As part of the Academy’s Ask an Expert series from our newsletter #TechKnow, Dr Steven Frisken FTSE answers a question from Emily and Sye, Vic.
We all tend to take for granted the ability to pick up a mobile phone almost anywhere and talk, or even video chat, with someone on the other side of the globe – apparently without any connecting wires.
And it may only be when things aren’t working so seamlessly, like a connection dropping out, that we wonder about the science behind these devices.
The basics
Many of us have experimented with a very crude form of telephony: a couple of tin cans linked by a taut piece of string.
When we speak, sound waves are converted via the tin can into mechanical vibrations, which travel down the wire. This process of taking a signal (in this case a sound wave from our voice) and converting it to a form suitable to travel over large distances underpins our mobile telephone network.
The amazing electromagnetic spectrum
Instead of strings to carry the signals, the mobile network uses technologies based on electromagnetic radiation. In this way, a signal may be carried through the air, cables or optical fibres.
Electromagnetic (EM) radiation is very familiar to us in the form of light, and we already know that light can travel over vast distances, like light from stars at night.
The other “colours” of the EM spectrum, determined by the different frequencies of waves, are just as amazing in their ability to be transmitted over long distances. Other non-colour frequencies are used to set-up networks to interconnect the billions of phone users.
The journey
Wireless signals, similar to radio waves, travel a short distance of a few kilometres to the nearest cellular-phone transmitter – an antenna that receives and transmits to many phones simultaneously.
From there, the signal usually travels wirelessly, via a cable or an optical fibre, to a telecom exchange. Optical fibres are thin strands of glass along which signals travel at the speed of light.
By the time the signal leaves the exchange it has already been converted several times, combined together with thousands of other voice calls, and then begins its often-long journey over the optical fibre network.
Optical fibres form an enormous network around the world, from country to country, and can direct a signal to another exchange anywhere in the world.
Along that journey, the infrared light will be switched in different directions, amplified many times and even re-transmitted at different colours of infrared light.
The fact that the signal can travel at the speed of light over these long paths means that in most cases the delays in hearing the other person are barely noticeable.
Once the signal reaches a telecom exchange near the receiving mobile phone, it is directed to the nearest cellular-phone transmitter.
At the same time, the signal travelling in the opposite direction – the other end of the call – needs to be established to allow your friend to communicate with you.
The systems controlling this flow of signals uses very sophisticated software models that need to account for many different simultaneous demands.
That means when everyone calls at the end of a concert to contact their friends, for instance, the network can become congested for a few seconds while the software tries to allocate connections to different phone calls.
Most of the time, incredibly, the signal arrives intact. To understand how, we need to consider a technology that has revolutionised communications over the last few decades.
Digital revolution
In our tin can experiment, the signal is transmitted as a form of vibration, but most communications today are digital.
Fast processors on a mobile phone measure the vibration of a tiny device called a transducer that converts your voice’s sound waves into a stream of numbers.
This series of numbers is converted into a series of bits (ones and zeroes).
As “ones” and “zeroes”, the signal can now be thought of as “on” and “off”. This on/off signalling can be easily converted into the different of forms of EM radiation, such as lasers pulsing light to transmit the signal.
What’s next?
One last thing to consider is that all the other content that we’ve become so dependent on – videos, navigation, social media – is also digitised and delivered along the same networks.
The plain old telephone system of 30 years ago has transformed into smart mobile phones today.
And there is plenty more change in store over the next few years as machines and devices start to “talk” directly with each other in what is becoming known as the Internet of Things.