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Parity Check

Last updated: September 1, 2022

What Does Parity Check Mean?

A parity check is an error-correction process in network communication that ensures data transmissions between communication nodes are accurate. In this process, the receiver agrees to use the same even parity bit or odd parity bit scheme as the sender. In an even parity check, parity bits ensure there are an even number of 1s and 0s in the transmission. In an odd parity check, there are an odd number of 1s and 0s in the transmission.

Once the source transmits data, the number of bits is checked by the receiver. If the number of received bits does not match what was agreed upon, it raises a red flag about the transmission's accuracy and future communication may be halted until the reason for the mismatch has been identified.

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Techopedia Explains Parity Check

Parity checking, which was created to eliminate data communication errors, has an easy to understand working mechanism. Parity bits are optional and there are no rules for where a parity bit has to be placed, but conventionally, parity bits are added at the end of the data transfer.

How Parity Checking Works

Imagine a data transfer that looks like this: 1010001. This example has an odd number of 1s and and even number of 0s.

When an even parity checking is used, a parity bit with value 1 could be added to the data’s right side to make the number of 1s even -- and the transmission would look like this: 10100011. If an odd parity check was used, the transmission would look like this: 10100010.

RAID

Redundant array of independent disks (RAID) also uses an enhanced form of parity check protection. A second set of parity data is written across all drives to avoid data loss in case of error.

When a RAID drive fails its parity check, data is rebuilt using parity information coupled with data on the other disks. The bits on the remaining drives are added up. If they add up to an odd number, the correct information on the failed drive has to be even, and vice-versa, for communication to continue.

Limitations

Parity checking is primarily used for communications, although more advanced protocols such as the Microcom Networking Protocols (MNP) and ITU-T V.42b has supplanted it as the standard in modem-to-modem communication.

Although parity checking provides a very basic method for detecting simple errors, it cannot, for example, detect errors caused by electrical noise changing the number of bits. It might happen, in fact, that both the receiving and sending bits are in error, offsetting each other.

Although the chance for this to happen in a PC is basically remote, in large computer systems where there’s an essential need to ensure data integrity, a third bit could be allocated for parity checking.

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