P

SL *C

Figure 1.4-3 Circuit for intraoffice call, (a): two-wire analog circuits.

Figure 1.4-3 Circuit for intraoffice call, (a): two-wire analog circuits.

1200 bits/second. The signal produced by a differential phase-shift keying (DPSK) modem is a single frequency with phase shifts. In the widely used V.26 modem [6], the frequency is 1800 Hz, and the phase shifts occur at a rate of 1200 shifts/second. The phase shifts can have four magnitudes, each of which represents the values of two consecutive bits in the digital signal. The modem thus transfers 2400 bits/second. More recently developed modems have transfer rates of 4800, 9600, 14,400, and 28,800 bits/second.

1.4.2 Analog Subscriber Lines [4,5]

Figure 1.4-3 shows a connection between two subscribers served by an analog local exchange. The subscriber lines (SL), and the path (P) across the exchange, are two-wire circuits.

The power of an electrical signal decreases as it propagates along the circuit. This attenuation becomes more severe with increasing circuit length.

The characteristics of the microphones and receivers in the Western Electric type 500 telephones (which are still regarded as the "standard" for telephones in the U.S.) are such that a listener receives a sufficiently strong acoustical signal when at least 1% of the electrical signal power produced by the talker's microphone reaches the listener's receiver. This corresponds to the attenuation in a circuit of about 15 miles. Most subscriber lines are less than 4 miles long, and there are no signal-strength problems in intraexchange calls.

1.4.3 Two-wire Analog Trunks

Two-wire trunks are similar to subscriber lines, and have similar attenuation characteristics. This limits the trunk length to about 10 miles.

1.4.4 Four-wire Analog Trunks

Long-distance trunks require amplification to compensate the signal attenuation. Amplifiers are unidirectional devices, and this is why long-distance trunks are four-wire trunks. A four-wire circuit consists of two amplified unidirectional two-wire circuits. In Fig. 1.4-4, hybrid circuits (H) at both ends of the trunk convert a two-wire circuit into a four-wire circuit, and vice versa. Amplifiers (A) are located at regular intervals along the two unidirectional circuits. The unidirectional circuits at an exchange that transfer signals to and

Exchange X

Exchange X

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