Frequency Shift Keying, or FSK, is a modulation technique that transmits digital signals over telephone lines, cables, optical fiber or wireless media. FSK data consist of two binary states, 0 and 1, represented by two different waves known as "space" to represent the 0s and "mark" to represent the 1s.
Frequency-shift keying is commonly used to transmit caller ID information, amateur radio transmissions, remote metering and emergency broadcasts. The technology evolved in the mid-1900s for use with mechanical teleprinters that processed data at 45 bits per second. In the 1970s, the technology evolved with the development of modems that could process FSK data at higher speeds.