Parity Check

Last Updated: October 6, 2020

Definition - What does Parity Check mean?

A parity check is the process that ensures accurate data transmission between nodes during communication. A parity bit is appended to the original data bits to create an even or odd bit number; the number of bits with value one.

The source then transmits this data via a link, and bits are checked and verified at the destination. Data is considered accurate if the number of bits (even or odd) matches the number transmitted from the source.

Techopedia explains Parity Check

Parity checking, which was created to eliminate data communication errors, is a simple method of network data verification and has an easy and understandable working mechanism.

As an example, if the original data is 1010001, there are three 1s. When even parity checking is used, a parity bit with value 1 is added to the data’s left side to make the number of 1s is even; transmitted data becomes 11010001. However, if odd parity checking is used, then parity bit value is zero; 01010001. If the original data contains an even number of 1s (1101001), then parity bit of value 1 is added to the data’s left side to make the number of 1s odd, if odd parity checking is used and data transmitted becomes 11101001.

The receiver agrees to use the same parity check as the sender, which is either odd or even. If this agreement is not properly configured, communication cannot occur. Once the data reaches the receiver, if data is transmitted incorrectly, the parity bit value becomes incorrect; thus, indicating an error has occurred during transmission.

Parity checking is used for communications, although more advanced protocols such as the Microcom Networking Protocols (MNP) and ITU-T V.42b is supplanted it as the standard in modem communication. It is still used for memory storage device testing, for example, to run memory checks when data is read.

Parity checking is a very basic method that can detect simple errors but 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.

Redundant array of independent disks (RAID) also use an enhanced form of protection based on parity that check horizontal and vertical parity. A second set of parity data is written across all drives to avoid 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 had to be even, and vice-versa.

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