 of nodes (i.e., switching centers) interconnected by circuit groups. There may be several direct circuit groups between a pair of nodes and these may be one-way or both-way (two-way). A simplified illustration of this idea is shown in Figure 8.9. Remember that a direct route consists of one or more circuit groups connecting adjacent nodes. We define an indirect route as a series of circuit groups connecting two nodes providing end-to-end connection via other nodes. An ISC is a node in a national network, which in all probability will have some sort of hierarchial structure as previously discussed. An ISC is also a node on the international network that has no hierarchical structure. It consists entirely of HU direct routes. 8.4.2 Logic of Routing 8.4.2.1 Routing Structure. Conceptually, hierarchical routing need not be directly related to a concept of a hierarchy of switching centers, as just described. A routing structure is hierarchical if, for all traffic streams, all calls offered to a given route, at a specific node, overflow to the same set of routes irrespective of the routes already tested. 10 The routes in the set will always be tested in the same sequence, although some routes Nodes Circuit groups Figure 8.9 A simplified network with circuit groups connecting pairs of nodes with one-way and both-way working. 10 Tested means that at least one free circuit is available to make a connectivity. This "testing" is part and parcel of CCITT Signaling System No. 7, which is discussed in Chapter 14. LOCAL AND LONG-DISTANCE NETWORKS Calling Called Figure 8.10 Hierarchical routing in a nonhierarchical network of exchanges. Note: All nodes are of equal status. may not be available for certain types of calls. The last choice route is final (i.e., the final route), in the sense that no traffic streams using this route may overflow further. A routing structure is nonhierarchical if it violates the previously mentioned definition (e.g., mutual overflow between circuit groups originating at the same exchange). An exampl