Basic principles

Basic principles



Figure 1: Schematic of a telephone installation.
A traditional landline telephone system, also known as "plain old telephone service" (POTS), commonly handles both control and audio signals on the same twisted pair (C) of insulated wires: the telephone line. The signaling equipment (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-lines are the wires, so having both received and sent voices on one pair of wires reduces the expense of wire-line service. Also, a twisted pair line rejects electromagnetic interference (EMI) and crosstalk better than a single wire or an untwisted pair. The strong microphone voice signal 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

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