6.1 Introduction

Having considered in Chapters 1 and 2 how a call is conveyed across one or more networks, this chapter is all about how the exchanges or more precisely, the switching units within the exchanges in a PSTN actually work. Section 6.2 considers the basic components of circuit-switching systems, often referred to as 'Voice switches'. For clarity, this chapter describes the arrangement for the fixed-network exchanges (also called 'wireline exchanges'); consideration is given to the additional elements that are included in a mobile network exchange in Chapter 9.

In Chapter 5 we looked at how a core transmission network carries a required capacity between two network nodes. But, how does a network operator determine what capacity is required to be carried? For example, are 90 circuits between Leicester and Coventry sufficient to ensure that the expected volume of calls between these exchanges can be carried satisfactorily, or could the operator save money by providing just 60 circuits? Section 6.3 considers how the size of a traffic route between two exchanges is determined, and how the flow of traffic through a network of many exchanges is routed so as to give maximum utilisation of the capacity while achieving the appropriate level of resilience (at the switching level). Finally, we consider a simple example of how a network is dimensioned at the switching level, and how this impacts the dimensioning of the Core Transmission Network.

6.2 Circuit-Switching Systems