frame relay frame
Frame Relay(FR) has a data frame consisting of a few data fields. Two 8 bit long flags mark the beginning and the end of the Frame Relay header. Between these two flags are the address field, the payload with the user data and a 2 byte long checksum field( FCS). The header is 2 to 4 bytes long and contains the Data Link Channel Identifier( DLCI), as well as some control bits and bits for overload indication.
Faulty packets, i.e. packets whose DLCI field was transmitted with errors, are requested again via the complete end-to-end connection. The DLCI field contains the control mechanisms for the connection and transmission. The numeric DLCI value identifies a specific virtual connection, which allows the frame relay network to uniquely identify the sending and receiving stations. The Frame Relay header also has several 1-bit data fields for congestion control, Forward Explicit Congestion Notification( FECN) and Backward Explicit Congestion Notification( BECN). One Command/Response bit (C/R), two bits for address extension, and another for Discard Eligibility( DE) for congestion events.
Within the variable information field, user data can be transmitted between 262 bytes and up to 8,192 bytes (8 KB). The checksum field following the user data comprises 2 bytes and is used to detect bit errors.