What is Multiplexing ?

      If every message had to move one after the other through every communication link in the global telecommunication network – like bicyclists on a narrow path – then communications would be hopelessly slow and very costly. But, if at some point, the bicyclist can be loaded onto trucks and sent down a multi-lane highway, traffic is enormously speeded. That, in effect, is what happens when many signals from telephones or computers reach a local exchange or central office on their way across a big city.

  Multiplexing allows multiple streams of electronic messages to be transmitted over the same connection in the time otherwise required for one message.

 Thus, multiplexing is the method of dividing a communication channel into many logical channels so that a number of independent signals may be simultaneously transmitted to it. This is a scheme that allows multiple logical signals to be transmitted simultaneously across a single physical channel. The electronic device that performs this task is known as a multiplexer.

         The multiplexer brings together several low speed communications lines, transforms them into one high speed channel and reverse the operation at the other end.

Programmable multiplexers are also available now a day. There are many applications in which several terminals are connected to a computer. If each terminal is operating at 300 bits per second over a communication line (channel) that can operate at 9600 bits per second, then we see a very inefficient operation. It has been found that the capacity of a channel exceeds that required for a single signal. A channel is an expensive resource. Hence, for its optimal utilization, the channel can be shared in such a way so as to simultaneously transmit multiple signals over it.

  
           A multiplexer takes several data communication lines or signals and converts them into one data communication line or signal at the sending location.

As seen as Figure, there are four terminals T1, T2, T3 and T4 connected to a multiplexer. The multiplexer takes the signals from their 4 terminals and converts them into one signal which can be transmitted over 1 communication channel. Then, at the receiving location, a de-multiplexer takes 1 large signal and breaks it into the original 4 signals.

  
          With multiplexing, it is possible for a single transmission medium to concurrently transmit data between several transmitters and receivers.

There are three methods to multiple channels. They are:

       
       1.      Frequency Division Multiplexing (FDM)

       
        2.      Time Division Multiplexing (TDM)

       
        3.      Wavelength Division Multiplexing (WDM)