parity definition

Contributor(s): Kim Hefner

In computers, parity (from the Latin paritas meaning equal or equivalent) refers to a technique of checking whether data has been lost or written over when it is moved from one place in storage to another or when transmitted between computers.

This is how parity works: An additional binary digit, the parity bit, is added to a group of bits that are moved together. This bit is used only for the purpose of identifying whether the bits being moved arrived successfully. Before the bits are sent, they are counted and if the total number of data bits is even, the parity bit is set to one so that the total number of bits transmitted will form an odd number. If the total number of data bits is already an odd number, the parity bit remains or is set to zero. At the receiving end, each group of incoming bits is checked to see if the group totals to an odd number. If the total is even, a transmission error has occurred and either the transmission is retried or the system halts and an error message is sent to the user.

The description above describes how parity checking works within a computer. Specifically, the Peripheral Component Interconnect bus and the I/O bus controller use the odd parity method of error checking. Parity bit checking is not an infallible error-checking method since it's possible that two bits could be in error in a transmission, offsetting each other. For transmissions within a personal computer, this possibility is considered extremely remote. In some large computer systems where data integrity is seen as extremely important, three bits are allocated for parity checking.

Parity checking is also used in communication between modems. Here, parity checking can be selected to be even (a successful transmission forms an even number) or odd. Users may also select no parity, meaning the modems will not transmit or check a parity bit. When no parity is selected (or defaulted), it's assumed there are other forms of checking that will detect any errors in transmission. No parity also usually means the parity bit can be used for data, speeding up transmission. In modem-to-modem communication, the type of parity is coordinated by the sending and receiving modems before the transmission takes place.

Parity and RAID

The concept of parity is also used in redundant array of independent disks (RAID) protection. Some RAID groups have one or more disk drives that contain parity information that allows them to rebuild data if a drive failure occurs. For example, double-parity RAID ( also known as RAID 6), stripes data across a set of at least four drives at a block level like RAID 5, but then writes a second set of parity data across all the drives. This approach guards against data loss in up to two failed drives. Drawbacks to double-parity RAID include the use of a complex controller, the cost of two extra drives for implementation and slower write transactions due to the extra parity set.

This was first published in May 2005

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