Types of Multiplexers and Multiplexing in Mobile Network

If you’re an electronics or computer science student, you’ve probably learnt a lot about multiplexers. Also known as a mux, this device allows multiple data lines to be crunched into a single line, no matter if the input lines are analog or digital. As evident, multiplexers allow heavy data flow without using too much bandwidth or taking up too much time. Instead of sending 10 lines of data one by one, a multiplexer can send them all at once, which obviously results in a far more efficient transfer. It can also be called a data selector. Now, since all that data is being crunched, it becomes necessary for the receiving end to make sense of it, so separate each strand of data, and that’s where a demultiplexer comes in. It does precisely the opposite of what a multiplexer does, and creates multiple data output lines from a single input line, originating from a multiplexer. Similar to the shortened word for multiplexer, it’s also called a demux. The simplest example is as follows a 2 input, single output MUX:

An electronic multiplexer is not only a data selector, but can also be used in circuits where multiple devices may need to access another single device. Think of it something similar to a smarter select line. It’s quite the versatile device, and is used in a whole range of devices all around the world, both commercial and industrial. Here’s all you need to know about them!

The History of Multiplexers

Even though multiplexing sounds like a very fancy new age idea, it in fact traces its origins back to the late 1800’s! It was first used in telegraph systems, which were ostensibly limited in scope and efficiency. Western Union is said to have implemented a form of multiplexing called duplexing by 1872. 2 years later, Thomas Edison invented something called ‘diplexing’, which allowed two separate messages to travel in one direction at the same time. It basically doubled operations and was a breakthrough. In some ways, it was the first rudimentary form of parallel processing. Although we’re talking about specific devices called multiplexers that do the job mentioned above, the concepts are pretty much the same in these historical cases.

French visionary engineer Jean-Maurice-Émile Baudot was the one who invented time division multiplexing and effectively propelled communications into the future. The unit ‘baud’ in communications is named after him. The Baudot distributor was a device created by him that allowed the transmission of multiple messages over a single telegraph line, separated by a time factor. Invented in 1894, it took the world by storm and made telegraph a much more viable means of communication. By the 1930’s, frequency division multiplexing had also been invented. It basically used a system of vacuum tubes and alternating current to vary frequencies and thus transmit signals simultaneously while being faster and more secure. A lot of these technologies went on to help create the telephone.

Types of Multiplexing

Multiplexing technologies are broadly classified into two categories, analog and digital. While both can be used in a circuit depending on the application and budget, for some applications, there’s only one type suitable. Under analog, we have:

• Frequency Division Multiplexing: A technique in which data streams of different frequencies are combined in a single channel over a communication medium. Basically, every piece of data under this is separated by frequencies, so this is how you can have multiple TV channels from a single frequency.
• Wavelength Division Multiplexing: As the name suggests, wavelength division multiplexing is a technique which allows different data streams to be communicated in a single line by varying wavelengths in the light spectrum. Developed specifically for optical fibers, it is the equivalent of frequency division multiplexing, optically.

Moving on to digital multiplexing, we have time division multiplexing, which separates data streams by a factor of time. So every data stream gets its time slot in a large set over a single channel, with fixed sequences. Abbreviated as TDM, it also has two types:

• Synchronous Time Division Multiplexing: In this technique, a single device gets only a fixed time slot to communicate, that can’t be changed. No other device can take that slot, and if the slot if missed, it has to wait for the next cycle.
• Asynchronous Time Division Multiplexing: This technique is the opposite of the above, with flexible time slots. It’s far more efficient because time slots are assigned to active channels or devices, so usually they don’t go to waste, and priorities can be changed.

So there you have it, a broad classification of multiplexing types and techniques, although you won’t see something very apparent in most daily uses.

Multiplexing in Mobile Networks

Although Frequency Division Multiplexing (FDM) is mentioned as analog and Time Division Multiplexing (TDM) is described as digital above, both these principles are used in modern mobile networks. You’ve probably heard of LTE. LTE stands for Long Term Evolution, which is also commonly termed as 4G in today’s world. Strictly speaking, it isn’t technically 4G because the increase in speed isn’t up to a certain number, but it is commercially considered so. Coming to the point, multiplexing is used extensively in 4G LTE. A few countries use FD-LTE and a few use TD-LTE. The performance is pretty much the same, it’s just that frequency bands differ. India, as opposed to the United States, uses TD-LTE, so unless you’re rocking a smartphone which packs in both antennas, you can’t use LTE in either country if the handset is bought from another.

In these networks also, the working principle is quite similar to the classic definition. There are certain differences though, because of how they work. FD-LTE uses frequency divided spectrum for an uplink and downlink channel, and has evolved from 3GPP technologies. Because it uses a paired spectrum separated by a guard band, it requires a diplexer to separate transmission and reception channel data. This drives up costs a little bit compared to TD-LTE.

TD-LTE is usually found in countries with larger populations because it’s cheaper to implement. Since it uses a single channel to transfer both lines of data separated only by guard time period, there’s no need for a diplexer. This also gives network providers the flexibility to increase or decrease uplink/downlink ratios, which cannot be changed in FD-LTE because of frequency band regulations. While TD-LTE is easy to implement and has a certain degree of flexibility, it is also prone to more cross slot interference. In short, both have similar performance, with their own pros and cons. The picture below perfectly illustrates how both standards work.

Multiplexers and their Versatility

As mentioned earlier, multiplexers can be both analog and digital. High speed logic gates can be made into multiplexers, and so can the regular transistors, relays or MOSFETs. Since they’re not just devices that combine data or select lines, they are used in a huge variety of applications, from the smallest switch to the mightiest server processor. It’s pretty obvious that almost every system, be it communications, processing or anything else works with multiple inputs, so the only thing that keeps everything together is a multiplexer. Here’s some multiplexer applications:

• Telephone lines
• Mobile phone networks (CDMA, TDD, FDD)
• Desktop, mobile and server processors
• Graphics processing units
• Cryptography
• Series to parallel and parallel to series converters
• Computer networking
• Digital cables for music, film projection, media and more

Basically, wherever there’s complex date being processed, there’s some form of multiplexing going on. Just imagine a single lane highway compared to a multi-lane one. We all know that the latter will allow more traffic to travel faster and much more efficiently, and that’s pretty much how it works for all these technologies and devices as well.

Advantages of Multiplexing

Now that you’re aware of the basic concept of multiplexers and the process, we can see how exactly it helps us overcome a lot of challenges. Without it, a lot of technologies we so depend on today would not be possible, or perhaps, not practical. Here’s some advantages of multiplexing:

• It reduces number of input/output lines
• It allows efficient communication
• It reduces costs because of less components required
• It simplifies circuits and designs
• Reduced complexity means applications can be developed further
• It helps with scaling
• It makes the system more reliable
• Allows parallel operation

Just imagine creating a mobile network for a country without multiplexing, it would be nothing short of impossible. Without multiple data lines, scaling and parallel processing, it simply would not be feasible to run a network where everything has to be queued. Apart from increasing stress on a single channel, processing requests at a reasonable rate would be out of the question!

Conclusion

In this fast paced world, people aren’t ready to wait for minutes, or even seconds in some cases, so just imagine no multiplexing. There would be no mobile networks, no fast computers, and by an extension, no advanced technologies. What started out as a simple device or technology to process 2 things at once has grown into something much, much larger. It only says how much technology can help and change the world!