d largely the same to this day and is made of twisted pair copper wires. As noted, these wires are inex- pensive and have excellent characteristics for transmission of voice signal for long distances. Voice Communications Local Regional exchange exchange International exchange Twisted pair copper wires Optical fibre links Local Regional exchange exchange International exchange Figure 3.6 A typical telephone call connected through a telecommunications network Telephone channel bandwidth (filter) for voice transmission Voice spectrum Frequency (Hz) 0 300 Figure 3.7 The voice signal frequency range This is because human voice frequency components are mostly limited to within a range of 300-3400 Hz (Figure 3.7), and the twisted copper wires can transmit this range of frequencies for tens of kilometres with little signal loss. Voice signal is filtered in the telephone SO the transmitted signal is confined to within 4 kHz to minimise the amount of added noise at the receiver. Usually each subscriber has a dedicated link to the local exchange, which is used to carry one voice call at a time. Frequency and Bandwidth To understand telecommunications better let us first describe signal frequency and bandwidth concepts. Modern telecommunications are carried out using electromagnetic waves. These waves are natural phenomena and are infinite in their range: from very low frequencies to very high frequencies. Although the range of frequency spectrum is infinite, only certain frequency ranges can be used for telecommunications as shown in Figure 3.8. The concepts of frequency and bandwidth are important in understanding how information signals are generated and communicated. Frequency is defined as the number of cycles per second: it is the number of times a waveform ebbs and flows each second. For example, Figure 3.9 shows a sine waveform with five cycles a second. The unit of frequency is hertz (Hz) in honour of Heinrich Hertz (1857-1894), who first experimented with the transmission of electromagnetic waves. Frequency is the