Working Principle The information in the computer is a digital signal composed of "0" and "1", but only analog electrical signals can be transmitted on the telephone line. Therefore, when two computers want to transmit data through telephone lines, a device is needed to convert the numbers into analog. This digital-to-analog converter is a modem. When the computer sends data, the modem first converts the digital signal into the corresponding analog signal, which is called "modulation". Before the modulation signal is transmitted to another computer by telephone carrier, the modem of the receiver is also responsible for restoring the analog signal to a digital signal that can be recognized by the computer. This process is called "demodulation". It is through such a digital-to-analog conversion process of "modulation" and "demodulation" that the remote communication between two computers is realized.