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Last Updated: Friday, 10 June, 2005, 16:24 GMT 17:24 UK
Home network on electricity wires
Spencer Kelly
By Spencer Kelly
Reporter, BBC Click Online

In order to listen to music or watch films in different rooms in the home, some people lay down cables or invest in a wireless network. But you probably have a data network already installed, as Spencer Kelly reports.

Spencer Kelly
Did you know about this data network in your home?
Even if your house, like mine, was built 40 years ago, or before the modern home computer was invented, the chances are it has a data network already built into the walls.

It is the electricity supply.

After all, this is a network of copper wires that runs to nearly every room in a house.

Just because it carries power does not mean it cannot be used to carry data too.

At recent tradeshows, companies were demonstrating just that capacity by sending computer data, music and even TV pictures across the mains.

Special modules plug straight into your electricity supply.

They take audio straight from a source like your CD player, and convert it into data which they then inject into the copper.

At the other end, another module pulls it out of the mains and turns it back into an audio signal.

So your CD player could be in one room and your speakers could be in another.

Super speeds

The idea of plugging your computer or stereo inputs directly into 240 volts of mains electricity sounds a little frightening.

Module
Special modules plug into your electricity supply and convert data
But the raw electricity is prevented from travelling up the data cable and into your computer, or your hand, by isolation transformers.

Products currently on sale can already achieve data speeds of up to 14Mbps - easily fast enough to stream television from one room to the next.

Some companies are claiming to be able to get 200Mbps out of power lines, which is phenomenally fast - much faster than, say, wi-fi.

It almost sounds too good to be true.

But there is a downside, as technology expert Barry Fox reveals.

"It sounds like a great idea, to send data into the home on power lines. The problem is radio interference.

"When you've got high frequencies going through wires that aren't shielded then those wires act an antenna because the carrier waves - the waves they're having to send down the power lines - are in the same frequency band used by short wave radio."

That means that using the mains as a home network could interfere with your enjoyment of shortwave radio, which is used by lots of broadcasters around the world, including the BBC World Service, and air traffic control.

Testing questioned

Surely any possible radio interference should have been spotted and dealt with?

Pylons
Mains electricity will not travel up the data cable
Most mains home networking products comply to a standard called Homeplug.

This standard was tested in 500 houses in the US, supposedly to spot any potential interference problems.

Chris Smales, of Magenta Solutions, an online networking and security equipment retailer in the UK, says: "There's been extensive testing in the field and it's not something we've experienced.

"In the trials of the 500 households nothing such as noticeable interference with home radio systems was experienced whatsoever."

In 2004, BBC researchers conducted an investigation into Homeplug-compliant power line communications, to see whether they did affect radio reception.

They found that as soon as data starts flowing, the radio signal is obliterated.

It turns out that the Homeplug standard only requires that devices avoid using radio frequencies that are used by amateur radio enthusiasts.

It does not mention anything about all the other frequencies that are used by broadcasters around the world.

This interference might not be restricted to just your home.

It could spread along the mains to any other nearby houses which use the same run of mains cable.

Data rate needs

So what of the future?

Jonathan Stott, from BBC Research & Development, says: "There's a lot of interest in transmitting much more information around the home. That means a bigger data rate.

Radio studio
You might not be able to listen to the shortwave BBC World Service
"There are only two things you can do to get a greater data rate over the same means of transmission, in this case the mains - either you send a stronger signal or you occupy a greater bandwidth.

"I don't think they actually can go any stronger, and if they do then it'll only cause more interference in the bands they are affecting. And if they increase the bandwidth then they're going to start interfering with FM radio."

We put these points to the Homeplug Alliance which said it had never been informed of any interference with a licensed frequency user.

It also said that future standards will be flexible enough to accommodate future regulatory requirements.

In other words, no-one has complained to them yet.

And maybe no-one will. Data over power lines does give you access to a high-speed data network that is already installed in your home.

You just might not be able to listen to the radio while you are using it.


Click Online is broadcast on BBC News 24: Saturday at 2030, Sunday at 0430 and 1630, and on Monday at 0030. A short version is also shown on BBC Two: Saturday at 0645 and BBC One: Sunday at 0730 . Also BBC World.



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