To understand much of the functionality of Frame Relay, it is helpful to understand the structure of the Frame Relay frame. Figure B-4 depicts the basic format of the Frame Relay frame, and Figure B-5 illustrates the LMI version of the Frame Relay frame.
Flags indicate the beginning and end of the frame. Three primary components make up the Frame Relay frame: the header and address area, the user -data portion, and the frame-check sequence (FCS). The address area, which is 2 bytes in length, is comprised of 10 bits representing the actual circuit identifier and 6 bits of fields related to congestion management. This identifier commonly is referred to as the data-link connection identifier (DLCI). Each of these is discussed in the descriptions that follow.
Standard Frame Relay frames consist of the fields illustrated in Figure B-4.
The following descriptions summarize the basic Frame Relay frame fields illustrated in Figure B-4.
Flags Delimits the beginning and end of the frame. The value of this field is always the same and is represented either as the hexadecimal number 7E or as the binary number 01111110.
Address Contains the following information:
- Forward-explicit congestion notification ( FECN ) is a single-bit field that can be set to a value of 1 by a switch to indicate to an end DTE device, such as a router, that congestion was experienced in the direction of the frame transmission from source to destination. The primary benefit of the use of the FECN and BECN fields is that higher-layer protocols can react intelligently to these congestion indicators. Today, DECnet and OSI are the only higher-layer protocols that implement these capabilities.
Backward-explicit congestion notification ( BECN ) is a single-bit field that, when set to a value of 1 by a switch, indicates that congestion was experienced in the network in the direction opposite of the frame transmission from source to destination.
Discard eligibility (DE) is set by the DTE device, such as a router, to indicate that the marked frame is of lesser importance relative to other frames being transmitted. Frames that are marked as "discard eligible" should be discarded before other frames in a congested network. This allows for a fairly basic prioritization mechanism in Frame Relay networks.
Data Contains encapsulated upper-layer data. Each frame in this variable-length field includes a user data or payload field that will vary in length up to 16,000 octets. This field serves to transport the higher-layer protocol packet (PDU) through a Frame Relay network.
Frame Check Sequence Ensures the integrity of transmitted data. This value is computed by the source device and is verified by the receiver to ensure integrity of transmission.
Frame Relay frames that conform to the LMI specifications consist of the fields illustrated in Figure B-5.
The following descriptions summarize the fields illustrated in Figure B-5.
Flag Delimits the beginning and end of the frame.
LMI DLCI Identifies the frame as an LMI frame instead of a basic Frame Relay frame. The LMI-specific DLCI value defined in the LMI consortium specification is DLCI = 1023.
Unnumbered Information Indicator Sets the poll/final bit to 0.
Protocol Discriminator Always contains a value indicating that the frame is an LMI frame.
Call Reference Always contains zeros. This field currently is not used for any purpose.
Message Type Labels the frame as one of the following message types:
Information Elements Contains a variable number of individual information elements (IEs). IEs consist of the following fields:
Frame Check Sequence (FCS) Ensures the integrity of transmitted data.
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