Basic Principles
A traditional landline telephone system, also known as "plain old telephone service" (POTS), commonly carries both control and audio signals on the same twisted pair (C) of insulated wires: the telephone line. The signaling equipment, or ringer, (see figure 1) consists of a bell, beeper, light or other device (A7) to alert the user to incoming calls, and number buttons or a rotary dial (A4) to enter a telephone number for outgoing calls. Most of the expense of wire-line telephone service is the wires, so telephones transmit both the incoming and outgoing voice channels on a single pair of wires. A twisted pair line rejects electromagnetic interference (EMI) and crosstalk better than a single wire or an untwisted pair. The strong outgoing voice signal from the microphone does not overpower the weaker incoming speaker signal with a sidetone because a hybrid coil (A3) subtracts the microphone's signal from the signal sent to the local speaker. The junction box (B) arrests lightning (B2) and adjusts the line's resistance (B1) to maximize the signal power for the line's length. Telephones have similar adjustments for inside line lengths (A8). The wire's voltages are negative compared to earth, to reduce galvanic corrosion. Negative voltage attracts positive metal ions toward the wires.
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