Gaussian minimum shift keying or GMSK is a kind of continuous phase frequency-shift keying. The baseband modulation is generated by starting with a bitstream 0/1 and a bit-clock giving a timeslice for each bit. This is the type of modulation used in Global System for Mobile Communications (GSM) – although that system includes differential encoding of the bitstream, which makes the overall modulation behave like a kind of continuous phase Binary Phase Shift Keying with constant envelope.