Frequency Division Multiplexing : FDM is an analog technique that is applied when he bandwidth of link is greater than the combined bandwidth of the signals to be transmitted. all of the signals along the same high speed link simultaneously with each signal set at a different frequency. For FDM to work properly frequency overlap must be avoided. Therefore, the link must have sufficient bandwidth to be able to carry the wide range of frequencies required. The demultiplexor at the receiving end works by dividing the signals by tuning into the appropriate frequency.
Frequency Division Multiplexing operates in a comparable way to radio broadcasting where a number of different stations will broadcast at the same time but on dissimilar frequencies. Listeners can then "tune" their radio so that it captures the frequency or station they want.