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THE EVOLUTION OF CIRCUIT SWITCHING The current circuit switched network concept has remained essentially unchanged from the original electromechanical Strowger exchange (see the Preface for an explanation of how this exchange came by its name). At its most basic level the telephone network comprises transmission paths and switching nodes. The design of a circuit switch is based on the ability to physically create a path (circuit) from one network element to another and to hold this path open for the duration of the interaction (call) | Next Generation Network Services Neill Wilkinson Copyright 2002 John Wiley Sons Ltd ISBNs 0-471-48667-1 Hardback 0-470-84603-8 Electronic 1 Circuit Switched Technologies 1.1 THE EVOLUTION OF CIRCUIT SWITCHING The current circuit switched network concept has remained essentially unchanged from the original electromechanical Strowger exchange see the Preface for an explanation of how this exchange came by its name . At its most basic level the telephone network comprises transmission paths and switching nodes. The design of a circuit switch is based on the ability to physically create a path circuit from one network element to another and to hold this path open for the duration of the interaction call . The second role of circuit switching is routing i.e. determining the path to take from the ingress point to the egress point in the network. This can be performed in multiple stages each switching stage being linked by transmission paths. In the Strowger exchange routing was performed on a step-by-step basis using the pulsed make and break signalling from the telephone dial to step electromechanical selectors. Figure 1.1 depicts a simple scenario of an own-exchange call i.e. a call that only involves one routing and switching stage . This is similar to what would have occurred in the second exchange opened in the UK. Called the official switch it was used as a private branch exchange by post office officials at the post office HQ. It can be clearly seen from this simple example how routing is taking place in a step-wise fashion and a physical path is being created at the same time. Clearly a more complex routing mechanism is required for national and international calls this is performed by multiple stages of switching and routing connected via transmission paths. In this way the hierarchical nature of the routing 4 CIRCUIT SWITCHED TECHNOLOGIES Figure 1.1 Strowger routing scheme -10 000-line four-digit numbering and thus numbering plan local transit and international