in Section 2.3 and sig- naling between exchanges in Section 2.6. 2.2 Operation of a Conventional Telephone The ordinary home telephone receives the electrical power that it needs for operation from the local exchange via two copper wires. This subscriber line, which carries speech signals as well, is a twisted pair called a local loop. The principle of the power supply coming from the exchange site makes basic tele- phone service independent of the local electric power network. Local exchanges have a large-capacity battery that keeps the exchange and subscriber sets operational for a few hours if the supply of electricity is cut off. This is essential because the operation of the telephone network is especially impor- tant in emergency situations when the electric power supply may be down. Figure 2.2 shows a simplified illustration of the telephone connection. Elements of the figure and operation of the subscriber loop are explained later in this chapter. Minor operational differences, particularly in the provi- sion of private branch exchangelautomatic branch exchange (PBX/PABX) sys- items, exist around the world, but the principles discussed in this chapter apply to the overwhelming majority of PSTN systems. 2.2.1 Microphone When we raise the hook of a telephone, the on/off hook switch is closed and current starts flowing on the subscriber loop through the microphone that is connected to the subscriber loop. The microphone converts acoustic energy The Telecommunications Network: An Overview Local telephone exchange Battery voltage - 48V On/off- Variable Variable Control air pressure air pressure, relay is heard as sound waves On/off hook Variable a sound switch generator Person current talking Switching Variable block matrix current Connections Variable to other magnetic field makes exchanges diaphragm vibrate Figure 2.2 Operation principle of a conventional telephone. to electrical energy. Originally telephone microphones were so-called carbon microphones that had diaphragms with small containers of carbon gra