Edited sentence: A variant of MSK called GMSK is used in the GSM mobile phone standard.Consequently, the waveforms used to represent a 0 and a 1 bit differ by exactly half a carrier period.FSK is commonly used in Caller ID and remote metering applications: see FSK standards for use in Caller ID and remote metering for more details.In MSK the difference between the higher and lower frequency is identical to half the bit rate.Minimum frequency-shift keying or minimum-shift keying (MSK) is a particularly spectrally efficient form of coherent FSK.This is the smallest FSK modulation index that can be chosen such that the waveforms for 0 and 1 are orthogonal