The topic of digital signal transmission is an enormous one, and cannot be covered completely in a book this size. The aim of this introduction is to set the material of later chapters into the general context of modern telecommunication systems, and to outline what will be covered in detail later and what will not. Some suggestions will also be made about how to use the book.

The Integrated Services Digital Network

In most countries, telecommunications services are evolving rapidly from a collection of separate, and largely incompatible, systems (telephone, telex, public and private data networks, and so on) towards a universal network, in which a wide variety of services are integrated using a common (digital) form of transmission.

Fig. 1.1 shows, in much simplified form, how two offices (or factories, or homes) might be interconnected through such an Integrated Services Digital Network (ISDN). At each office, digital signals from a number of different audio, video or data terminals are connected, via appropriate network terminating equipment (NTE), to an exchange termination (ET) within the ISDN. Whatever their origin, the signals are transmitted to their destinations over digital links which may include optical fibres, metallic cables, terrestrial or satellite microwave channels, and so on. To the network it is completely immaterial whether the signal carries a telephone conversation, computer data, or the reading on an electricity meter.

A universal network should eventually prove more flexible and cheaper than a number of separate networks, each providing only a narrow range of services.