The art of multiplexing
Behind every worldwide event there is always a new invention providing the means so the human kind can change the course of the nature as they like.
Not any different, multiplexed systems had a very special role in the human history. Most of the times unnoticed by the untrained eye, multiplexing has become something extremely important to the evolution of our communication systems all around the world.
First of all, it is important to get the context behind the need of it.
It all started with the telegraph, the first technology that provided fast communication across large distances almost instantly.
Developed in the late 1830`s by Samuel Morse and other inventors, the telegraph revolutionised long-distance communication at the time.
It worked by transmitting electrical signals over a wire laid between stations.
Around 1844, Morse sent his first telegraph message, from Washington, D.C., to Baltimore, Maryland. By 1860's, a telegraph line had been laid across the Atlantic Ocean from the U.S. to Europe.
In addition to helping invent the telegraph, Morse developed a code (bearing his name) that assigned a set of dots and dashes to each letter of the English alphabet and allowed for the simple transmission of complex messages across telegraph lines.
By designing the electric telegraph and unified language that managed to connect all four corners of our world, Morse had a very important role in the world history.
Morse’s invention had a huge influence on several worldwide events, its historical impact can be compared to the internet.
Following the massive adoption growth of the telegraph, the need for a huge network infrastructure was a big problem in the late 1840's. Mostly because the demand of a physical wire for each communication channel was a painful reality at the time.
No problem, We got it!
By the end of 1850's, about ten years after the first telegraph public demonstration, over the UK, the telegraph lines covered around 1,800 miles of railway network.
Within 25 years, Britain was connected to 650,000 miles of telegraph wire and 30,000 miles of submarine cables. More than 20,000 towns and villages were part of the UK network.
New kid in town
An initiative of the French telegraph service in 1874 resulted on what today it would be called a synchronous time division multiplex system.
The purpose was to create a printing telegraph, later invented by Emile Baudot, carrying its inventor’s name.
It used certain printing details from the David Hughes first attempt on the printing telegraph, a distributor invented by Bernard Meyer and the five unit code devised by Gauss and Weber. Baudot combined these, together with original ideas of his own, to produce the final multiplex system.
Bautod’s invention brought new paradigms to the whole telegraphy system. His multiplex system allowed the telegraph to send and receive several messages simultaneously through the same line.
Although the multiplexed telegraph brought energy to the telegraphy, that didn’t stop it of getting obfuscated by telephone invention.
The replacement
The telegraph fell out of widespread use, but yet it leaves the scene with a special addition to technology. The concept of multiplexing implemented by Baudot laid the groundwork for the communications revolution that led to later innovations.
Call me … maybe
In the 1870s, two inventors Elisha Gray and Alexander Graham Bell both independently designed devices that could transmit speech electrically.
Both men rushed their respective designs to the patent office within hours of each other, Alexander Graham Bell patented his telephone first.
“Alexander Graham Bell entered into a famous legal battle over the invention of the telephone, which Bell won.”
In 1877, the first telephone line was constructed, the first switchboard was created and the first telephone exchange was in operation. Three years later, almost 50 thousand telephones were in use.
By 1900 there were nearly 600,000 phones in Bell’s telephone system; that number shot up to 6 million by 1910.
Soon after massive adoption of the telephone, the challenge was to connect all the calls, because once again each line needed one wire.
The need for a suitable distribution system led the phone companies at the time to use switch boards in order to connect calls and centralise the “network” distribution.
Switch Boards
The switch boards were the early 60s response to the distribution needs at the time. It was possible to connect lines individually, task accomplished manually by switchboard operators who connected calls by inserting a pair of phone plugs into the appropriate jacks.
In order to support and identify lines each jack had a light above it that lit when the telephone receiver was lifted and also a tag near to identify the client number. Each pair of plugs was part of a cord circuit with a switch associated that let the operator participate in the call as long as needed.
Operators
The switchboard operator work was very stressful because they had to answer the calls and connect lines with the jacks, most of times they took care of multiple lines simultaneously.
The first operators were teenage boys, who didn’t have very nice communication skills. Telephone companies soon began hiring girls, in order to present a more gentle "image" to customers, besides that women salaries were very cheaper compared to man at the time.
Centrals
The regional centrals were the end of every cable coming from all telephones near the station. It was really a massive number of lines in the most populated cities, so a main distribution frame took care of the incoming lines, with the responsibility to deliver those lines to several switchboards stations, which were handled by the operators 24 hours a day, 7 days a week.
Evolution
By design, the whole system had a lots of limitations, starting from the need for wires from the client to the central up to the switchboard operators flow, but it was the provided technology at the time.
Following the technology revolution, global communication systems had to implement more sophisticated solutions to the distribution problems. This is where the multiplexing come handy, because it allows systems to converge multiple data streams to a single data stream by splitting data fast enough it would not get noticed or affect the sender and the receiver.
