Multiplexing

Multiplexing is a fundamental method used in telecommunications and computer networking to combine multiple signals into one. These signals can be analog or digital. By combining them, many users can share a single transmission medium, such as a cable or a radio wave. This process is essential because physical transmission media are often scarce resources.

Multiplexing allows a single high-data-rate link to carry many low-data-rate signals simultaneously. It effectively divides the total capacity of a communication channel into several logical channels. Each logical channel carries one specific message or data stream.

The mechanism of multiplexing follows a specific sequence to ensure data integrity. First, a device called a multiplexer, or MUX, receives several incoming signals. The MUX combines these individual streams into a single composite signal. This combined signal is then transmitted over a communication channel.

At the receiving end, the process must be reversed to recover the original data. A device called a demultiplexer, or DEMUX, performs this task. It extracts the original individual channels from the single incoming stream. This allows the data to reach its intended destination correctly.

There are several distinct types of multiplexing categorized by how they separate signals. Frequency-division multiplexing, or FDM, is an analog technology. It achieves signal combination by sending signals in different, distinct frequency ranges over one medium. This is how traditional radio and television broadcasting works.

In contrast, time-division multiplexing, or TDM, is a digital technology. It uses time rather than frequency to separate data streams. TDM works by sequencing small groups of bits or bytes from each input stream one after another. If this happens fast enough, the receiver does not notice the gaps between different signals.

Other specialized methods exist for different physical media. Wavelength-division multiplexing, or WDM, is a variant used in optical communications. It uses different colors of light to carry data through fiber-optic cables.

Code-division multiplexing, or CDM, allows several channels to share the same frequency spectrum simultaneously. In this method, each channel transmits bits as a unique, coded sequence of pulses called chips. This is used in the Global Positioning System, or GPS. Finally, space-division multiplexing uses separate physical conductors or antenna elements to create different channels. In wireless systems, this can involve multiple-input multiple-output, or MIMO, schemes.

The history of multiplexing shows how it evolved from simple wires to complex digital systems. The technology originated in telegraphy during the 1870s. In 1874, Thomas Edison developed the quadruplex telegraph. This device could transmit two messages in each direction at the same time, totaling four messages on one wire. Later, in 1910, George Owen Squier is credited with developing telephone carrier multiplexing. These early advancements in telegraphy and telephony laid the groundwork for modern high-speed communications.

Multiplexing provides significant efficiency in modern infrastructure. In telephony, a customer's line often ends at a remote concentrator box. Here, the line is multiplexed with others to reach a central switching office. This allows many calls to travel much further using significantly fewer wires. In digital systems, TDM is still used as a backbone for many national fixed-line telephony networks in Europe. For example, an E1 port provides 30 or 31 speech timeslots for signaling and voice channels.

This allows massive amounts of data to move through narrow-band exchanges.

Multiplexing also connects to broader concepts like multiple access methods. While multiplexing happens at the physical layer, multiple access methods allow several transmitters to share a medium. For example, TDM can be extended into time-division multiple access, or TDMA. Statistical multiplexing can become carrier-sense multiple access, or CSMA. These systems ensure that many devices, from wireless routers to massive satellite networks, can communicate without constant interference or the need for infinite physical wires.