Multiplexing

Multiplexing is not a new concept, but it keeps delivering new and ever-more-impressive results. The idea goes back to the 1860s when the electric telegraph was invented.At that time, telegraph messages could only be sent one at a time and only in a single direction. A breakthrough came in France in 1874 when Emile Baudot invented a multiplex system. The system was much more efficient and could simultaneously transmit several messages on the same line.

From those simple beginnings, multiplexing is used today in a wide variety of applications, some far beyond the world of telecommunications. In this article, we'll trace that history to learn about different types of multiplexing. We'll consider the technology's benefits and challenges and zero in on its use in streaming applications.

What Is Multiplexing?

In its simplest form, multiplexing is a way to send multiple signals over the same communication channel at once. Multiple signals or data streams are fed into a multiplexer (sometimes called a muxer) and combined into a single composite signal or data stream. It's that multiplexed signal that we transmit over the communication channel.

At the other end, a demultiplexer (aka demuxer) separates the combined signal into the original signals or data streams. The demuxer then passes each signal to the device for which it was intended.

A helpful analogy is a zip file. It contains multiple compressed files. Once you unzip it, you can access the individual files. Multiplexing works a lot like that zipping and unzipping process.

The Evolution of Multiplexing

The first version of multiplexing is what we now call time-division multiplexing (TDM), specifically synchronous TDM. It's synchronous because the multiplexer assigns a fixed time slot to each connected device. Each device waits for its assigned time slot to send or receive data.

Telephony

When multiplexing was invented in France, Alexander Graham Bell and Elisha Gray found a way to send and receive voices over telephone lines. It wasn't long before multiplexing was needed there too, but synchronous TDM wasn't fit for purpose.

And so, asynchronous TDM was born. Instead of using pre-defined time slots, it allocates them dynamically based on the demand for each analog signal or data stream. But it wasn't perfect because the time slot a user gets depends on how many users are currently active.

That led to the development of frequency-division multiplexing (FDM), which divides a channel into smaller channels using different frequencies. It worked so well that it was used in many areas, including radio, TV, and first-generation cell phones.

Data Communications

The internet brought an explosion of data, driven by demand for streaming video, social media, online gaming, and other bandwidth-intensive applications. That tested the boundaries of TDM and FDM.

With synchronous time-division multiplexing, more signals could be sent at the same time, and time slots could be divided more easily. Asynchronous TDM needed to be able to control the dynamic allocation of time slots. Frequency-division multiplexing, meanwhile, needed ever-more and narrower guard bands to cut interference between neighboring frequency bands. But that also reduced the frequency spectrum available to send and receive data.

Enter space-division multiplexing (SDM), which exploits the spatial dimension to transmit multiple independent data streams simultaneously. SDM works by physically separating the transmission paths for different data streams with antennas, optical fibers, or other spatially distinct paths.

SDM significantly increased the capacity and efficiency of communication systems. Its widespread adoption in modern cellular networks and wireless communication systems illustrates this perfectly.

Optical Fiber

However, when optical fiber was the communication medium, SDM couldn't fully maximize the available bandwidth. As the volume of spatial modes increased, so did the complexity of managing crosstalk and interference between modes. That'll always lead to signal degradation and poor data integrity, especially over long distances.

The solution was wavelength-division multiplexing (WDM), which exploits the vast spectrum of wavelengths available in optical fibers. WDM transmits multiple signals at different wavelengths over a single fiber, massively increasing the capacity of optical communication systems by allowing multiple independent data streams to be transmitted at the same time.

Meanwhile, engineers were experimenting with another property of light: polarization. This led to the invention of polarization-division multiplexing (PDM). By transmitting data on different polarization states of light, PDM effectively doubles the data capacity of each wavelength channel.

While each of these techniques is powerful in its own right, the combination of space-, polarization-, and wavelength-division multiplexing is greater than the sum of the parts. Integrating these different multiplexing techniques has led to a wide range of hybrid systems that meet growing demands for high-capacity data transmission.

Moving to the Next Level

The world will soon reach the limits of what SDM, WDM, and PDM can do together. Scientists have been working on the next step. It's called orbital angular momentum (OAM) multiplexing, and it exploits another unique property of light.

This property is also known as vortex beams, which have a helical phase front and can be distinguished by their topological charge (TC). Each OAM mode with a different TC is at a right angle from the others. This lets multiple data streams be sent at the same time over the same frequency band without interference.

OAM can be combined with WDM and PDM to use wider frequency bands. A recent study demonstrated this, achieving wireless data transmission speeds of 938 Gbps. The combination can push wireless data transmission speeds into the terabit/s range.

Beyond Data Communications

Multiplexing is a powerful technique that extends beyond data telecommunications. It's widely used in various scientific and medical fields, including biological research, genetic sequencing, and real-time polymerase chain reaction (PCR).

