trend away from this structure, or, at least, there will be a reduction of the number of levels. In Figure 1.9d there are five levels. The highest rank or order in the hierarchy is the class 1 center (shown as 1 in the figure), and the lowest rank is the class 5 office (shown as 5 in the figure). The class 5 office (switch), often called an end office, is the local serving switch, which was discussed above. Remember that the term office is a North American term meaning switching center, node, or switch. In a typical hierarchical network, high-usage (HU) routes may be established, regard- less of rank in the hierarchy, if the traffic intensity justifies. A high-usage route connectivity is the same as a direct route. We tend to use direct route when discussing the local area, and we use high-usage routes when discussing a long-distance or toll network. 1.3.7.1 Rules of Conventional Hierarchical Networks. One will note the back- bone structure of Figure 1.9d. If we remove the high-usage routes (dashed lines in the figure), the backbone structure remains. This backbone is illustrated in Figure 1.10. In the terminology of hierarchical networks, the backbone represents the final route from which no overflow is permitted. Let us digress and explain what we mean by overflow. It is defined as that part of the offered traffic that cannot be carried by a switch over a selected trunk group. It is that type of traffic that met congestion, which we called blockage above. We also can have overflow of a buffer (a digital memory), where overflow just spills, and is lost. INTRODUCTORY CONCEPTS Figure 1.9d A typical hierarchical network. This was the AT&T network around 1988. The CCITT recommended network was very similar. In the case of a hierarchical network, the overflow can be routed over a different route. It may overflow on to another HU route or to the final route on the backbone. See Figure 1.10. A hierarchical system of routing leads to simplified switch design. A common expres- sion used when discussing hierarchical routi