Multiplexing in Data Communication

In this lecture, we shall discuss an important process in a communication system known as multiplexing. In this lecture, we shall discuss what is multiplexing and what are the types of multiplexing and then proceed with detailed aspects of multiplexing.

Multiplexing

• Multiplexing may be defined as a technique that allows many users to share a common communication channel simultaneously.

Types of Multiplexing

• There are two major types of multiplexing techniques. they are as under:

1. Frequency Division Multiplexing (FDM)
2. Time Division Multiplexing (TDM)

1. Frequency Division Multiplexing (FDM)

Definition of FDM:

• Thie technique permits a fixed frequency band to every user in the complete channel bandwidth. Such frequency slot is allotted continuously to that user.

• As an example, consider that that channel bandwidth is 1 MHz. let there be ten users, each requiring up to 100 kHz bandwidth. Then the complete channel bandwidth of 1 MHz can be divided into ten frequency bands, i.e. each of 100 kHz and every user can be allotted one independent frequency band. This technique is known as Frequency Division Multiplexing (FDM).

Main Application Area:

• FDM technique is mainly used for the modulated signal. This is due to the fact that a modulated signal can be laced in any frequency band by just changing the carrier frequency.

• however, at the receiver, these frequency multiplexed signals can be separated by the use of tuned circuits of their respective frequency band. And for every band, there are independent tuned circuits and demodulators.

2. Time Division Multiplexing (TDM)

Definition of TDM

• The pulse is present for a short duration and for most of the time between the two pulses, no signal is present. This free space between the pulse can be occupied by pulses from other channels. This is known as Time Division Multiplexing (TDM). Thus Time Division Multiplexing makes maximum utilization of the transmission channel.

Comparison with FDM

• We can say that in FDM, all the signals are transmitted simultaneously over the same communication medium, and the signals occupy frequency slots. However, In TDM the signals to be multiplexed are transmitted sequentially one after the other. Each signal occupies a short time slot as shown in the figure below. Thus, the signals are isolated from each other in the time domain, but all of them occupy the same slot in the frequency spectrum. Therefore in TDM, the complete bandwidth of the communication channel is available to each signal being transmitted.

Concept of Frame in TDM

• At this stage, it may be noted that in the context of TDM, we define one important term, i.e., frame. One frame corresponds to the time period required to transmit all the signals once on the transmission channel. This has been shown in the figure. Here, we have a total of four message signals to be transmitted. hence, one frame will correspond to the time period required to transmit all four signals once on the channel. The TDM system can be used to multiplex analog or digital signals, however, it is more suitable for the digital signal multiplexing.

Frequently Asked Question on Multiplexing