ion. The switch connected cable from the call's originator to cable at the call's receiver. At first it was operated manually, by a now-quaint figure known as a "switchboard operator" who plugged wires into sockets. Figure 1-2 Star topology. SWITCH Signalling in the Circuit-Switched Network Old though it may be, this technology is still in use and still referred to as circuit switching. Obviously, circuit switching facilitates the addition of new users to the system, because it reduces the "cost" to a single cable between the switch and the new telephone set. Management of the system is streamlined. The switch still has to handle a number of circuits, but the user equipment-the telephone set-remains simple. The next step in circuit-switching is to connect together several switches building a circuit-switched network (see Figure 1-3). In such a network, each user's telephone set is connected to the switch nearest to it. That switch is connected to another, and SO on across the system. In telephony, switches are also referred to as exchanges. Therefore, the closest switch to a telephone set is called its local exchange. Calls between two telephones are routed first to the local exchange of the caller and then traverse other exchanges until reaching the local exchange of the callee, which can announce the call to him by ringing his telephone. Circuit- switching overcame the limitations of the first telephone system and remains the dominant technology for voice transmissions even now. But circuit-switching technology evolved repeatedly from that very first net- work, and at present, several hierarchical levels of switches are imple- mented all together to make up the PSTN, a worldwide communication network. Figure 1-3 Hierarchical topology. Chapter 1 Characteristics of Circuit-Switching Besides voice transport, circuit-switching is regularly used to transport dif- ferent types of traffic. Circuit-switched networks transport data between computers and control signals between terminals, for instance. However, no matter