signal's bandwidth before it is transmitted. Generally the more information contained within a signal, the more the bandwidth. For example, an audio signal (such as music) may have a bandwidth of up to 20 kHz and a TV video signal a bandwidth of 7 MHz. Broadband Telecommunications Technologies and Management Figure 3.11 A sound clip in the time (top) and frequency (bottom) domains Circuit Switching The term circuit switching was coined to describe telecommunications systems which can only start after a link has been established. The switching of intermediate links between a caller and a receiver through a switchboard or an automatic exchange establishes such an end-to-end connection which generally remains until the call ends. Inter-exchange Telecommunications Because of the good conductivity properties of twisted copper wire in the 300-3400 Hz fre- quency range, the required power for transmission of voice electrical signal is quite low. These links have been traditionally used to connect a subscriber's premises to the local exchange. The necessary power to transmit and receive voice signal is generally supplied through the local exchange and no local power source is necessary at the customer's premises, which is why the telephones work even when electricity is out. Commonly many subscribers served by a local exchange need to connect to subscribers of other local exchanges, and therefore the links connecting the exchanges need to carry multiple calls simultaneously. The twisted pair copper wires were generally set up to carry one tele- phone call. Therefore, multiple pairs of wires or media capable of carrying multiple calls are needed to connect the exchanges together. The latter solution uses a technology known as multiplexing, where two or more connections are carried over the same physical medium. To understand signal multiplexing we first define modulation and demodulation technol- logies. These technologies are fundamental to telecommunications and were among the early developments of this field and are in