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A wireless repeater.

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 without degradation.

The term "repeater" originated with telegraphy and referred to an electromechanical device used to regenerate telegraph signals. Use of the term has continued in telephony and data communications.

In telecommunication, the term repeater has the following standardized meanings:

1. An analog device that amplifies an input signal regardless of its nature (analog or digital).
2. A digital device that amplifies, reshapes, retimes, or performs a combination of any of these functions on a digital input signal for retransmission.

(Source: from Federal Standard 1037C in support of MIL-STD-188)

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 digipeater is a blend meaning "digital repeater", particularly used in amateur radio. Store and forward digipeaters generally receive a packet radio transmission and then retransmit it on the same frequency, unlike repeaters that receive on one and transmit on another frequency.

Repeaters are often used in trans-continental and trans-oceanic cables, because the attenuation (signal loss) over such distances would be completely unacceptable without them. Repeaters are used in both copper-wire cables carrying electrical signals, and in fibre optics carrying light - see Optical communications repeater.

Repeaters are used in radio communication services - see Radio Repeater. Radio repeaters often transmit and receive on different frequencies. A special subgroup of those repeaters is those used in amateur radio - see amateur radio repeater.

Repeaters are also used extensively in broadcasting, where they are known as translators or boosters. See broadcast translator for more.

When providing a point-to-point telecom link using radio beyond line of sight, one uses repeaters in a microwave radio relay. A reflector, often on a mountaintop, that relays such signals around an obstacle, is called a passive repeater or Passive Radio Link Deflection.

In optical communications the term repeater is used to describe a piece of equipment that receives an optical signal, converts that signal into an electrical one, regenerates it, and then retransmits an optical signal. Since such a device converts the optical signal into an electrical one, and then back to an optical signal, they are often known as Optical-Electrical-Optical (OEO) repeaters.

Before the invention of electronic amplifiers, mechanically coupled carbon microphones were used as amplifiers in telephone repeaters. The invention of the audion tube made transcontinental telephony practical. In the 1930s vacuum tube repeaters using hybrid coils became commonplace, allowing the use of thinner wires. In the 1950s negative impedance gain devices were more popular, and a transistorized version called the E6 repeater was the final major type used in the Bell System before the low cost of digital transmission made all voiceband repeaters obsolete.