plant, is illustrated in Figure 8.1. 8.2.1 The Evolving Local Network Over 65% of the investment in a PSTN is in the local network. More cost-effective and efficient means are now being implemented to connect a subscriber to the local serving exchange. In Section 4.3.8, concentrators and remote switching were introduced. The digital subscriber line (DSL) concept was covered in Section 6.10. With ISDN/BRI we bring digital service directly to the subscriber premise. If the two B-channels are combined (see Section 12.4.6.2), 128-kbps service can be provided. Users of the Internet desire downstream ¹ bit rates in excess of this value, up to 1.544 Mbps Downstream in the direction local serving switch to the subscriber. Fundamentals of Telecommunications, Second Edition, by Roger L. Freeman ISBN 0-471-71045-8 Copyright © 2005 by Roger L. Freeman LOCAL AND LONG-DISTANCE NETWORKS Local network Local network Long-distance network Figure 8.1 The general structure of a PSTN. or better. The shorter we make a conventional subscriber loop, the greater the bit rate it can support. 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 fiber-wire-pair system. It may be a node for wireless local loop employing point-to-multipoint radio. It may provide an add-drop 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 o