 illustrates a typical traffic profile of bursty traffic over a frame relay network. Such a traffic profile is also typical of a LAN. Of course the primary employment of frame relay is to interconnect LANs at a distance. With conventional leased data circuits we have to pay for the bit rate capacity whether it is used or not. On the other hand, with frame relay we only pay for the "time" used. Billing can be handled in one of three ways: Time (minutes) Figure 12.9a A typical traffic profile of a public switched telephone network (PSTN). ENTERPRISE NETWORKS II: WIDE AREA NETWORKS Discard excess Maximum Transmit if possible committed information Transmission guaranteed Figure 12.9b Typical bursty traffic of a frame relay circuit. Note the traffic levels indicated. CIR (committed information rate) is the data rate subscribed to by the user. This rate may be exceed for short bursts during peak period(s) as shown in Figure 12.9b. 2. We can just pay a flat rate. 3. We can pay per packet (i.e., frame). Now turn to Figure 12.9b. Note that on the right-hand side of the figure there is the guaranteed transmission bit rate equivalent to the CIR. Depending on the traffic load and congestion, during short periods the user may exceed the CIR. However, there is a point where the network cannot sustain further increases in traffic without severe conges- tion resulting. Traffic above such levels is arbitrarily discarded by the network without informing the originator. 12.5.6 Congestion Control: A Discussion Congestion in the user plane occurs when traffic arriving at a resource exceeds the net- work's capacity. It can also occur for other reasons such as equipment failure. Network congestion affects the throughput, delay, and frame loss experienced by the end-user. End-users should reduce their offered load in the face of network congestion. Reduction offered load by an end-user may well result in an increase in the effective throughput available to the end-user during congestion. Congestion avoidance procedures, including option