Error Control

When data is transmitted over a cable or channel, there is always a chance that some of the bits will be changed (corrupted) due to noise, signal distortion or attenuation. If errors do occur, then some of the bits will either change from 0 to 1 or from 1 to 0. Error Control allows the receiver to inform the sender of any frames lost or damaged in transmission and coordinates the retransmission of those frames by the sender. Error control is divided in two main categories:

Error Detection It allows a receiver to check whether received data has been corrupted during transmission. It can, for example, request a retransmission. Error Correction This type of error control allows a receiver to reconstruct the original information when it has been corrupted during transmission. In the data link layer, the term error control refers primarily to methods of error detection and retransmission. Error control in the data link layer is often implemented simply: Any time an error is detected in an exchange, specified frames are retransmitted. This process is called automatic repeat request (ARQ). Error control in the data link layer is based on automatic repeat request, which is the retransmission of data.

There are 2 ways to correct found errors: • Forward error correction (FEC is accomplished by adding redundancy to the transmitted information using a predetermined algorithm. Each redundant bit is invariably a complex function of many original information bits. The original information may or may not appear in the encoded output; codes that include the unmodified input in the output are systematic, while those that do not are nonsystematic.) and • Automatic repeat request (ARQ) ( in which the receiver detects transmission errors in a message and automatically requests a retransmission from the transmitter. Usually, when the transmitter receives the ARQ, the transmitter retransmits the message until it is either correctly received or the error persists beyond a predetermined number of retransmissions.A few types of ARQ protocols are Stop-and-wait ARQ, Go-Back-N ARQ and Selective Repeat ARQ.).