Checksum

A checksum is a value calculated from a block of data by applying an agreed calculation to it. The sender performs that calculation, sends the resulting checksum value together with the data, and the receiver repeats the same calculation on the data it received. The two values are then compared. If they are different, the data has changed during transmission and an error has been detected. If they are the same, no error has been detected — which is not the same thing as the data being guaranteed correct.

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.