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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.
The term "repeater" originated with telegraphy in the 19th century, 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:
- An analog device that amplifies an input signal regardless of its nature (analog or digital).
- A digital device that amplifies, reshapes, retimes, or performs a combination of any of these functions on a digital input signal for retransmission. A repeater that includes the retiming function is also known as a regenerator
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.
Repeaters are used to boost signals in coaxial and twisted pair cable and in optical fiber lines. An electrical signal in a cable gets weaker the further it travels, due to energy dissipated in conductor resistance and dielectric losses. Similarly a light signal traveling through an optical fiber suffers attenuation due to scattering and absorption. In long cable runs, repeaters are used to periodically regenerate and strengthen the signal. A network hub is one such device.
An optical communications repeater is a piece of equipment that receives an optical signal, converts that signal into an electrical one, regenerates it, and then retransmits it as an optical signal. In contrast, optical amplifiers, which amplify the light beam directly, are often used in transcontinental and submarine communications cables, because the signal loss over such distances would be unacceptable without them.
Radio repeaters are used in radio communication services such as Commercial or Amateur Radio. A radio repeater consists of a radio receiver connected to a transmitter. The radio signal is received, amplified and retransmitted, usually on a different frequency. Higher radio frequencies are limited to line-of-sight transmission, their range is blocked by mountains and the curvature of the Earth, so repeaters are located on hills and mountains, to retransmit the signal beyond the obstruction. Radio repeaters are also used extensively in broadcasting, where they are known as broadcast relay stations. These extend the broadcast coverage area to remote communities, outside the range of the main broadcast station.
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.
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.
Extraneous noises and other type of natural interference can cause the repeater to undesirably retransmit such a signal. To reduce this issue a tone (code) filter such as CTCSS can be added to the repeater's receiver. To access this type of arraignment the (input) user's signal would require the tone to be transmitted, along with other intelligence such as a (audio) voice transmission at the same time.
- Makes it easy to expand a network over a large distance.
- Connection between various types of media [e.g. fibre optic, UTF, coaxial cable] is possible.
- Traffic cannot be filtered to ease congestion.
- A repeater cannot work across multiple network architectures.
Before the invention of electronic amplifiers, mechanically coupled carbon microphones were used as amplifiers in telephone repeaters. After the turn of the century it was found that negative resistance mercury lamps could amplify, and they were used. The invention of audion tube repeaters around 1916 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. Frequency frogging repeaters were commonplace in frequency-division multiplexing systems from the middle to late 20th century.
See also 
- Loring, A. E. (1878). A Hand-book of the Electro-Magnetic Telegraph. New York: D. Van Nostrand. pp. 53–54.
- This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).
- Sungook, Hong (2001). Wireless: From Marconi's Black-Box to the Audion. MIT Press. p. 165. ISBN 0262082985.