Multiplexing was developed in the early 1870s, but it’s become much more applicable to digital telecommunications in the late 20th century. Today, frequency division multiplexing (FDM), time division multiplexing (TDM), and wavelength division multiplexing (WDM) have become an extremely important asset to telecommunication processes and has greatly improved the way that we transmit and receive independent signals over AM and FM radio, telephone lines, and optical fibers.

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Understanding Multiplexing

Telecommunications such as radio, telephone, and television all use a method called multiplexing--shortened to "muxing"--to transmit and receive information. Multiplexing was designed to send numerous analog signals or digital streams through one common transmission line. Multiplexers, or shortened to MUX, amalgamate signals that are being transmitted from multiple devices through a transmission line.

The integrated circuit reads and analyzes each individual signal, or digital data stream, being inputted into the MUX and then assigns each a fixed-length time slot. Once assigned, the MUX now has what is called a single composite signal and transmits a piece of data from each slot during its fixed-length time slot through the high-speed transmission line. On the other end of the high-speed transmission line, the composite signal is reanalyzed and separated by a demultiplexer, or DEMUX. The figure below shows the flow of which digital data is transmitted and received from one device to another through the use of a single multiplexer and demultiplexer on FDM, TDM, and WDM systems. 




Frequency Division Multiplexing

FDM deals primarily with analog message signals rather than digital data streams. It is a scheme in which the entire bandwidth that is available in a data source is divided amongst sub-channels which each have a different frequency. Each sub-channel then carries separate signals through a transmission line, or an aggregate channel. The sub-channels can travel independently through a transmission line or they can travel simultaneously besides one another. These two travelling types represent two types of transmissions that we use every day.

Multiplexing through radio broadcasting, whether it's amplitude modulation or frequency modulation (AM and FM), generates a station that you can tune into. We can choose to listen to just one station because each data stream that is being transmitted belongs to an individual radio station and is regulated over a different provider. If this were not true, there would be overlapping between each station causing an unwanted static noise. Conversely from TDM, if a digital signal is trying to be transmitted, it needs to be converted to analog form first before it can be interpreted across a transmission line.

While multiplexing across Cable TV is similar to radio broadcasting, all channels are being transmitted simultaneously while a TV receiving them “tunes in” to a specific data stream channel. There is no interference between each channel because each signal is spaced far enough apart in frequency that the separate channels do not overlap. This scheme of data is normally transmitted through coaxial cable, optical fiber or using a radio transmitter. 

What is Time Division Multiplexing?

The method of combining more than one independent data streams into a single data signal and transmitting that single data signal through a multiplexer to a demultiplexer is known as time-division multiplexing. TDM differs from FDM and WDM because of its alternating pattern of transmission through the single data signal. Each individual signal that is transmitted through the multiplexer, is periodically broadcasted entirely on the transmission for a brief duration.

When first implemented in the late 1800s, time-division multiplexing was being used with the application of telegraphs. TDM was primarily being used to create a more simplified way of directing numerous telegrams sent by Hughes telegraph machines simultaneously. The concept behind using time division multiplexing was to take multiple broadcasts of telegraphs and synchronously transmit them at the same time, using a common transmission line to other Hughes telegraph machines. This was the beginning of long distance broadcasting of information through a single telephone communication line.

While TDM manipulates digital data, telephone circuits produce analog data signals. In order for the multiplexing to work correctly, a codec decoder device is needed in order to process the analog data. The codec decoder converts the analog format to a quantized, discrete time format. Once the codec has converted analog data to digital, the data is then multiplexed together using TDM. Once the data passes through the single transmission line, a demultiplexer is needed to take that single data signal and send it among many devices. 

TDM Operating With Network Bandwidth

The same concept of multiplexing that was developed for the communication between numerous Huges telegraphs over a large distance, is now being used vastly in closed-switched networks, like the public switched telephone network (PSTN). Time-division multiplexing has been further developed since its creation, and can now partition a network’s bandwidth into smaller bandwidth parts. The focus of this new operation is to minimize the amount of bandwidth a number of devices use on a system’s network. Although the same term, multiplexing, is used as it was with telegraphs, its convention of sending data has been revised and altered, thus higher-quality data can be sent from device to device. This process of telecommunication has been engineered to establish a simplistic and economical way for companies to build swift networks that interlink devices to one another over vast geographic locations.

Standard TDM systems transfer segments to other devices by giving them a unique fixed-time slot through the network. If X, Y, and Z represent data transmitting devices, data from X is sent to the MUX, then data from Y is sent to the MUX, and finally data from Z is sent to the MUX. This sequence is repeated until there is no more data being sent from each device. Although the data is just being sent from “point A to point B,” there are still a few different ways the TDM systems can be schemed to work more efficiently for different tasks.

Common TDM systems use one of two conventional multiplexing schemes: Bit-Interleaved or Byte-Interleaved. The structure’s fixed-time slot are either given a bit (either 1 for true or 0 for false) or a byte that can be up to 8 bits long to represent an integer, symbol, or a character. For a Bit-Interleaved scheme, the structure’s fixed-time slots are given a bit (either a 1 for a true statement or 0 for a false statement). 


Transmitting Through Wavelength Division Multiplexing

This technique of multiplexing has shown to be more useful to telecommunication companies in the late 20th century because of the capacity of data streams that can be sent through fiber cables. Transmitting through WDM is possible because the method combines numerous data signals on laser beams at different infrared wavelengths along transmission lines. WDM uses fiber optic cables to transmit a large number of data streams, which is favored over the conventional use of FDM and TDM systems. This system is similar to FDM, but alternatively, its method takes place on the infrared (IR) end of the electromagnetic spectrum. The figure below illustrates each data stream channel being combined into a white light that is transferred over a single fiber optic cable.


At the beginning of the system, a laser is controlled by a single set of data signals and at the receiving and of the system there are infrared-sensitive filters that direct each signal to its destination. At the multiplexer, each data stream being passed through the fiber cable has a different level of energy, translating to a different IR wavelength. Once combined at the multiplexer through a prism, it is transmitted through a shared fiber optic cable and they are split again with another prism at the demultiplexer.

Hopefully this article has provided you with enough information in understanding the basic applications, concepts, and designs of how multiplexing is used in telecommunication processes. If you have any questions or feedback, be sure to leave a comment!