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A repeater is a simple physical layer add-on device for extending an electrical, wireless, or optical transmission system over a greater distance. It amplifies signals and transmits the boosted signal on the next cable or wireless segment. Repeaters are necessary because signal strength diminishes over distance, a condition known as attenuation. Repeaters are also known as line conditioners.

Repeaters on analog transmission lines amplify the incoming signal as is. If the signal is distorted in some way, the distortion is boosted with the rest of the signal. A digital repeater will convert the incoming analog to digital and then forward it as a clean digital signal, although this technique is prone to error if the incoming signal is distorted. Reducing cable distance between repeaters can reduce signal problems. An analog signal can be transmitted about 18,000 meters before a repeater is necessary. A digital signal can be transmitted about 6,000 meters before a repeater is necessary.

Repeaters are used to extend a network segment to reach distant nodes. Expanding the network (as opposed to physically extending it) is something that should be done with repeaters or routers. Think of repeaters as connections to distant workstations rather than as a way to add more workstations.

Optical amplifiers boost signals through the magic of light physics. Photons are injected into the existing signal. This has the effect of boosting the signal. See "Fiber-Optic Cable" and "Optical Networks" for more information.

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