ne or several DS1 or E-1 bit streams. In this case the DSL would be on two two-wire pairs, one for downstream and one for upstream. Figure 8.2c is the same as Figure 8.2b, but in this case the DSL is carried on two fibers plus a spare in a fiber-optic cable configuration. Figure 8.2d illustrates the use of a fiber-optic or wire-pair bus to feed several RSUs. Figure 8.2e shows a simple self-healing ring architecture employing either SONET or SDH (see Section 9.4). In sparsely populated rural areas, point-to-multipoint full-duplex radio systems are particularly applicable. Such systems would operate in the 2- or 4-GHz band with 10-, 20-, or 30-mile links. Access to the system would probably be time division multiple access. However, in densely populated urban areas, the 23-, 25-, or 38-GHz frequency bands should be considered, keeping link length well under 3.5 miles (5.5 km). Because of expected high interference levels from nearby users, code division multiple access should be considered for this application. The TDMA and CDMA access schemes are described in Chapter 18. 8.2.2 What Affects Local Network Design? There are a number of important factors that will influence the design of a local network. Among these factors are: Subscriber density and density distribution. Breakdown between residential and business subscribers. 8.2 MAKEUP OF THE PSTN Code: Main cable Branch feeder (primary distribution) Distribution cable (secondary distribution) Aerial wire or rural distribution wire Branch splice or cabinet Terminal or distribution point Figure 8.2a Conventional subscriber distribution plant. Local serving switch Figure 8.2b A digital subscriber line (DSL) carrying a DS1 or E-1 configuration feeds an RSU. The RSU provides the necessary functions for subscriber loop interface. Local serving Fiber optic cable exchange Figure 8.2c A fiber-optic pair feeds an RSU. This configuration is particularly useful for nests of subscribers beyond 12,000 ft (3700 m) from the local exchange. LOCAL AND LONG-DISTANCE NETWORKS Local s