I know that FDM actually uses a division of total frequency of the carrier channel to separate into several smaller frequency channels. I want to know that whether these small channels are actually having their own waves over the communication medium or the carrier wave is modulated in such a way that it works for all the smaller channels? If they are having their own waves over the communication channel then please explain how can two waves travel over the same medium. I always assume signals to be like AC current...how can two AC current travel over the same wire!!!