Also, although the term predictable lies in a gray area between best effort and guaranteed, it is the type of service most likely to be served by most performance mechanisms, as we see in Chapter 8. For example, suppose a device requires capacity (bandwidth) between 4 and 10 Mb There must be a way to communicate this request across the network, a way to measure and derive the level of resources needed to support this request, a way to determine whether the required resources are available, and a method to control the information flow and network resources to keep this service between 4 and 10 Mb Capacity (or bandwidth) is a finite resource within a network. For example, the performance of a 100 Mb FE connection between two routers is bounded by that technology. If we were to look at the traffic flows across that 100 Mb connection, we would see that, for a common besteffort service, capacity would be distributed across all of the traffic flows. As more flows were added to that connection, the resources would be spread out until, at some point, congestion occurs. Congestion would disrupt the traffic flows across that connection, affecting the protocols and applications for each flow. What is key here is that, in terms of resource allocation, all traffic flows have some access to resources. This is shown in Figure 1.29. In this figure available capacity (dashed curve) decreases as the number of traffic flows increases. Correspondingly, the loading on the network (solid curve) from all of the traffic flows increases. However, at some point congestion affects the amount of user traffic being carried by the connection, and throughput of the connection (heavy curve) drops. As congestion interferes with the endtoend transport of traffic, some protocols (e.g., TCP) will retransmit Service Characteristics 41 Capacity of Fast Ethernet Connection (Mb) 50 Number of Concurrent Traffic Flows 100 Resource Loading Capacity Available Throughput Congestion Begins to Load Network FIGURE 1.29 The Performance of a Fast Ethernet Connection under BestEffort Conditions traffic. The difference between the loading and the throughput curves is due to retransmissions. This is undesirable, for while the connection is being loaded, only a percentage of that loading are successfully delivered to destinations. At some point all of the traffic on that connection could be due to retransmissions and throughput would approach zero. This approach is used in besteffort networks. In contrast, consider a traditional telephony network. Calls are made on this network, and resources are allocated to each call. As more calls are added to the network, at the point where all of the resources have been allocated, additional calls are refused. The exiting calls on the network may suffer no performance degradation, but no new calls are allowed until resources are available. Call admission control (CAC) is a mechanism to limit the number of calls on a network, thereby controlling the allocation of resources.