TABLE OF CONTENTS The costs of a telecommunications network are minimised by exploiting the significant economies of scale offered by the above-described transmission systems. In general, the cost per circuit carried reduces as the capacity of the transmission system increases; thus, it pays to multiplex as many channels as possible on to each transmission link. In this section we consider the standard ways in which channels carrying telephone calls or data communications are assembled into multiplexed composite signals the payloads for conveyance over digital transmission systems, i.e. copper cable, coaxial cable, optical fibre, microwave radio or satellite, as appropriate. (It should be noted that the multiplexed payloads first used in telecommunications networks were analogue and based upon FDM a technique described in Chapter 3. However, these systems are now obsolete and so they are omitted from this book for brevity. A description of FDM transmission payload systems may be found in [6].)
The primary multiplexed payload created by assembling a group of PCM channels forms the basic building block of all digital transmission and switching networks used throughout the World. Thus, it is an important entity to understand. There have been several versions of the PCM primary multiplex payload, but there are now just two versions standardised by the ITU (previously known as the 'CCITT'): the 30-channel standard, used in Europe, Asia, and elsewhere regionally and on all international links; and the 24-channel DS1 standard,...
