MODEM This is an acronym for modulator-demodulator. A modem is a device or program that enables a data transmission over telephone lines. While computer information is stored digitally, data sent over telephone lines is transmitted are in the form of analog waves. A modem converts from one form to the other. To faciliate compatibility between modems and computers, a standard interface, called RS-232, has been established so that any computer can be be connected with any modem adhering to the standard. Modems in the form an expansion board that are placed in a vacant expansion slot are called onboard or internal modems. There are a number of protocols for formatting the data transmitted through the modem,

While the modem interfaces are standardized, a number of different protocols exist for formatting data to be transmitted over telephone lines, like the CCITT1 V.34 and the X25 for high speeds. Usually a modem has built-in support for the most common protocols.

The characteristics of a modem, in addition to the formatting protocols, include:
The modem transmission speed for sending and receiving data. For slow rates, the unit of measurement is called a baud. The slowest rate (and at one time the only one) is 300 baud. When higher speeds are attained, speed is measured in bits per second (bps). Even higher speeds are available when the data is compressed before transmission. In some areas telephone lines cannot reliably transmit at high speeds.
Today most modems can switch between voice and data transmission. In voice mode, the modem acts like a telephone with onboard speaker and microphone. In data mode, the modem transmits only data.
Ths feature allows the computer to receive calls automatically.
As mentioned before greater speeds of data transmission are obtained by compressing the data before it is sent. Obiously, the receiving modem must have the capability of decompressing the data, using the same rules.
Fax capability
Most modern modems can also be used for sending and receiving fax messages.
1 = Comité Consultatif International Téléphonique et Télégraphique or International Telegraph and Telephone Consultative Comittee.