Parity Check

The most common approach is parity checking, which involves counting all the 1 bit in the data and adding one more bit to make the total number of 1 bit even (even parity) or odd (odd parity). The extra bit is called parity redundant bit. The method is also called vertical redundancy check (VRC).
Figure 3.4 illustrates the transmission of binary data unit 1100010 (ASCII character b). Adding together the number of is gives us 3, an odd number. Before transmitting, parity generator counts the is and appends the parity bit, that is, 1 to the end. The total number of is now four, an even number. Now the sender transmit: the entire expanded unit, that is, 11000101. When it reaches the destination, the receiver puts all eight bits through an even-parity checking function.
If the receiver sees 11000101 and counts the total number of is in the data unit that is, four, an even number, the data unit passes. If the data unit has been damaged during transmission and the receiver receive

 
11100101 in place of 110001001, the parity checking function counts the is to get 5 which is an odd number. The receiver knows that an error has been introduced into the data somewhere and therefore rejects the whole unit.
Some systems use odd-parity checking, where the number of Is should be odd The principle is the same; only calculation is different.
For example, suppose the sender wants to send the word "world". In ASCII, the five characters are represented as:
Each of the first four characters has an even number of is So the parity bit is 0. The last character `d' however has three Is (an odd number), so the parity bit is 1 to make the total number of Is even. Figure 3.5 illustrates the actual bits received by the receiver when the signal is not corrupted in transmission. The receiver counts the Is in each character and comes up with an even number (6, 6, 4, 4) and the data will be accepted.

                 
The receiver counts the 1s in each character and comes up with even and odd numbers (7, 6, 5, 4) indicating an error. The receiver asks for retransmission. Parity check can detect all signal-bit errors. It can detect burst errors only if the total number of errors in each data unit is odd.

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