In a Frame-Relay network, we can ask to routers during congestion in the network cloud to reduce the CIR to a lower value. So when a Service Provider becomes congested and needs to discard frames, it first discards frames with their Discard Eligible (DE) bit set to 1. The Service Provider also can request that the sending router slows its transmission rate by marking the Backward Explicit Congestion Notification (BECN) bit to 1, in a frame going back to the sender.
When this occurs , if the router is configured to respond to BECNs, the router reduces its CIR by 25 percent. If the router receives another BECN in the next time interval, it decreases its transmission rate by 25 percetnof the current rate and this can happen until the rate drops to the router’s configured minimum CIR.
Please also take in mind that sometimes , you can encounter the situation where the sender doesn’t receive anything or insufficient traffic back. In such a situation , the Service Provider can’t mark the BECN bit in a frame going back to the sender. To remedy this situation, the Service Provider can mark then the Forward Explicit Congestion Notification (FECN) bit to 1 in a frame which is destined for the receiver. Then if the receiver is configured to respond to FECN, it generates a Q.922 test frame which will be sent back to the sender and will then offer the opportunity to the Service Provider to mark the frame with the BECN bit and so will ask then to the sender to slow the traffic.
As a general rule , if a sender has slowed its transmission rate because of BECNs, 16 timing intervals must elapse before the sender begins to increase its transmission rate again.