For a frame relay network to be able to detect and then signal congestion, it is necessary for each frame handler to monitor its queueing behavior. If queue lengths begin to grow to a dangerous level, then either forward or backward explicit notification or a combination should be set to try to reduce the flow of frames through that frame handler. The frame handler has some choice as to which logical connections should be alerted to congestion. If congestion is becoming quite serious, all logical connections through a frame handler might be notified. In the early stages of congestion, the frame handler might just notify users for those connections that are generating the most traffic.

In one of the frame relay specifications, an algorithm for monitoring queue lengths is suggested; this is shown in Figure 20.15. A cycle begins when the outgoing circuit goes from idle (queue empty) to busy (nonzero queue size, including the current frame). If a threshold value is exceeded, then the circuit is in a state of incipient congestion, and the congestion avoidance bits should be set on some or all logical connections that use that circuit. Describe the algorithm in words and explain its advantages.