A Computer Modem (which stands for modulator-demodulator) is a device that encodes digital information over an analog signal (such as a phone line or radio), and also decodes the analog signal to recover the digital information. The most familiar example is a typical computer (voiceband) modem that turns the digital '1s and 0s' of a personal computer into sounds that can be transmitted over the telephone lines. You can have one PC Modem that sends and receives information from the Internet, or a pair of modems can communicate via the phone line to form a closed network. Modems are generally classified by the amount of data they can send in a given time, normally measured in bits per second, or "bps". They can also be classified by Baud, the number of distinct symbols transmitted per second. Typically, the fastest modem that can be used over regular phone lines is a 56k baud modem.
Fewer users now use regular dial-up connections, so the need for slower modems is far less. However, faster PC modems are used by Internet users every day, such as: cable modems and DSL modems (ADSL modems). In other forms of 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 optical fibers. Most intercontinental data links now use optical modems transmitting over undersea optical fibers. Optical modems routinely have data rates in excess of a billion (1x109) bits per second.