This is another modulation technique (see Frequency Modulation) used to transmit data. It shifts the frequency above and below the carrier frequency (representing 1 and 0, respectively).
The form of frequency modulation that uses two separate audio frequencies to transmit binary ones and zeros.
The technique used by the CCITT V.12 and Bell 103 communication protocols where `0' and `1' bits are assigned to a specific frequencies in transmit and receive directions. Signalling rate equals bit rate.