A frame check sequence (FCS) refers to the extra error-detecting code added to a frame in a communications protocol. Frames are used to send upper-layer data and ultimately the application data from a source to a destination.
The detection does not imply error recovery; for example, Ethernet specifies that a damaged frame should be discarded, but at the same time does not specify any action to cause the frame to be retransmitted. Other protocols, notably the Transmission Control Protocol (TCP), can notice the data loss and initiate error recovery.
Overview
All frames and the bits, bytes, and fields contained within them, are susceptible to errors from a variety of sources. The FCS field contains a number that is calculated by the source node based on the data in the frame. This number is added to the end of a frame that is sent. When the destination node receives the frame the FCS number is recalculated and compared with the FCS number included in the frame. If the two numbers are different, an error is assumed and the frame is discarded. The sending host computes a cyclic redundancy check on the entire frame and appends this as a trailer to the data. The receiving host recomputes the cyclic redundancy check on the frame using the same algorithm, and compares it to the received FCS. This way it can detect whether any data was lost or altered in transit. It may then discard the data, and request retransmission of the faulty frame.
The FCS is often transmitted in such a way that the receiver can compute a running sum over the entire frame, together with the trailing FCS, expecting to see a fixed result (such as zero) when it is correct. For Ethernet and other IEEE 802 protocols, this fixed result, also known as the magic number or CRC32 residue, is 0xC704DD7B. When transmitted and used in this way, FCS generally appears immediately before the frame-ending delimiter.
By far the most popular FCS algorithm is a cyclic redundancy check (CRC), used in Ethernet and other IEEE 802 protocols with 32 bits, in X.25 with 16 or 32 bits, in HDLC with 16 or 32 bits, in Frame Relay with 16 bits, in Point-to-Point Protocol (PPP) with 16 or 32 bits, and in other data link layer protocols.