 was divested. LOCAL AND LONG-DISTANCE NETWORKS The United States has a two-level hierarchy: the local exchange carrier (or LATA [local access and transport area]) and the interexchange carrier network. Our concern here is the interexchange carrier network, which is synonymous with the long-distance network. So the question remains: how many hierarchical levels in the long-distance or toll network? There will be "trandem" exchanges in the network, which we will call transit exchanges. These switches may or may not be assigned a higher hierarchical level. Let us assume that we will have at least a two-level hierarchy. Factors that may lead to more than two levels are: Geographical size Telephone density, usually per 100 inhabitants5 Long-distance traffic trends Political factors (such as Bell System divestiture in the United States, privatization in other countries) The trend toward greater use of direct HU (high-usage) routes tends to keep the number of hierarchical levels low (e.g., at two levels). The employment of dynamic routing can have a similar effect. We now deal with fan-out. A higher-level exchange, in the hierarchical sense, fans out to the next lower level. This level, in turn, fans out to still lower levels in the hierarchy. It can be shown that fan-outs of six and eight are economic and efficient. Look at this example. The highest level, one exchange, fans out to six exchanges in the next level. This level, in turn fans out to eight exchanges. Thus there is connectivity to 48 exchanges (8 X 6), and if the six exchanges in the higher level also serve as third-level exchanges, then we have the capability of 48 + 6, or 54 toll exchanges. Suppose that instead of one exchange in the highest level, there were four intercon- nected in mesh for survivability and improved service. This would multiply the number of long-distance exchanges served to 48 X 4 = 192, and if we use the 56 value it would be 56 x 4 = 224 total exchanges. In large countries we deal with numbers like this. If we assign a long-distance 