TABLE OF CONTENTS The concept of a telephone call being routed through a succession of exchanges was first introduced in Chapter 1, extended to the interconnection of several different operators' networks in Chapter 2 and considered from the switching systems' viewpoint in Chapter 6. We are now in a position to examine how the control of such call routeings across the networks (PSTN and others) is achieved. To this end, this chapter first considers the mechanism of signalling between nodes across the networks, and then the principal features of call switching control are introduced.
Signalling may be defined generically as the extension of control information from one network node or user to another over one or more links. It is employed in two distinct domains: between the user and the serving node (subscriber concentrator switch) - known as 'user' or 'subscriber' signalling, and between network nodes (e.g. exchanges) - known as 'inter-exchange' or 'inter-nodal' signalling. The latter may extend across network boundaries, perhaps concatenating several operators' networks in the case of an international call. Fig. 7.1 shows the domain of subscriber and inter-exchange signalling.
Network operators create a clear demarcation within the exchanges between subscriber and inter-nodal signalling in order to ensure network integrity. It is important that the control that an individual user can have on the network seizing of links, routeing of calls, etc. - is carefully constrained to avoid a rogue subscriber deliberately or...
