Overview

The Integrated Services Digital Network (ISDN) represents the overhaul and redesign of our conventional telephone network from an analog system to an end-to-end digital network. This new, completely digital-based network is capable of transmitting voice and data communications over a single telephone line using inexpensive and conventional twisted-pair cable (i.e., standard copper telephone wire). A brief overview of various fundamental ISDN concepts was provided in Chapter 7 as part of our discussion of WAN technologies and services. In this chapter, we extend this discussion and provide more detailed information about ISDN.

1. What is ISDN?

   The Integrated Services Digital Network is a carrier service that is offered by telephone companies (telcos) and designed to transmit voice and nonvoice (e.g., computer data, fax, video) communications on the same network. The advantage ISDN offers over other services is that separate connections are not needed for these different transmissions. Thus, instead of having a telephone line for voice communications, a second telephone line for fax or computer dialup connections, and a coaxial cable link for video communications, a single ISDN connection will support all of these transmissions. That is, ISDN integrates all of these services into a single system. ISDN service (and hence, an ISDN connection) is completely digital from end-to-end. This represents both a departure from and an improvement in today's conventional telecommunication services, which use a hybrid of analog and digital technologies. (See Figure 11-1.) Furthermore, ISDN's technology permits standard twisted-pair wiring to carry circuit- or packet-switched digital data. ISDN...