High Quality Content by WIKIPEDIA articles! A frame check sequence (FCS) refers to the extra checksum characters added to a frame in a communication protocol for error detection and correction. Frames are used to send upper-layer data and ultimately the user application data from a source to a destination. The data package includes the message to be sent, or user application data. Extra bytes may be added so frames have a minimum length for timing purposes. LLC bytes are also included with the Data field in the IEEE standard frames. The LLC sublayer takes the network protocol data, which is an IP packet, and adds control information to help deliver the packet to the destination node. Layer 2 communicates with the upper layers through LLC. (OSI model)