Editing Frame Relay (section)

=== Protocol data unit ===
Each Frame Relay [[protocol data unit]] (PDU) consists of the following fields:

# '''Flag Field'''. The flag is used to perform high-level data link synchronization which indicates the beginning and end of the frame with the unique pattern 01111110. To ensure that the 01111110 pattern does not appear somewhere inside the frame, [[bit stuffing|bit stuffing and destuffing]] procedures are used.
# '''Address Field'''. Each address field may occupy either octet 2 to 3, octet 2 to 4, or octet 2 to 5, depending on the range of the address in use. A two-octet address field comprises the EA=ADDRESS FIELD EXTENSION BITS and the C/R=COMMAND/RESPONSE BIT.[[File:En-tête Relais de Trame.png|thumb|Address field (2 octets)]]
## '''DLCI'''-Data Link Connection Identifier Bits. The [[DLCI]] serves to identify the virtual connection so that the receiving end knows which information connection a frame belongs to. Note that this DLCI has only local significance. A single physical channel can [[multiplexing|multiplex]] several different virtual connections.
## '''FECN, BECN, DE''' bits. These bits report congestion:
##* '''FECN'''=Forward Explicit Congestion Notification bit
##* '''[[BECN]]'''=Backward Explicit Congestion Notification bit
##* '''DE'''=Discard Eligibility bit
# '''Information Field'''. A system parameter defines the maximum number of data bytes that a host can pack into a frame. Hosts may negotiate the actual maximum frame length at call set-up time. The standard specifies the maximum information field size (supportable by any network) as at least 262 octets. Since end-to-end protocols typically operate on the basis of larger information units, Frame Relay recommends that the network support the maximum value of at least 1600 octets in order to avoid the need for segmentation and reassembling by end-users.
# '''Frame Check Sequence (FCS) Field'''. Since one cannot completely ignore the bit error-rate of the medium, each switching node needs to implement error detection to avoid wasting bandwidth due to the transmission of ''err''ed frames. The error detection mechanism used in Frame Relay uses the [[cyclic redundancy check]] (CRC) as its basis.