The CBR Service ATM SERVICE CATEGORIES

94 CONGESTION CONTROL IN ATM NETWORKS as the decision to accept or reject a new connection is based purely on whether its peak bit rate is less than the available bandwidth on the link. Let us consider, for example, a non-blocking switch with output buffering see Figure 3.10, and suppose that a new connection with a peak bit rate of 1 Mbps has to be established through output link 1. Then, the new connection is accepted if the link’s available capacity is more or equal to 1 Mbps. In the case where nonstatistical allocation is used for all of the connections routed through a link, the sum of the peak bit rates of all of the existing connections is less than the link’s capacity. Peak bit rate allocation can lead to a grossly underutilized link, unless the connections transmit continuously at peak bit rate. Statistical Bandwidth Allocation In statistical bandwidth allocation, the allocated bandwidth on the output link is less than the source peak bit rate. In the case where statistical allocation is used for all of the connections on the link, the sum of the peak bit rates of all of the connections can exceed the link’s capacity. Statistical allocation makes economic sense when dealing with bursty sources, but it is difficult to implement effectively. This is due to the fact that it is not always possible to characterize accurately the traffic generated by a source and how it is modified deep in an ATM network. For instance, let us assume that a source has a maximum burst size of 100 cells. As the cells that belong to the same burst travel through the network, they get buffered in each switch. Due to multiplexing with cells from other connections and scheduling priorities, the maximum burst of 100 cells might become much larger deep in the network. Other traffic descriptors, such as the PCR and the SCR, can be similarly modified deep in the network. For instance, let us consider a source with a peak bit rate of 128 Kbps. Due to multiplexing and scheduling priorities, it is possible that several cells from this source can get batched together in the buffer of an output port of a switch. Let us assume that this output port has a speed of, say 1.544 Mbps. Then, these cells will be transmitted out back-to-back at 1.544 Mbps, which will cause the peak bit rate of the source to increase temporarily Another difficulty in designing a CAC algorithm for statistical allocation is due to the fact that an SVC has to be set up in real-time. Therefore, the CAC algorithm cannot be CPU intensive. This problem might not be as important when setting up PVCs. The problem of whether to accept or reject a new connection can be formulated as a queueing problem. For instance, let us consider again our non-blocking switch with output buffering. The CAC algorithm has to be applied to each output port. If we isolate an output port and its buffer from the switch, we will obtain the queueing model shown in Figure 4.9. Output port New Connection Existing Connections Figure 4.9 An ATM multiplexer.