nter, a "Level 1" or "Level 2 center." Center T, of course, is a long- distance transit exchange with a fan-out of four; these four local exchanges (A, B, C, and D) connect to T. 6 The entire national geographic area is made up of such small segments as shown in Figure 8.5, and each may be represented by a single exchange T, which has some higher level or rank. Figure 8.5 (a) Areas and (b) exchange relationships. 60f course, in the United States, T would be the POP (point of presence). LOCAL AND LONG-DISTANCE NETWORKS The next step is to examine traffic flows to and from (originating and terminating) each This information is organized and tabulated on a traffic matrix. A simplified example is illustrated in Table 8.1. Care must be taken in the preparation and subsequent use of such a table. The convention used here is that values (in erlangs or ccs) are read from the exchange in the left-hand column to the exchange in the top row. For example, traffic from exchange 1 to exchange 5 is 23 erlangs, and traffic from exchange 5 to exchange 1 is 25 erlangs. It is often useful to set up a companion matrix of distances between exchange pairs. The matrix (Table 8.1) immediately offers candidates for HU routes. Nonetheless, this step is carried out after a basic hierarchical structure is established. We recommend that a hierarchical structure be established at the outset, being fully aware that the structure may be modified or even done away with entirely in the future as dynamic routing disciplines are incorporated (see Section 8.4). At the top of a country's hierarchy is (are) the international switching center(s). The next level down, as a minimum, would be the long-distance network, then down to a local network consisting of local serving exchanges and tandem exchanges. The long-distance network itself, as a minimum, might be divided into a two-layer hierarchy. Suppose, for example, that a country had four major population centers and could be divided into four areas around each center. Each of the four major populatio