TO CONVEY DATA, SOME ASPECT OF A SIGNAL SHOULD BE VARIED. THERE ARE several different characteristics of a signal which can be made to change in a controlled way, such that data is imprinted on it. Modulation is process of imprinting data onto electric current or radio wave.
Modulation is accomplished by varying amplitude, frequency, or phase of a wave. Another method is to transmit the series of pulses, whose duration, amplitude, or spacing is made to change according to the data to be conveyed.