A mechanism used during a file transfer to determine whether transmission errors have occurred, and to correct those errors, if possible. Some programs or transmission protocols request a retransmission of the affected block of data if such an error is detected. More complex protocols attempt to both detect and correct transmission errors.

A critical aspect in data transmission is the determination of whether the received data is error-free. To do this some form of error protection, or error detection and correction is utilised. The numbers of errors at the output of the receiver can be reduced by adding additional information to the message to increase the receiver's decision-making ability. This additional, redundant information increases the overhead that is included in the transmission.

The most simplified method of error detecting or checking is parity. The parity is set to either even or odd by the transmitting device and the receiving device is set accordingly. The transmitting device sets the parity bit to a level that makes the total number of 1's in the character always even (even parity) or always odd (odd parity)