Modem

A Modem is a device which enables a computer to transmit data over analog telephone lines. Modem is short for modulator-demodulator. A modem converts the digital information in a computer into analog data. Traditional modems can operate at a top speed of 56Kbps, though most do not go that fast on today's phone lines.
A lot of companies still rely on dial-up access using modems for connectivity, though companies are increasingly moving to other methods of connectivity including cellular modem cards and high speed Internet access (such as DSL or cable modem).
A modem operates in the Physical Layer (layer 1) of the OSI model.