In order to visualize what multiplexing provides, the image (on the left), represents, on a very simple way, the purpose of multiple channels talking simultaneously through a single channel. The example can be interpreted as 5 telephone lines connected though a unique wire, talking with each other in real time, with no knowledge of other people sharing the line.
This is only possible because of multiplexing, that basically split “parts” of the conversations and transmit them very fast, on a frequency that it is not noticed by the human ear.
Sync
In order to ensure that no one is hearing other people`s calls is guaranteed by a “timer” between the sender and receiver. Without ensuring the timer sync between the two sides, the system as whole cannot be trusted, this may, and will, cause people to hear other people`s calls, once the data is splitted based on small “time packages”.
It is possible to imagine the effect of this new concept on the communication systems all around the world, providing the means to start a revolution on how distribution is made.
Finally, multiplexing
Multiplexing is everywhere, it is not limited to telecom, actually it is behind the concept that powers your TV screen, network router and so many other things around you. It is nice to understand the theory behind it, even that it may seem complex, it is actually very simple if understood from the scratch.
Multiplexing is a method by which multiple analog message signals or digital data streams are combined into one signal over a shared medium.
As the historical records show, the aim to share the resource is based on massive distribution, because the shared medium is an expensive resource. For example, in telecommunications, several telephone calls may be carried using one wire.
Behind the scenes
A device that performs the multiplexing is called a multiplexer (MUX) and a device that performs the reverse process is called a demultiplexer (DEMUX or DMX).
MUX
The mux responsability is to “compress” the inputs into a single data stream, making possible to alternate between the inputs over a channel switcher.
- CHO — channel zero, can be interpreted as a call
- CH1 — channel one, on the example, another call
- SEL — channel switcher
- OUT — selected channel
As data is streamed from each channel, the switcher controls which one should be streamed at the given time.
DEMUX
The demux responsability is to “uncompress” the data stream, handling each streamed signal to its own channel.
- SEL — channel switcher
- IN— incoming signal
- CHO — channel zero
- CH1 — channel one
Putting all together
As the signal is streamed from each channel, the MUX chooses which one should be streamed at the given time, as on the other side, the DEMUX chooses which should receive the streamed channel, based on the channel switcher and some time handler device, more specifically a clock.
The image above provides a better way to understand that the multiple signals are being lined up, in order to be carried over by the shared medium. The switcher is constantly controlling which channel is being transmitted and will handle the signal delivering to that specific channel.
Everywhere
To have an idea of how deep multiplex is present on your life, at this exact moment that you are reading this, whether is on a phone, notebook, desktop or even a kindle, there is no escape, your screen is multiplexed, you network uses multiplexing concepts, your keyboard, and so on.
Keyboards
Most of your physical keyboards, of any kind of device has a multiplexed interface behind it. Otherwise it would require a input and a wire for each key.
Multiplexing allows to use a matrix in order to read pressed keys.
In the example, the rows(R1, R2, R3, R4), the columns(C1, C2, C3, C4) control each key individually, providing a simple way to read them. For instance, if the column C4 has a positive signal, the rows R1, R2, R3 and R4 represent respectively the keys A, B, C and D.
In most of the cases behind the hardware there is a micro-controller, or even on computers, a driver, that handles the search for the pressed key, by sending signals to the columns, and reading signals from the columns.
Screens
A simple full HD screen (1920x1080) is composed by RGB LEDs, that are actually 3 LEDs, one for each base color (Red, Green and Blue).
A FullHD screen should have around 6 million LEDs.
If each one of those must have a connection, how much power and money would be spent just to turn on this screen?
Again, that is when multiplexing comes handy, it allows the screen to be smart enough to understand that those LEDs doesn’t have to be always on, or at the same time.
The whole purpose of a screen is to provide a suitable image to the human eye, that is known to be capable to percept render changes at 13 to 15 frames per second (fps, or Hz).
Which means that those LED’s can be off roughly up to 60 ms and we would not actually notice.
Most of high-definition televisions refresh at 60 Hz or even higher frequencies, at this rate human perception of motion is entirely seamless.
More information about the limits of Human Perception here.
As the image on the left shows, a multiplexed implementation of a LED matrix consists on each LED being turned on by a line and a column.
Which implies that if a column already has another line connected, trying to turn on two specific LEDs on the same line or column at the same time will cause side effects.
This imposes the premise that only a single LED will be turned on at any given time, but the frequency happens to make your eyes believe that they are all on at the same time.
More
Multiplex is a very vast field of study, it has a lot of segments, shapes, theories and applications. So it is very hard to aggregate much on this simple article. If you found it interesting and want to learn about it, here are some interesting books:
