 mission. In Problems of Randomness in Communication Engin- Performance in an Optical Communication System, IEEE eering (ed. Cattermole and O'Reilly) Pentech Press, 1984. Transactions on Communications, 32, pp. 163-169. Telephone networks JE Flood OBE DSc FInstP CEng FIEE Aston University Contents 29.1 Introduction 29/3 Network architecture 29/4 Network standards 29/8 Transmission 29/8 Integrated digital neworks 29/10 Network services 29/12 Numbering and charging 29/12 Routing 29/15 Dimensioning 29/16 29.10 References 29/19 This page intentionally left blank Introduction 29/3 29.1 Introduction A public switched telecommunication network (PSTN) contains transmission links between different locations known as nodes (Flood, 1975). A node may be either a subscriber's station, which originates and terminates calls, or a switching centre (i.e. an ex- change). Thus, any two subscribers may communicate by means of the appropriate combination of transmission links connected together at switching nodes for the duration of their call. Although other services are growing rapidly, telephony is still the predominant service provided by public telecommunication oper- ators (PTOs). Thus, most of the traffic carried on their networks is voice traffic. It is uneconomic to provide entirely separate networks for other services, so these make use of transmission links in the telephone network as bearers for their traffic. For example, a single telephone channel can convey 24 multiplexed telegraph channels, or a variety of data channels operating at different digit rates. Conversely, the bandwidth which could accommodate over a thou- sand telephone channels can, instead, be allocated to a single tele- vision channel. Consequently, the design of telecommunication networks is mainly dictated by the needs of telephony, as discussed in this chapter. The transmission links in a national network act as common bearers for a variety of different services, as shown in Figure 29.1. A subscriber has access to these services over his local-line l