Parity Check

A parity check works by adding one extra bit — the parity bit — to each group of bits before transmission. Under even parity, the parity bit is set so that the total number of 1 bits in the group is even. Under odd parity, it is set so that the total number of 1 bits is odd. Sender and receiver must have agreed in advance which is being used. After transmission the receiver counts the 1 bits in each group: if the count no longer matches the agreed parity, an error has been detected.

Automatic repeat query (ARQ) is a method of establishing that data has been received without error. It uses an error-detection method — such as a checksum or a parity check — on the received data, and then acts on the result. If the data passes the check, the receiver returns a positive acknowledgement. If it fails, the receiver returns a negative acknowledgement, and the sender retransmits. If no acknowledgement arrives at all before the sender's timeout expires, the sender assumes the data did not get through and retransmits anyway. The process repeats until the data is acknowledged or the system stops after the number of attempts it permits.