Multiplexing - To combine multiple signals (analog or digital) for transmission over a single line or media. A common type of multiplexing combines several low-speed signals for transmission over a single high-speed connection.
Multiplexing is done by using a device called multiplexer (MUX) that combines n input lines to generate one output line i.e. (many to one). Therefore multiplexer (MUX) has several inputs and one output.
At the receiving end, a device called demultiplexer (DEMUX) is used that separates signal into its component signals. So DEMUX has one input and several outputs.
Concept of Multiplexing
As shown in fig multiplexer takes 4 input lines and diverts them to single output line. The signal from 4 different devices is combined and carried by this single line. At the receiving side, a demultiplexer takes this signal from a single line & breaks it into the original signals and passes them to the 4 different receivers.
Advantages of Multiplexing
If no multiplexing is used between the users at two different sites that are distance apart, then separate communication lines would be required as shown in fig.
This is not only costly but also become difficult to manage. If multiplexing is used then, only one line is required. This leads to the reduction in the line cost and also it would be easier to keep track of one line than several lines.
The following are several examples of different multiplexing methods:
Frequency Division Multiplexing (FDM) - Short for frequency division multiplexing, a multiplexing technique that uses different frequencies to combine multiple streams of data for transmission over a communications medium. FDM assigns a discrete carrier frequency to each data stream and then combines many modulated carrier frequencies for transmission. For example, television transmitters use FDM to broadcast several channels at once.
Time Division Multiplexing (TDM) - Short for Time Division Multiplexing, a type of multiplexing that combines data streams by assigning each stream a different time slot in a set. TDM repeatedly transmits a fixed sequence of time slots over a single transmission channel. Within T-Carrier systems, such as T-1 and T-3, TDM combines Pulse Code Modulated (PCM) streams created for each conversation or data stream.
Wavelength Division Multiplexing (WDM) - Short for wavelength division multiplexing, a type of multiplexing developed for use on optical fiber. WDM modulates each of several data streams onto a different part of the light spectrum. WDM is the optical equivalent of FDM.