A terminal or computer produces digital signals,which are simply the presence or absence of an electric pulse.The state of being on or off represents the binary number 1 & O,respectively.Some communications lines accept digital transmission directly and the trend in the communications industry is toward digital signals.However most telephone lines through which this digital signals are sent were originally built for voice transmission,& voice transmission requires analog signals.

Digital Transmission sends data as distinct pulses either on or off, in much the same way that data travels through the computer. However, most communications media are not digital. Communication devices such as telephone lines, coaxial cables and microwave circuits are already in place for voice transmission. The easiest choice for most users is to piggyback on one of these. Thus, the most common communication device is the ANALOG TRANSMISSION, a continuous electric signals in forms of wave to be sent over analog lines, a digital signal must first be converted to an analog form. It is converted by altering an analog signal called "carrier wave" which has alterable characteristics. One such characteristic is the amplitude or height of the wave which can be increase to represent the binary number 1. Another characteristic can be alterd is the frequency or number of times a wave repeats during a specific time interval, frequecy can be increase to represent a one.

Conversion from digital to analog signals is called modulation and the reverse process---reconstructing the original message at the other end of the transmission is called demodulation. An extra device is needed to make the conversations --- a modem.