The parity check method is used fro error detection. It is also known as VRC method.
(a) In this method, a redundant bit called parity bit is added to each data.
(b) This method may include even parity or odd parity. Even parity means that total number of Is in data (including parity bit) is even. Odd parity means that the total number of 1s in data (including parity bit) is odd.
(c) For example, if source wants to transmit a data unit 11001111 using even parity to the destination. The source will first pass this unit to even parity generator.
(d) The parity generator will count the number of Is in data unit and will odd parity bit. In this example, since the number of 1s in data unit is five, the parity generator appends a parity bit 1 to this data unit making the total number of 1seven i.e. six.
(e) This data along with parity bit is then transmitted across the network. In this case 11001111 will be transmitted.
(f) At the destination, this data is passed to parity checker. The parity checker counts the number of 1s in data. If the number of ones comes out to be odd, for e.g., five or seven then destination will come to know that there is some error in the data. The receiver then rejects such erroneous data unit.
(g) VRC can detect all single bit error.
(h) It can also detect burst errors but only in those cases where number of bits changed is odd i.e. if number of bits changed are 1,3,5, 7 …. etc.
(i) The major disadvantages of this method is that it is not able to detect burst error if number of bits changes are even i.e., if 2, 4, 6,8 … etc.