e of hierarchical routing in a nonhierarchical network of exchanges is shown in Figure 8.10. 8.4.2.2 Routing Scheme. A routing scheme defines how a set of routes is made available for calls between pairs of nodes. The scheme may be fixed or dynamic. For a fixed scheme the set of routes in the routing pattern is always the same. In the case of a dynamic scheme, the set of routes in the pattern varies. 8.4.2.2.1 Fixed Routing Scheme. Here routing patterns in the network are fixed, in that changes to the route choice for a given type of call attempt require manual intervention. If there is a change it represents a "permanent change" to the routing scheme. Such changes may be the introduction of new routes. 8.4.2.2.2 Dynamic Routing Scheme. Routing schemes may also incorporate frequent automatic variations. Such changes may be time-dependent, state-dependent, and/or event- dependent. The updating of routing patterns make take place periodically, aperiodically, predetermined, depending on the state of the network, or depending on whether calls succeed or fail in the setup of a route. In time-dependent routing, routing patterns are altered at fixed times during the day or week to allow for changing traffic demands. It is important to note that these changes are preplanned and are implemented consistently over a long time period. In state-dependent routing, routing patterns are varied automatically according to the state of the network. These are called adaptive routing schemes. To support such a routing scheme, information is collected about the status of the network. For example, each toll exchange may compile records of successful calls or outgoing trunk occupancies. This 8.4 TRAFFIC ROUTING IN A NATIONAL NETWORK Local Centralized Distributed to database all exchanges Network information Routing decision Local Centralized exchange processor Figure 8.11 Adaptive or state-dependent routing, network information (status) versus routing decisions. information may then be distributed through the network to other exchanges 