Multiplexing and Demultiplexing Explained with Types

This tutorial explains the types of multiplexing and demultiplexing in detail. Learn what the multiplexing is and how it works in computer networks.

Multiplexing is a process that allows multiple signals to travel simultaneously over a single communication channel or path. Multiplexing in computer networks increases the amount of data that can be transmitted in a given time-span over a given bandwidth.

Multiplexing divides a given path logically into several short paths and then uses each path to transmit the data of an individual node. The following image shows an example of this concept.

example of multiplexing

In multiplexing, two devices are mainly used; a multiplexer and a demultiplexer. Both devices work on both ends of the path. A multiplexer works on the transmitting side and a demultiplexer works on the receiving side.

A multiplexer merges signals of all nodes and loads them on the medium/path. When these signals arrive at the demultiplexer, the demultiplexer separates all the signals and passes them to their respective nodes.

There are several types of multiplexing, depending on the technique used to merge signals. Some common and most widely used techniques are explained below.

Time Division Multiplexing (TDM)

In this technique, time intervals are used to merge signals. The multiplexer creates time-slots equal to the number of sending nodes and then assigns a separate time-slot to each node. Each node can send data only in its designated time-slot. If a node has no data to send, it sends nothing in its time-slot. If a node has more data to send, it must have to wait till the next time-slot.

Let’s take a simple example. Suppose four nodes (A, B, C, and D) are connected to a network over a single channel. The multiplexer creates four time-slots (1, 2, 3, and 4) and assigns a time-slot to each node; time-slot 1 to node A, time-slot 2 to node B, time-slot 3 to node C, and time-slot 4 to node D.

The following image shows this example.

Time Division Multiplexing

This technique is not more efficient because it reserves a time-slot for each participant node, regardless of whether a participant node has any data to send or not. A node that rarely sends data can waste too much bandwidth by keeping its specified time-slot empty in each data cycle.

Statistical Multiplexing (SM)

Statistical multiplexing works similar to TDM. It also divides a data cycle into time-slots and assigns a separate time-slot to each node. After assigning time-slots, it actively monitors the transmission. If a node does not have any data to send, it assigns the time-slot of that node to the next node.

Statistical Multiplexing

This technique is more efficient than the TDM because it utilizes each time-slot.

Frequency Division Multiplexing (FDM)

This multiplexing technique is used in analog communication. This technique works in two steps. In the first step, it divides the communication channel into sub-channels and assigns a separate sub-channel to each node.

In the second step, it modulates the frequency of the carrier wave of each node. A carrier wave is a simple analog wave that does not contain any data. A node uses a carrier wave to transmit digital signals over an analog channel.

To know more about the carrier wave and how it is used in analog transmission, you can check this tutorial.

Data Modulation in Computer Networks

Changing the frequency of the carrier wave does not affect the transmitted data. By changing the frequency of the carrier wave, this technique can transmit multiple waves simultaneously over a single path.

The following image shows an example of this technique.

Frequency Division Multiplexing

This technology is mainly used by telephone companies to provide internet facilities through existing telephone lines. A human can hear signals of frequency 300–3400 Hz. Telephone companies implement FDM to subdivide telephone lines and send data signals in frequencies greater than 3400 Hz. Since a human cannot hear frequencies above 3400 Hz, data transmission above this frequency does not affect voice communication over the telephone.

Wavelength Division Multiplexing (WDM)

This technology is used in fiber-optic connections to carry multiple light signals simultaneously over a single fiber-optic cable. This technology can work over any type of fiber-optic cable.

Fiber-optic cable uses a light beam to transmit data. WDM divides this light beam into up to 40 different light beams of different wavelengths or colors and then assigns a separate light beam to each node. Since each beam uses a different wavelength or color, it does not overlap or blend with the other beams.

The following image shows an example of WDM.

Wavelength Division Multiplexing

There are two popular types of WDM; DWDM and CWDM.

Dense Wavelength Division Multiplexing (DWDM)

DWDM uses more wavelengths for signaling than the original form of WDM. Because of this, a signal fiber cable can carry between 80 to 160 channels. This technology uses costly transceivers equipment. Due to cost, usually, this technology is only used on high-bandwidth or long-distance WAN links.

Coarse Wavelength Division Multiplexing (CWDM)

CWDM was developed in an effort to lower the cost of the transceiver equipment. This technology uses cheaper transceivers equipment. In this technology, channels are spaced more widely and signals are not amplified. Because of these, the effective distance of CWDM is less than the original from WDM. Through CWDM, a signal fiber cable can carry between 8 to 16 channels.

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