Since the telephone is a duplex (bidirectional) communication system, the wire pair carries two audio signals, one going in each direction. In an analog telephone line consisting of a pair of wires, it consists of an amplifier circuit made of transistors which use power from a DC current source to increase the power of the alternating current audio signal on the line. They are most frequently used in trunklines that carry long distance calls. This is used to increase the range of telephone signals in a telephone line.

In computer networking, because repeaters work with the actual physical signal, and do not attempt to interpret the data being transmitted, they operate on the physical layer, the first layer of the OSI model a multiport Ethernet repeater is usually called a hub. Use of the term has continued in telephony and data communications. The term 'repeater' originated with telegraphy in the 19th century, and referred to an electromechanical device (a relay) used to regenerate telegraph signals. Since it amplifies the signal, it requires a source of electric power. A repeater is an electronic device in a communication channel that increases the power of a signal and retransmits it, allowing it to travel further. Similarly, the farther from a radio station a receiver is, the weaker the radio signal, and the poorer the reception. So with a long enough wire the call will not be audible at the other end. The longer the wire is, the more power is lost, and the smaller the amplitude of the signal at the far end.

For example, when a telephone call passes through a wire telephone line, some of the power in the electric current which represents the audio signal is dissipated as heat in the resistance of the copper wire. When an information-bearing signal passes through a communication channel, it is progressively degraded due to loss of power.

Video repeater