- A parity bit, or check bit, is a bit added to a string of binary code that indicates whether the number of 1-bits in the string is even or odd.
- Parity bits are used as the simplest form of error detecting code.
- There are two variants of parity bits: even parity bit and odd parity bit.
Even parity bit
- In even parity bit, sending data consist must be even number of 1-bit count. In these case it will add parity bit zero other wise one.
7 bits of data |
(count of 1-bits) | 8 bits including parity |
---|---|---|
0000000 | 0 | 00000000 |
1010001 | 3 | 10100011 |
1101001 | 4 | 11010010 |
1111111 | 7 | 11111111 |
Odd parity bit
- In Odd parity bit, sending data consist must be odd number of 1-bit count. In these case it will add parity bit zero other wise one.
7 bits of data |
(count of 1-bits) | 8 bits including parity |
---|---|---|
0000000 | 0 | 00000001 |
1010001 | 3 | 10100010 |
1101001 | 4 | 11010011 |
1111111 | 7 | 11111110 |
Error detection
- The below table shows the successful transmission while sending data from A to B.
Type of bit parity | Successful transmission scenario |
---|---|
Even parity |
A wants to transmit data: 1001 A computes parity bit value: 1+0+0+1 (mod 2) = 0 A adds parity bit and sends data: 10010 B receives data: 10010 B computes parity: 1+0+0+1+0 (mod 2) = 0 B reports correct transmission after observing expected even result. |
Odd parity |
A wants to transmit data: 1001 Acomputes parity bit value: 1+0+0+1 + 1 (mod 2) = 1 A adds parity bit and sends data: 10011 B receives data: 10011 B computes overall parity: 1+0+0+1+1 (mod 2) = 1 B reports correct transmission after observing expected odd result. |
- The below table shows the failed transmission scenario while sending data from A to B.
Type of bit parity error | Failed transmission scenario |
---|---|
Even parity
Error in the second bit |
A wants to transmit data: 1001 A computes parity bit value: 1+0+0+1 = 0 A adds parity bit and sends data: 10010 ...TRANSMISSION ERROR... B receives data: 11010 B computes overall parity: 1+1+0+1+0 = 1 B reports incorrect transmission after observing unexpected odd result. |
Even parity
Error in the parity bit |
A wants to transmit data: 1001 A computes even parity value: 1+0+0+1 = 0 A sends data: 10010 ...TRANSMISSION ERROR... B receives data: 10011 Bcomputes overall parity: 1+0+0+1+1 = 1 B reports incorrect transmission after observing unexpected odd result. |
History
- A "parity track" was present on the first magnetic tape data storage in 1951.
- Parity in this form, applied across multiple parallel signals, is known as a transverse redundancy check.
- This can be combined with parity computed over multiple bits sent on a single signal, a longitudinal redundancy check.
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