s economic to provide a direct route between them. If beneficial, since traffic can still reach its destination, whereas it there is little traffic between two exchanges, it is more economic to could not if there were only the direct route. However, the large combine this with traffic to other destinations to produce a large amount of additional traffic offered to the backbone route can cause amount of traffic over a common route to a tandem switching centre. severe congestion resulting in the loss of calls to destinations served The correct solution obviously depends on the cost of the circuits as only by that route (which could be to most other exchanges in the well as the amount of traffic. If circuits are cheap it is less expensive network). A solution to this difficulty is trunk reservation (Ingham, for them to be lightly loaded than to incur the cost of switching 1989; Katzschner, 1973). A proportion of the circuits on the final equipment in a tandem exchange. Thus, many direct routes are route is reserved for calls which can only take that route. Thus, these provided between local exchanges in a small area with a high calls still obtain a reasonable grade of service when the overflow subscriber density but not to more distant exchanges. traffic offered to the route is abnormally large. In some networks, automatic alternative routing is used (Clos, Sophisticated automatic alternative routing can cause congestion 1954). Direct routes are under-provided with circuits; when all originating at one part of the network to spread as traffic overflows circuits on a direct route are busy, traffic overflows to a fully-pro- to other routes. This is obviously undesirable, SO network manage- vided tandem route through a switching centre at a higher level in ment (Macurdy, 1973; IEEE, 1988) is being introduced. A central the hierarchy. An under-provided direct route is called a high-usage management centre can monitor the traffic on different routes and, route and the fully-provided indirect route to which its traffic fi