Two wires, or conductors are needed to complete the circuit between the telephone transmitter and receiver. Some exchange cables carry thousands of pairs of wires.
If every call needed a separate pair of conductors for transmission throughout the telephone network, the simultaneous transmission of thousands of calls from one exchange to another would be unmanageable. A pair of ordinary copper wires can be made to handle only a limited number of calls at once because they are designed for low-frequency current. Higher frequencies would allow more simultaneous calls, but unless a different design of cable is used, the signal radiates away and loses strength.
Most trunk lines between telephone exchanges are now coaxial cables, in which the signal is confined to prevent loss of strength and interference. Instead of a pair of wires, each coaxial cable has a central copper wire with an outer copper conductor that sounds it like a sleeve. They can handle high frequencies and carry thousands of calls. Built in amplifiers boost the signals about every 1¼ miles (2 km).
Using a technique known as frequency multiplexing, the electric signals corresponding to the voice sound waves are modulated – that is, combined with an electromagnetic carrier wave in the same way as radio waves. A number of carrier waves of different frequencies are then sent along the same pair of conductors.
At the receiving exchange, the signals are separated from the carrier wave by a process called demodulation. The other then filtered to the correct receiver.
Picture Credit : Google