Explain in a sentence or two the main difference between FDM, TDM and WDM.

Answer

FDM:  is a form of signal multiplexing where multiple base band analog signals are modulated on different frequency carrier waves and added together to create a composite signal. It has a mere concern with the analog signal. FDM is also used to combine multiple signals before final modulation onto a carrier wave. In this case the carrier signals are referred to a sub carrier: an example is, where frequency division multiplexing is used as to allow multiple users to share a physical communication channel, is called frequency division multiple access (FDMA).

TDM: Is a type of digital or rarely analog multiplexing in which two or more signals or bit streams are transferred apparently simultaneously as sub-channels in one communication channel, but physically are taking turns on the channel. The time domain is divided into several recurrent timeslots of fixed length, one for each sub channel. One TDM frame consists of one time slot per sub channel.