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.
Note that detection does not also mean error recovery. Ethernet defines that the errored frame should be discarded, but Ethernet takes no action to cause the frame to be retransmitted. Other protocols, notably TCP, can notice the lost data and cause error recovery to occur.[2]