 incorporate new techniques such as ATM, and such techniques will 3 P4. increase propagation delays. Over the next ten years a rapidly increasing number of customers Richards, D.L. (1973) Telecommunications by Speech, Butterworth will be served by digital exchange lines where the network bound- Heinemann, London. Principles of switching systems Eur Ing S F Smith BSc (Eng) CEng FIEE Northern Telecom Europe D G Bryan DFH CEng FIEE Telecommunication Consultant Contents Introduction 31/3 Structure of telephone systems 31/4 Teletraffic 31/8 Analogue switching 31/9 31.5 Digital switching 31/12 31.6 Stored program control 31/15 31.7 Operational security 31/16 31.8 Bibliography 31/17 This page intentionally left blank Introduction 31/3 31.1 Introduction The simplest form of telephone system consists of two terminations INTERNATIONAL and a metallic connection between them. However, this arrange- Other ment is clearly not viable for more than a very small number of National TRANSIT terminals and the concept of switching is introduced, whereby each Networks terminal is connected to a central location (exchange) where means is provided to interconnect any two of the total population of terminations. Within an exchange, more than one connection will be required at any one time but not all terminations will be wanting service at the same time. Practical systems are designed to provide a number of links which is less than the number of terminations determined on a TANDEM + LOCAL statistical basis. Figure 31.1 shows such an arrangement, based on a matrix of crosspoints where each crosspoint is some form of on/off switch such as a manually operated key, an electromechanical relay or a transistor. LOCAL (with side routes) Crosspoint EXCHANGE Links Figure 31.2 Exchange hierarchy other exchanges and some exchanges exist solely for this purpose and have no terminations of their own. This leads to a hierarchical arrangement of exchanges as shown in Figure 31.2. Terminations The various types of call routing are shown in Figure 31.3 an