In telecommunication, the term repeater has the following standardized meanings:
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 in radio communication services. Radio repeaters often transmit and receive on different frequencies. A special subgroup of those repeaters is those used in amateur radio.
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. A microwave repeater in a communications satellite is called a transponder.
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. Frequency frogging repeaters were commonplace in frequency-division multiplexing systems from the middle to late 20th century.