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What is a modem? What does it do?

The word "modem" is short for "modulator-demodulator" which is essentially what it does. A computer moves data around on parallel wires by applying one or another voltage to each wire (representing 0's and 1's) and the voltage is read at the other end of the wires. This is digital communication. In order for multiple computers to communicate over greater distances, a method was developed to exchange data over a world-wide pre-existing network: the telephone network. Telephone transmissions are analog in nature. Instead of streaming ones and zeroes with varying voltage over multiple parallel wires, signals are sent as analog wave frequencies (sounds) over a single wire.

The device that translates voltages into frequencies (digital into analog) and back again is called a modem. (Actually, a modulator converts digital to analog and a demodulator converts analog to digital.) In order for one computer to exchange data with another via a telephone line, the sending computer must activate its modem and dial the receiving computer. The receiving computer's modem must answer the call, "handshake" to establish communications, and signal its computer to be ready to receive. When this is done, the modems are in a state referred to as "connected". The two computers can exchange data, which the modems translate and pass through the telephone network.


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