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Error Detection and Correction in Data link Layer
Computer Applications (IRDA37)
Annamalai University
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Error Detection and Correction in Data link Layer
Data-link layer uses error control techniques to ensure that frames, i. bit streams of data, are transmitted from the source to the destination with a certain extent of accuracy.
Errors
When bits are transmitted over the computer network, they are subject to get corrupted due to interference and network problems. The corrupted bits leads to spurious data being received by the destination and are called errors.
Types of Errors
Errors can be of three types, namely single bit errors, multiple bit errors, and burst errors. Single bit error − In the received frame, only one bit has been corrupted, i. either changed from 0 to 1 or from 1 to 0.
Multiple bits error − In the received frame, more than one bits are corrupted.
Burst error − In the received frame, more than one consecutive bits are corrupted.
Error Control
Error control can be done in two ways Error detection − Error detection involves checking whether any error has occurred or not. The number of error bits and the type of error does not matter. Error correction − Error correction involves ascertaining the exact number of bits that has been corrupted and the location of the corrupted bits.
For both error detection and error correction, the sender needs to send some additional bits along with the data bits. The receiver performs necessary checks based upon the additional redundant bits. If it finds that the data is free from errors, it removes the redundant bits before passing the message to the upper layers.
Error Detection Techniques
There are three main techniques for detecting errors in frames: Parity Check, Checksum and Cyclic Redundancy Check (CRC).
Parity Check
The parity check is done by adding an extra bit, called parity bit to the data to make a number of 1s either even in case of even parity or odd in case of odd parity. While creating a frame, the sender counts the number of 1s in it and adds the parity bit in the following way In case of even parity: If a number of 1s is even then parity bit value is 0. If the number of 1s is odd then parity bit value is 1. In case of odd parity: If a number of 1s is odd then parity bit value is 0. If a number of 1s is even then parity bit value is 1.
On receiving a frame, the receiver counts the number of 1s in it. In case of even parity check, if the count of 1s is even, the frame is accepted, otherwise, it is rejected. A similar rule is adopted for odd parity check. The parity check is suitable for single bit error detection only.
Error Detection and Correction in Data link Layer
Course: Computer Applications (IRDA37)
University: Annamalai University
