Principles Digital Communication System & Computer Networks (Charles River Media Computer Engineering)

28.4 CONGESTION CONTROL

If there is a sudden increase in traffic, it is possible that a switch may experience congestion; its buffer may be full or it may not have enough memory to process any more frames. In such a case, the two bits FECN and BECN are used to tell the other switches about the congestion.

Figure 28.4: Congestion control through FECN and BECN.

When a switch experiences congestion, it changes the FECN bit from 0 to 1 and sends the frame to the switch toward the destination. It also changes the BECN bit from 0 to 1 and sends the frame toward the source to notify the source that congestion is being experienced. Note that it is the responsibility of the higher layer to reduce the number of frames if the congestion notification is obtained.

If there is congestion in a Frame Relay switch, the two fields FECN and BECN are used to notify the other switches so that those switches will reduce their speed.

28.4.1 Committed Information Rate

When a virtual connection is established, the average data capacity of the VC can be specified in terms of the bit rate that is assured by the service provider. At the time of buying the Frame Relay service, the subscriber generally specifies the committed information rate (CIR) to the service provider. The VC must maintain the minimum CIR. If the subscriber is pumping data at a rate faster than CIR, the switch can discard some frames. This is done based on the DE (discard eligibility) bit. The DE bit is set to 1 by the switch if the frame is above the CIR. If DE =1, the frame can be discarded by any of the other switches on route to the destination if that switch experiences congestion.

Committed information rate (CIR) is the minimum data rate that has to be supported by a virtual connection. At the time of buying the Frame Relay service, this parameter is specified by the subscriber.