Frequency-shift keying is the simplest (binary) form of frequency modulation
(FM), which is used for digital modulation in which the
two binary values are represented by two different frequencies near
the carrier frequency. Normally the carrier is shifted low for
a 1 (called a mark in old telegraph terms) and
high for a 0 (space).
Other topics in our resources on GSM related to Frequency Shift Keying include: