Modulate a computer with digital signals into analogue signals are converted for transmission over the telephone. This is modulation, and this is the task of the modem, an acronym for modulation, demodulation. The first form of modulation used in ADSL is applied, the amplitude modulation (AM), we also know from the radio. The values of the square wave of the digital signal by the modem multiplied by the values of the analog carrier. At each place where the digital signal the value zero, the values of the analog carrier to zero. This results in a wave movement that occasionally alternates with sections of the value zero. These flat parts correspond to the flat parts of the square wave of the digital signal and the value’0 ‘, while the parts with the original wave to the value’1′. When the analog signal must be digitized, you know the modem on the other side of the line which parts the value should be zero and the value one.