ng and multiple-star configurations is that lower-rank exchanges home on higher-rank exchanges. If a call is destined for an exchange of lower rank in its chain, the call proceeds down the chain. In a similar manner, if a call is destined for another exchange outside the chain (the opposite side of Figure 1.9d), it proceeds up the chain and across. When high-usage routes exist, a call may be routed on a route additional or supplementary to the pure hierarchy, proceeding to the distant transit center10 and then descending to the destination. Of course, at the highest level in a pure hierarchy the call crosses from one chain over to the other. In hierarchical networks, only the order of each switch in the hierarchy and those additional links (high-usage routes) that provide access need be known. In such networks, administration is simplified, and storage or routing information is reduced when compared to the full-mesh type of network, for example. 10 A transit center or transit exchange is a term used in the long-distance network for a tandem exchange. The term tandem exchange is reserved for the local network. 1.3 INTRODUCTORY TOPICS IN TELECOMMUNICATIONS Subscriber Subscriber Figure 1.10 The backbone of a hierarchical network. The backbone traces the final route. 1.3.7.2 The Trend Away from the Hierarchical Structure. There has been a decided trend away from hierarchical routing and network structure. However, there will always be some form of hierarchical structure into the foreseeable future. The change is brought about due to two factors: transmission and switching. Since 1965, transmission techniques have taken leaps forward. Satellite communications allowed direct routes some one-third the way around the world. This was followed by the introduction of fiber-optics transmission providing nearly infinite bandwidth, low loss, and excellent performance properties. These transmission techniques are discussed in Chapter 9. In the switching domain, the stored program control (SPC)¹ 11 switch had the computer brains