The Basic Principles

A traditional fixed telephone system, also known as "Plain Old Telephone Service" (POTS), is generally both audio and control signals on the same twisted pair (C) son isolated: the telephone line. Signaling equipment or the ring (see Figure 1) consists of a bell, buzzer or other lamp (A7) to alert the user to incoming calls, and number buttons or a rotary dial (A4) to enter a phone number on outgoing calls. spending most of wireline telephone service is the son, so telephones transmit both incoming and outgoing voice channels on a single pair of son. A twisted pair line rejects electromagnetic interference (EMI ) and crosstalk better than a single wire or twisted pair. loud signal coming from the microphone does not overpower the weaker signal arrival speaker with a local effect due to a hybrid coil (A3) subtracts the signal from the microphone to the signal sent to the local speaker. junction box (B) lightning arrest (B2) and adjusts the resistance of the line (B1) to maximize the signal strength for the length of line. Telephones have similar adjustments for line lengths in (A8).'s son voltages are negative compared to earth, to reduce galvanic corrosion. negative voltage attracts positive metal ions toward the son.