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Monday, December 14, 2009

Error Detection and Correction

Error detection and correction are techniques to ensure that data is transmitted without errors, even across unreliable media or networks. Error detection is the ability to detect the presence of errors caused by noise or other impairments during transmission from the transmitter to the receiver. Error correction is the additional ability to reconstruct the original, error-free data.
Because of the extremely low bit-error rates in data transmissions, most error detection methods and algorithms are designed to address the detection or correction of a single bit error.

Data Detection Methods :
Errors introduced by communications faults, noise or other failures into valid data, especially compressed data were redundancy has been removed as much as possible, can be detected and/or corrected by introducing redundancy into the data stream.

- Parity Schemes
A parity bit is an error detection mechanism that can only detect an odd number of errors. The stream of data is broken up into blocks of bits, and the number of 1 bits is counted. Then, a "parity bit" is set (or cleared) if the number of one bits is odd (or even). (This scheme is called even parity; odd parity can also be used.) If the tested blocks overlap, then the parity bits can be used to isolate the error, and even correct it if the error affects a single bit.
There is a limitation to parity schemes. A parity bit is only guaranteed to detect an odd number of bit errors (one, three, five, and so on). If an even number of bits (two, four, six and so on) are flipped, the parity bit appears to be correct, even though the data is corrupt.

- Checksum
A checksum of a message is an arithmetic sum of message code words of a certain word length, for example byte values, and their carry value. The sum is negated by means of ones-complement, and stored or transferred as an extra code word extending the message. On the receiver side, a new checksum may be calculated from the extended message. If the new checksum is not 0, an error has been detected.

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