it is marked by a pulse that launches signal along the very path that later will transport voice. In-band Figure 1-7 Callee's The callee is busy local exchange and there is signalling in the backward direction. Caller's local exchange Chapter 1 Figure 1-8 FDM system. FDM link FDM table: Voice channel A = Frequency 1 Voice channel B = Frequency 2 Voice channel C = Frequency 3 signalling is still used at present in some countries, mainly for international connections. Although FDM reduced the cost of telephone systems by putting many connections on the same cable, it proved to be an expensive technology. FDM uses analog filters to keep different channels separate. The filters are costly to maintain. They have to be checked periodically-as often as once a month-to keep them working in the required frequency. Analog Transmission From the first, voice transmission was always analog. Another way to say this is that transmission consisted of sending continuous signals, such as voice, without regard to their contents (see Figure 1-9). The signal to be transmit- ted is represented by an electromagnetic wave that moves through the cable, bearing the information. A recipient needs to recover the original signal from the electromagnetic wave received in order to possess the message. Continuous signals have different frequency components, but every sig- nal concentrates most of its energy within a certain range of frequencies- the so-called spectrum of the signal. Frequency components outside the spectrum contain little energy, and thus contribute less to the interpreta- tion of a certain signal. As a result, it is usually necessary to transmit only Signalling in the Circuit-Switched Network Figure 1-9 Analogue transmission of voice. the frequency components inside the spectrum for recovery of the original signal, or, to be precise, a signal very similar to the original. In analog transmission, the electromagnetic wave obtained from a signal contains all frequency components of the spectrum. The standard spectrum for voice tr