8.5 SUMMARY

Virtual circuit networks like ATM seek to combine the gains from statistical multiplexing that packet switching creates with the guaranteed performance that circuit switching offers. When they succeed in this goal, virtual circuit networks will be able to reap the benefits of economies of scale, service integration, and network externalities. In order to succeed, however, network engineers must solve a number of control problems concerning admission, routing, flow and congestion, and resource allocation.

Problems of routing and admission are similar to those that occur in circuit-switched networks; flow- and congestion-control problems arise in packet-switched networks. And we reviewed the approaches developed in the context of circuit and packet switching.

Problems of resource allocation are peculiar to virtual circuit networks. (In circuit-switched networks each connection gets a fixed resource, whereas in datagram networks no resources are allocated to a connection.) The major difficulty in designing resource-allocation mechanisms is that they depend on (1) characteristics of the user traffic, (2) the available resources, and (3) the quality of service guaranteed to the user. Of these three items, the first is the most difficult to characterize, and we presented two approaches.

The deterministic approach is being pursued by the ATM Forum. It is easy to implement, but its worst-case assumptions will give very conservative answers, leading to significant underutilitzation of the network. The statistical approach at the present time is still being developed in research laboratories. It is far from standardization, but progress is rapid. We have described the most important...