VAISHUMAHAVIJAY

There is 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, while 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 is an extended address (EA) bit. If this bit is one, 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 bit marked “C/R” 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 that 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 from frames encountering a congested path. The notion behind both of these bits is that the FECN or BECN indication can be promoted to a higher-level protocol that can take flow control action as appropriate. (FECN bits are useful to higher-layer protocols that use receiver-controlled flow control, while 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.