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1.4 ANALOG TRANSMISSION 19Figure 1.4-2. Circuit representations: (a) bidirectional two-wire or four-wire circuit and (b) twounidirectional two-wire circuits forming a bidirectional four-wire circuit.two-wire or four-wire. When discussing the two unidirectional circuits of afour-wire circuit, representation (b) is used.Data Transmission on Analog Circuits. In the 1960s, subscribers also beganto use the telephone network for transmission of digital data, and the network hasbecome a telecommunication network. Digital data are converted by modems intoa form that fits within the 300–3400 Hz band of analog circuits. There are severalmodem types. Frequency-shift keying (FSK) modems convert the zeros and onesof the digital bit stream into two voiceband frequencies, for example, 1300 and1700 Hz. With FSK, data can be sent at speeds of 600 or 1200 bits/second. Thesignal produced by a differential phase-shift keying (DPSK) modem is a single frequencywith phase shifts. In the widely used V.26 modem [6], the frequency is1800 Hz, and the phase shifts occur at a rate of 1200 shifts/second. The phaseshifts can have four magnitudes, each of which represents the values of two consecutivebits in the digital signal. The modem thus transfers 2400 bits/second. Morerecently developed modems have transfer rates of up to 56 kb/second.1.4.2 Analog Subscriber Lines [4,5]Figure 1.4-3 shows a connection between two subscribers served by an analog localexchange. The subscriber lines (SLs) and the path (P) across the exchange aretwo-wire circuits.The power of an electrical signal decreases as it propagates along the circuit. Thisattenuation becomes more severe with increasing circuit length.The characteristics of microphones and receivers are such that a listener receivesa sufficiently strong acoustical signal when at least 1% of the electrical signalpower produced by the talker’s microphone reaches the listener’s receiver. Thiscorresponds to the attenuation in a circuit of about 15 miles. Most subscriberFigure 1.4-3. Circuit for intraoffice call. (a), Two-wire analog circuits.