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Sec. 8–2 Telephone Systems 573
The sequence of events that occur when placing a local phone call will now be
described with the aid of Fig. 8–2 and Table 8–1. The calling party—the upper telephone set
in Fig. 8–2—removes the telephone handset; this action closes the switchhook contact (offhook)
so that DC current flows through the caller’s telephone line. The current, about 40 mA,
is sensed at the CO and causes the CO to place a dial-tone signal (approximately 400 Hz) on
the calling party’s line. The calling party dials the number by using either pulse or touchtone
dialing. Today, most phone systems use touch-tone dialing, which consists of two tones that
denote a dialed digit (see Table 8–1). If pulse dialing is used, the DC line current is interrupted
for the number of times equal to the digit dialed (at a rate of 10 pulsess). For example, there
are five interruptions of the line current when the number 5 is dialed. Upon reception of the
complete number sequence for the called party, the CO places the ringing generator (90 V
RMS, 20 Hz, on 2 s, off 4 s) on the line corresponding to the number dialed. This rings the
phone. When the called party answers, DC current flows in that line to signal the CO to
disconnect the ringing generator and to connect the two parties together via the CO circuit
switch. Direct current is now flowing in the lines of both the called and the calling party,
and there is a connection between the two parties via transformer coupling, † as shown in
Fig. 8–2. When either person speaks, the sound vibrations cause the resistance of the carbon
microphone element to change in synchronization with the vibrations so that the DC line current
is modulated (varied). This produces the ac audio signal that is coupled to the headphone
of the other party’s telephone. Note that both parties may speak and hear each other simultaneously.
This is full-duplex operation.
In modern telephone sets, the carbon microphone element is replaced with an electret
or dynamic microphone element and an associated IC amplifier that is powered by the battery
voltage from the CO, but the principle of operation is the same as previously described.
The telephone system depicted in Fig. 8–2 is satisfactory as long as the resistance of
the twisted-pair loops is 1300 Ω or less. This limits the distance that telephones can be
placed from this type of CO to about 15,000 ft for 26-gauge wire, 24,000 ft for 24-gauge,
and 38,000 ft or about 7 miles, if 22-gauge twisted-pair wire is used. Historically, 19-gauge
wire was used in rural areas so that phones may be located up to 20 miles away from the CO
without remote terminals.
Supplying a dedicated wire pair from each subscriber all the way to the CO is expensive.
In applications where a large number of subscribers are clustered some distance from the
CO, costs can be substantially reduced by using remote terminals (RT). (See Fig. 8–3.)
Remote terminals also allow telephones to be placed at any distance from the CO, as we will
describe subsequently.
The circuit for a typical RT is illustrated in Fig. 8–4. The POTS line card supplies
battery voltage and ringing current to the subscriber’s telephone. The two-wire circuit that
carries the duplex VF signals to and from the subscriber is converted into a four-wire circuit
that carries two one-way (simplex) transmit and receive signals by the use of a hybrid circuit.
The hybrid circuit is a balanced transformer circuit (or its equivalent electronic circuit)
that provides isolation for the transmit and receive signals. As shown in Fig. 8–4b, the
hybrid circuit acts as a balanced Wheatstone bridge where Z 1 Z 3 = Z 2 Z 4 . Thus, the voltage
† The transformers are called repeat coils in telephone parlance.