Modem

A modem (a portmanteau word constructed from modulator and demodulator) is a device that modulates an analog carrier signal (sound), to encode digital information, and that also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data.
The most familiar example of a modem turns the digital '1s and 0s' of a personal computer into sounds that can be transmitted over the telephone lines of Plain Old Telephone System (POTS), and once received on the other side, converts those sounds back into 1s and 0s.
Far more exotic modems are used by internet users every day. In telecommunications, "radio modems" transmit repeating frames of data at very high data rates over microwave radio links. Some microwave modems transmit more than a hundred million bits per second.
Optical modems transmit data over optic fibers. Most intercontinental data links now use optic modems transmitting over undersea optical fibers. Optic modems usually use interferometric filters called etalons to separate different colors of light, and then individually turn the pulses of each color of light into electronic digital data streams. Optical modems routinely have data rates in excess of a billion (1x10^9) bits per second. Their bandwidths are currently limited by the thermal expansion of the etalons. Heat changes an etalon's size and thus its frequency.
Modems can be used over any means of transmitting analog signals, from driven diodes to radio.