Electronic circuitry in the modem makes the modulation and demodulation capability possible. At higher data rates more complex modulation techniques are used, phase modulation being common, The actual modulation techniques are usually of more interest to design engineers than users .The end-users of modems are primarily concerned with other characteristics of modems, such as speed. This is the rate at which the modem can send data over the telecommunications channel. Modems used in the dial-up telephone network have two wires, the same as a standard telephone set. One pair of wires is used for sending data and the other pair for receiving it.
Modems come in all sizes and shapes.
They can be mounted in a large box that sits on a desk or can be on a small printed circuit board mounted directly inside a terminal. Regardless of their size or shape, they perform the same basic functions. Modems have to be compatible with each other .At lower speeds, modems from different vendors are often compatible .At higher speeds, and compatibility is often more of a problem since different vendors often use different modulation techniques.