Back in the CIR section of this chapter, you looked at how the discard eligibility (DE) bit is used to “mark” frames that are transmitted above the CIR on a Frame relay virtual circuit. Making certain frames discard-eligible provides a way to determine which frames should be discarded first if a Frame Relay switch is experiencing congestion. However, Frame Relay also uses two additional techniques to help make end devices aware of congestion on the Frame Relay network – Forward Explicit Congestion Notification (FECN) and Backwards Explicit Congestion Notification (BECN).
- FECN. If a Frame Relay switch is experiencing congestion, it will set what is known as the FECN bit in the Frame Relay header. This bit is used to notify the receiving device that the network is experiencing congestion and that it should expect some delay in the receipt of frames. Ultimately, this information should be passed to higher-layer protocols for processing.
- BECN. By the same token, a switch experiencing congestion will also set the BECN bit in frames that are being sent back to the original sender. BECN alerts the sender to a congestion situation, effectively letting the sender known that it should throttle back the rate at which it sends data. In the same way, this information should be passed to higher-layer protocols for processing.