How does one telephone wire transfer all of that data?

03 September 2012
Presented by Hannah Critchlow.

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We find out how it is possible for a single telephone wire to carry information for a telephone conversation, file downloads to a computer, WiFi access, and allow us to watch a film and listen to the news. Plus we ask can we make an infinitely powerful laser simply by adding more mirrors?

In this episode

00:00 - How can I use one telephone line for multiple uses?

Using one telephone wire, we can have a conversation with a distant relative, download emails, stream live TV and, of course, catch up with the latest Naked Scientists Podcasts - all at the same time! So how can a simple pair of copper wires handle so much activity?

How can I use one telephone line for multiple uses?

When you talk into a phone the vibrations in the air that make up the sound are converted into vibrations in the electrical current carried by the copper wire of the telephone line. In the same way that your voice is made up of different pitches, this electrical vibration can be a wide-range of different frequencies. But according to Mark Smith, Network Engineer and Telecommunications Consultant, a telephone call uses a narrow range of frequencies...

Mark - Basically, there's a pair of wires from the local exchange, all the way to your phone; but the telephone conversations are actually quite a low-frequency signal: they only go up to 4 kilohertz.

Hannah - This leaves a lot of other frequencies that can transfer data above the range of human hearing. Using an electronic filter, you can transfer data at frequencies above the range that you can hear. A modem or "modulator-demodulator" converts digital data into these vibrations. As this is done at vibrations of at least 25,000 hertz, you can send voice and internet data along the same wire at the same time, without interfering with your telephone conversation.

Mark - The modem uses all the frequencies up to about 10 megahertz and it divides the frequency into bands, a bit like the spectrum of rainbow; and based upon how good your line is, it allocates different frequencies to different parts of a band. The things like YouTube, file transfer and watching your television, they're all data that needs to go as noughts and ones over the line. All that data's split into packets of data and it's sent over a line using a very special modem that decides the maximum amount of data it can get over that and works out how to do that.

With things like video for example, if the line is good enough, you'll always get that, but if the line isn't good enough, then it will start to break up. If you're transferring things like files, if the line is really good, it would be really quick, and if the line is not good, it will be very slow.

Hannah - On the forum, Evanau adds that these packets of data each have an address on the front, and a return address, like letters in the postal service. The network equipment uses this address to send each packet to the right destination. This means that, even though the packets are sent mixed up and out of sequence, the data doesn't get scrambled...

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