A parity bit is an extra bit transmitted with data item select to give the resulting bit odd or even parity. For example an even parity data packet 10100001 has parity bit 1 as it has odd number of 1's. An odd parity data packet 10010111 has parity bit 0 as it has odd number of 1's.
Let us take another example, if noise or other interference produces an error one of the bits in the data will be modified from a 1 to a 0 or from a 0 to a 1. Thus the parity of giving bits will be large.
Suppose original parity and data is 10100001+1 (even parity). After interference the incorrect data is 10101001+1 and it has convert an odd parity.
LIMITATIONS OF PARITY CHECKING:
Parity can only check errors that modify in odd number of bits for example the parity and original data is 10100001+1 (even parity) and the incorrect data is 10101011+1 (even parity). We can see that even no. of bits have been modified due to noise so parity checking could not check this error. Parity generally is used to check on bit error.
ALTERNATIVE ERROR DETECTION METHODES:
In addition to parity checking alternative error detection functions have been produced. These functions differ from each other by the following respects.