LOCAL AND LONG-DISTANCE NETWORKS
The general structure of a PSTN.
or better. The shorter we make a conventional subscriber loop, the greater the bit rate it
We will incorporate a new, generic device in the subscriber plant. It is called a remote
subscriber unit (RSU). It distributes service to the subscriber plant customer. The basic
service it provides is POTS using a typical subscriber line interface card (SLIC). The
RSU has a large group of optional capabilities listed below:
It may or may not have a local switching capability.
In most cases the RSU will carry out a concentrator function.
It may be a fiber-to-wire interface point for a hybrid fiberwire-pair system.
It may be a node for wireless local loop employing point-to-multipoint radio.
It may provide an adddrop multiplex (ADM) capability on a SONET or SDH
self-healing ring (SHR).
Several of these RSU concepts are illustrated in Figure 8.2. Figure 8.2a shows the
conventional subscriber distribution plant and identifies its various functional parts.
Figure 8.2b illustrates a digital subscriber line feeding (DSL) an RSU. The DSL might
consist of one 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.
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.