Error Detection Codes



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Binary information transmitted through some form of communication medium is subject to external noise that could change bits from 1 to 0, and vice versa. An error detection code is a binary code that detects digital errors during transmission. The detected errors cannot be corrected but their presence is indicated. The usual procedure is to observe the frequency of errors. If errors occur infrequently at random, the particular erroneous information is transmitted again. If the error occurs too often, the system is checked for malfunction.

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parity bit: The most common error detection code used is the parity bit. A parity bit is an extra bit included with a binary message to make the total number of 1's either odd or even. A message of three bits and two possible parity bits is shown in Table 3-7. The P(odd) bit is chosen in such a way as to make the sum of 1's (in all four bits) odd. The P(even) bit is chosen to make the sum of all 1's even. In either case, the sum is taken over the message and the P bit. In any particular application, one or the other type of parity will be adopted. The even-parity scheme has the disadvantage of having a bit combination of all O's, while in the odd parity there is always one bit (of the four bits that constitute the message and P) that is 1. Note that the P(odd) is the complement of the P(even).

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parity generator: Dunng rransfer of information from one location to another, the parity bit is handled as follows. At the sending end, the message (in this case three bits) is applied to a parity generator, where the required parity bit is generated. The message, including the parity bit, is transmitted to its destination. At the receiving end, all the incoming bits (in this case, four) are applied to a parity checker that checks the proper parity adopted (odd or even). An error is detected if the checked parity does not conform to the adopted parity. The parity method detects the presence of one, three, or any odd number of errors. An even number of errors is not detected.

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