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Sec. 8–2 Telephone Systems 571
Telephone
handset
Central
office
Telephone
handset
Headphone
Headphone
i(t)
Carbon
microphone
Telephone
line
Battery
Telephone
line
Carbon
microphone
i(t)
Figure 8–1
Voice
No voice
t
Historical telephone system.
The simple two-wire telephone system shown in Fig. 8–1 has three important advantages:
(1) it is inexpensive; (2) the telephone sets are powered from the CO via the telephone
line, so no power supply is required at the user’s location; and (3) the circuit is full duplex. †
The two-wire system has one main disadvantage: Amplifiers cannot be used, since they
amplify the signal in only one direction. Consequently, for distant telephone connections, a
more advanced technique—called a four-wire circuit—is required. In a four-wire circuit, one
pair (or one optical fiber) is used for signals sent in the transmit direction, and another is used
for arriving signals in the receive direction.
Modern Telephone Systems and Remote Terminals
A simplified diagram for the classic analog local-loop system that is used today is shown in
Fig. 8–2. The local switching office connects the two parties by making a hardwire connection
between the two appropriate local loops. This is essentially a series connection, with the
earphone and carbon microphone of each telephone handset wired in a series with a battery
(located at the telephone plant). Figure 8–2 illustrates this analog local-loop system, which is
said to supply plain old telephone service (POTS). The wire with positive voltage from the
CO is called the tip lead, and it is color coded green. The wire with negative voltage is called
the ring lead and is color coded red. The terms tip and ring are derived from the era when a
plug switchboard was used at the CO. These leads were connected to a plug with tip and ring
contacts. This jack is similar to a stereo headphone jack that has tip, ring, and sleeve contacts.
The Earth ground lead was connected to the sleeve contact.
† With full-duplex circuits, both parties can talk and listen simultaneously to the other person.