3.3 Layer 2 Protocol Overview - WAN Protocols
3.3.7 Frame Relay frame format
A Frame Relay frame is shown in Figure . The Flags fields delimit the beginning and end of the frame. Following the leading Flags field are 2 bytes of address information (Figure ). Ten bits of these 2 bytes make up the actual circuit ID (called the DLCI, for data-link connection identifier). The 10-bit DLCI value is the heart of the Frame Relay header. It identifies the logical connection that is multiplexed into the physical channel. In the basic (not extended by LMI) mode of addressing, DLCIs have local significance; that is, the end devices at two different ends of a connection may use a different DLCI to refer to that same connection. Figure provides an example of the use of DLCIs in non-extended Frame Relay addressing.

In Figure , assume two PVCs, one between Atlanta and Los Angeles and one between San Jose and Pittsburgh. Los Angeles uses DLCI 12 to refer to its PVC with Atlanta, whereas Atlanta refers to the same PVC as DLCI 82. Similarly, San Jose uses DLCI 12 to refer to its PVC with Pittsburgh. The network uses internal proprietary mechanisms to keep the two locally significant PVC identifiers distinct.

At the end of each DLCI byte (Figure ) is an extended address (EA) bit. If this bit is 1, the current byte is the last DLCI byte. All implementations currently use a 2-byte DLCI, but the presence of the EA bits means that longer DLCIs may be agreed upon and used in the future.

The command/response bit following the most significant DLCI byte is currently not used.

Finally, 3 bits in the 2-byte DLCI provide congestion control. The forward explicit congestion notification (FECN) bit is set by the Frame Relay network in a frame to tell the DTE receiving the frame that congestion was experienced in the path from source to destination. The backward explicit congestion notification (BECN) bit is set by the Frame Relay network in frames traveling in the opposite direction of frames encountering a congested path. The notion behind both of these bits is that the FECN or BECN indication can be acted on by a higher-level protocol that can take flow-control measures as appropriate. (FECN bits are useful to higher-layer protocols that use receiver-controlled flow control, whereas BECN bits are significant to those that depend on emitter-controlled flow control.)

The discard eligibility (DE) bit is set by the DTE to tell the Frame Relay network that a frame has lower importance than other frames and should be discarded before other frames if the network becomes short of resources. Thus, it represents a very simple priority mechanism. This bit is usually set only when the network is congested.

Note: Frame Relay troubleshooting is covered in Chapter 8.