erving exchange Figure 8.2d A fiber-optic or wire-pair cable bus feeds several RSUs. Local serving exchange Figure 8.2e A fiber-optic cable self-healing ring feeds RSUs. Further breakdown among types of business subscribers as a function of white-collar and blue-collar workers. For example, a large insurance company will have much greater calling activity than a steel mill with the same number of workers. Cultural factors. It follows that in regions of high subscriber density typical of urban areas, there would be many exchanges, each with comparatively short subscriber loops. Such a network would trend toward mesh connectivities. The opposite would be in rural regions where we would expect few exchanges, and a trend toward a greater use of tandem working. Long 8.3 DESIGN OF LONG-DISTANCE NETWORKS route design would be the rule rather than the exception. Suburban bedroom communities present still a different problem, where we would expect calling trends toward urban, industrial centers. Many calls in this case will be served by just one exchange. Judicious use of tandem working would be advisable. Cultural factors can give the network designer insight into expected calling habits. The affluence of a region may well be a factor. However, consider countries where affluence goes hand-in-hand with volunteerism, an organizational spirit, and a socialization factor. Certain cultures encourage strong family ties, where one can expect high intrafamily calling activity. 8.3 DESIGN OF LONG-DISTANCE NETWORKS 8.3.1 Introduction A long-distance network connects an aggregate of local networks. Any subscriber in the nation should be able to reach any other subscriber. Likewise, any subscriber in the nation should be able to connect with any other subscriber in the world. So we have need not only of local and long-distance (toll) exchanges, but international exchanges as well. 8.3.2 Three Design Steps The design of a long-distance network involves basically three considerations: 1. Routing scheme given inlet and outlet points an