

Advantage:-
LRC of n bits can easily detect burst error of n bits.

Disadvantages:-
If two bits in one data units are damaged and two bits in exactly same position in another data unit are also damaged , the LRC checker will not detect the error.

Cyclic redundancy check:-

CRC or Cyclic Redundancy Check is a method of detecting accidental changes/errors in communication channel.
CRC uses Generator Polynomial which is available on both sender and receiver side. An example generator polynomial is of the form like x3 + x + 1. This generator polynomial represents key 1011. Another example is x2 + 1 that represent key 101.
A cyclic redundancy check is an error detecting code common in digital networks.
CRC is more powerful than VRC and LRC in detecting errors.
• It is not based on binary addition like VRC and LRC. Rather it is based on binary division.




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