ing switch and circuit capacity. A solution is to send delay tables usage and minimising packet transit delay, if decisions can be made only when events deem it necessary: when something in the net- by the switches on a packet-by-packet basis. work has changed. If some links in the network belong to one or more public PSNs, A new route, a new active end point device, increased delay time load sharing through dynamic routeing can ensure that circuits paid on a route, could all constitute an event in such a strategy. A stable, for according to traffic load are optimised for maximum cost effec- constant network, will therefore not be burdened with unnecessary tiveness. delay table transmissions. Datagram PSNs are particularly suitable for DAR as no informa- 54.6.3 Distributed adaptive routeing tion about virtual calls needs to be kept within the switches and the volume of network related information on the move is therefore A version of dynamic routeing assigns a cost value to each circuit manageable. In contrast, the data necessary to decide how and when based on available bandwidth, link loading, transit delay and to switch packets is found within the switches on the route of a throughput. The number of links or hops in the route is also assigned virtual call; if packets are to take a different route, the virtual call a value. The optimum path provides the overall lowest cost between information also has to be moved. Packet switching equipment 54/15 DAR for a virtual circuit network is best provided by running the packet-mode services, X.32 for PSTN and circuit-switched public virtual call protocol above the datagrams. The datagrams convey the data networks accessed by X.25 devices, X.75 gateways and virtual call information between end point devices, while only the X.3,X.28 and X.29 for asynchronous devices. end point devices need to know anything about the virtual call The range also provides transparent communications between protocol. BT's international data network operates in this way. otherwise incompatible