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repeater
In telecommunications, a repeater is an electronic device that receives a signal and retransmits it at a higher level or higher power, or onto the other side of an obstruction, so that the signal can cover longer distances. It is a generic term that refers to several different types of devices; a ''telephone repeater'' is an amplifier in a telephone line, an ''optical repeater'' is an optoelectronic circuit that amplifies the light beam in a optical fiber cable; and a ''radio repeater'' is a radio receiver and transmitter that retransmits a radio signal. A broadcast relay station performs an analogous role in broadcast radio and television.
==Overview==
When an information-bearing signal passes through a communication channel, it is progressively degraded due to loss of power. 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. The longer the wire is, the more power is lost, and the smaller the amplitude of the signal at the far end. So with a long enough wire the call will not be audible at the other end. Similarly, the farther from a radio station a receiver is, the weaker the radio signal, and the poorer the reception. 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. Since it amplifies the signal, it requires a source of electric power.
The term "repeater" originated with telegraphy in the 19th century, and referred to an electromechanical device (a relay) used to regenerate telegraph signals. Use of the term has continued in telephony and data communications.
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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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