We need multiplexing because several persons when try to send their data at the same time there are chances that data gets lost due to collision and resending the data again.

So its better to do multiplexing over the data or the medium which carries the information by using different multiplexing techniques.

Time-division multiplexing (TDM) is a method of  transmitting and receiving the data based on time intervals at each end of the transmission line so that each signal appears on the line only a fraction of time in an alternating pattern.

TDM is aplplied primarily on digital signals but it can also be applied on analog signal as well.

We divide the medium into the time slots for sending and receiving data.

Each sender who wants to send data according to the time slot alloted within the time frame i.e. frame of an optimal size which can be transmitted in given time slot.

TDM orks onthe synchronized way means at  the  both ends, i.e. Multiplexer and De-multiplexer are timely synchronized and both switch to next channel simultaneously.