Parity Bits

A parity bit is a single bit added to a binary data transmission used to indicate if whether the 0's and 1's within that data transmission is an even or odd number. The parity bit is used in parity error checking to find errors that may occur during data transmission.

Even parity can be more clearly explained by means of an example. Consider the transmitted message 1010001, which has three ones in it. This is turned into even parity by adding a one, making the sequence 1 1010001, so that there are four ones (an even number). If the transmitted message has the form 1101001, which is already an even number, a zero is added to sustain the even parity. The resulting message is 0 1101001, so that an even number of ones remains in the transmitted message.

Even parity checking is also used in testing memory storage devices. However, in order for this to work, the sender and receiver should agree to use even parity checking as the basic error detection technique. If a single bit is switched during transmission, parity checks can detect that the data is corrupted. However, even parity may fail to detect errors introduced when an even number of bits in the same data unit is altered due to electrical noises.

Parity checking is the most basic form of error detection in communications. Although it detects many errors, it is not foolproof, because it cannot detect situations in which an even number of bits in the same data unit are changed due to electrical noise. There are many other more sophisticated protocols for ensuring transmission accuracy.

Parity checking is used not only in communications but also to test memory storage devices. Many PCs, for example, perform a parity check on memory every time a byte of data is read.