Frame Check Sequence

A frame check sequence (FCS) refers to the extra checksum characters added to a Frame in a communication protocol for error detection and correction.

The sending host computes a checksum on the entire frame and sends this along.The receiving host computes the checksum on the frame using the same algorithm, and compares it to the received FCS. This way it can detect whether any data was lost on altered in transit. It may then discard the data, and request retransmission of the faulty frame. A cyclic redundancy check is often used to compute the FCS.

FCS is used in X.25, HDLC, Frame Relay, and other data link layer protocols.