In telecommunication, frequency-change signaling is a signaling method in which one or more discrete frequencies correspond to each desired significant condition of a code. In Telecommunication, signalling (UK spelling or signaling (US spelling has the following meanings The use of signals for controlling communications Frequency is a measure of the number of occurrences of a repeating event per unit Time. In Communications a code is a rule for converting a piece of Information (for example a letter, Word, Phrase, or It may be used in both supervisory signaling and data transmission. Line signaling is a class of Telecommunications signaling protocols. Data transmission is the transfer of Data from point-to-point often represented as an Electro-magnetic Signal over a physical point-to-point or point-to-multipoint In the latter role, it is often called frequency modulation (FM).
The transition from one set of frequencies to the other may be a continuous or a discontinuous change in the frequency or phase. Frequency is a measure of the number of occurrences of a repeating event per unit Time. The phase of an oscillation or wave is the fraction of a complete cycle corresponding to an offset in the displacement from a specified reference point at time t = 0
Source: Federal Standard 1037C and MIL-STD-188
See also: Frequency-shift keying