Two level FSK signalling is well understood and used freely in the communications world. In this case one bit is sent per frequency level ( or symbol ). For example two frequencies can be chosen to be transmitted or received where frequency f1 represents a “0” bit and a frequency f2 represents a “1” bit. In this case each symbol ( frequency) contains one bit of information. However there is no reason to use only two level signalling. Four frequencies may be used. In this case each frequency ( symbol) can represent two bits. For example “00” can be represented by a frequency f1, “01” by frequency f2, “10” by frequency f3 and “11” by frequency f4.. Each symbol in this system represents two bits increasing the information sent. In this manner multilevel FSK can be used to advantage.