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A repeater is a device that takes the signal off the cable at one interface, retimes it and sends it out on the other. Inside a repeater are two or more tranceivers and some logic that connects the tranceivers together.
A repeaters function is to forward each incoming signal, without modification or unnecessary delay, to all ouput ports. This means it also send out collisions, fragments, jam signals, and bursts of noise.
Repeaters can have two or more ports. A good example of a multiport repeater is a HUB in ethernet networks.
Repeater ports do not have to be identical. You could repeat a copper signal to a fiber signal, but that is only possible when it is the same topology. So you could repeat 10Base-T signals to 10Base-F, but it is impossible to build an ethernet to token ring repeater, such a device is called a bridge.
Since repeaters only consist of tranceivers and they do not have any knowledge about the signals they are repeating, this device is placed at OSI layer 1.
Note on Repeaters:
A special kind of repeaters are called Fan-Out unit or Delni (Digital Equipment Corporation). These devices are repaters but without the PHY (tranceiver) so you only have AUI connectors on the in and output ports.