Modems use frequency shift keying to send digital information over a telephone line. The sending modem modulates the digital data into a signal that can be deciphered by the phone line, while the receiving modem demodulates the signal and turns it into digital data.
With frequency shift keying, a separate tone is used for separate bits. Once a terminal's modem dials up the computer's modem, the terminal's modem then becomes the originate modem and sends a 1,070-hertz tone for a 0 and a 1,270-hertz tone for a 1. The computer's modem is known as the answer modem and sends a 2,025-hertz tone for a 0 and a 2,225-hertz tone for a 1. Since the two modems send out different tones, they are able to use the lines at the same time, which is known as full-duplex operation.
To make modems faster, a technique that is more sophisticated than frequency shift keying had to be utilized. Phase-shift keying was the first new technique that was used along with quadrature amplitude modulation. Both of the new techniques make it possible for an immense amount of data to fit into the 3,000 hertz of bandwidth that is available on a regular voice-grade phone line.