Real-time PCR allows the simultaneous amplification of two or more target genes in the same reaction. This is particularly useful in clinical settings where sample volume is limited. At the same time, it saves on reagents and minimizes pipetting errors. It also improves precision by amplifying multiple genes in the same well, reducing variability.

The technique is also used elsewhere in the science field. For example, sample multiplexing, which is also called multiplex sequencing, allows large numbers of libraries to be pooled and sequenced simultaneously during a single run.

These applications use multiplexing for the same reasons that it's so prevalent in data communications. Evaluating multiple experimental elements at once increases throughput, eliminates buffering, reduces costs, and enhances the precision and comprehensiveness of data analysis. This improves the efficiency and effectiveness of scientific research and clinical diagnostics.

Benefits of Multiplexing

Multiplexing techniques are critical in a world of rapidly expanding data. Data needs to be transmittable to be useful, and this technology allows us to move large amounts of data quickly and efficiently. At the same time, it also provides other benefits.

Efficiency

Multiplexing techniques improve data communications efficiency by optimizing available bandwidth, reducing latency, and cutting costs. Sending many digital signals or data streams over one communication channel ensures the medium is used to its full potential.

These techniques promote low latency, which is the reduction (or elimination) of delays experienced by data as it travels from the source to the destination. It does this through efficient signal processing and reduced overhead. This maximizes infrastructure use (cables, frequency allocations, or optical fibers) and ensures resources are used efficiently, improving performance and cutting costs.

Quality of Experience

Real-time communication apps use multiplexing techniques to improve bandwidth and resources, improving the users' Quality of Experience.

This is related to but different from Quality of Service (QoS). Where QoE is about the experience, QoS is usually a contractual obligation with measurable thresholds. But both have the effect of reducing buffering or slow streams and providing reliable connections for users, which supports high-quality viewing and listening experiences.

Challenges of Multiplexing

As we've seen, each technique mentioned above has its problems. Most of these problems helped create a better method. But, there are other challenges and disadvantages to consider.

Implementation

Setup requires considerable time, resources, and technical expertise. The work doesn't stop there; the hardware and software need ongoing maintenance and occasional upgrades.

The payoff is usually found in improved efficiency and bandwidth utilization, which leads to cost savings. With careful planning, spending money on good equipment, and working with experienced professionals, the benefits will usually outweigh the cost.

Interference

With the evolution of different multiplexing techniques, interference can be a problem. Compressing multiple data streams into a single communications channel can cause interference between neighboring channels. This degrades signal quality, reduces data rates, and increases error rates.

Each technique has built-in avoidance strategies. Tactics such as guard bands, precise synchronization, orthogonal carriers, equalization, guard intervals, pilot tones, and frequency hopping are all used to reduce interference.

How Multiplexing Improves Streaming

We've seen that multiplexing techniques are being used far beyond data communications. One area where they're particularly useful is improving streaming quality, particularly for live event streaming, video-on-demand, and web conferencing, among many other applications.

And it makes perfect sense to us that multiplexing delivers a smoother streaming experience, especially at very high resolutions. There are three main reasons for that:

1. Optimal Bandwidth Management is essential for ensuring that streaming services can deliver high-quality content without interruptions. It involves using compression techniques to reduce the volume of data being transmitted. It also uses adaptive bitrate streaming to adjust the quality of the media stream based on the prevailing network conditions. It also uses network slicing to create multiple virtual networks within a single physical network.

2. Greater Efficiency and technological advances, like new video codecs, can provide greater efficiency than multiplexing alone. For example, new streaming protocols send media more quickly and safely. Artificial Intelligence-driven algorithms can change video quality based on real-time network conditions.

3. Improved Connectivity is crucial for delivering a seamless streaming experience. Technology like 5G mobile networks improves streaming with faster speeds and shorter delays and cloud computing makes content delivery easier and more efficient.

Gaining a Global Edge

The world of business today is global. Audio and video need to be accessible from anywhere without any degradation in quality. Multiplexing appears to be up to the challenge.

Other technologies allow you to take it even further. Multiplexing is a cornerstone in meeting our growing demands for high-quality video streams, seamless conferencing, and robust data transmission across global edge networks. By enabling multiple signals to share a single transmission medium, multiplexing optimizes bandwidth and improves our systems' scalability and flexibility.

Paring multiplexing with newer technologies like 5G, artificial intelligence, and the Internet of Things promises to push the boundaries even further. These advances will enable faster, more reliable, and more intelligent networks capable of supporting the ever-increasing volume of data we generate. Multiplexing isn't just a tool for communication today. It's a foundational technology that will support and help shape communication in the years ahead.

Frequently Asked Questions