Multiplexing is the method by which multiple signals can be combined into one signal to save power or bandwidth.
There are two primary methods of multiplexing used in communcations systems.
Frequency Division Multiplexing is where each frequency is assigned to a particular sender.
Time Division Multiplexing is where each time slot is assigned to a particular message sender.




To understand TDM a little better, consider a 24 voice channel T1 line. There are 25 total time slots in each transmission cycle. Each voice channel gets one time slot while the final position is given to a synchronizing bit. This allows simultaneous transmission of multiple channels that can be relatively easily decoded at the receiving stations.
Frequency Division Multiplexing allows multiple signals at different carrier frequencies to be broadcast simultaneously. So long as each carrier signal is significantly different than each other carrier signal, no overlap will occur. When designing such a system, the Fourier-domain representation of each signal must be carefully considered to ensure sufficient space exists for each signal. Standard broadcast FM is the most commonly encountered form of FDM.
Next we’ll look the associated distortion that sometimes happens whenever we’re working with our communications signals.