Mains data 'threat to aircraft'

Data-over-the-mains systems that promise home networks operating at up to 200Mbits/sec using standard household wiring could present a major threat to aircraft communications, according to a BBC expert.

There may be a way to avoid them interfering with public and amateur radio reception at ground level - but most current technologies do not appear to use it, says Jonathan Stott, who has written a number of papers on the subject for the BBC’s R&D Labs.

XDSL links, which, like Power Line Communication (PLC), send a high-frequency signal over lines designed for low frequencies, offer some protection against interference. Mains wiring scores top marks for convenience but is not a good choice for passing data because it behaves exactly like a transmitting aerial.

Indeed tests have shown that 14Mbits/sec links based on the Homeplug specification used in most systems curently on sale in Britain are as much wireless as wired.

A German researcher found that both the data connection and the interference it caused continued even after he pulled the plug out...the unplugged wire was acting as a transmitting aerial and the far end as a receiver.

Stott, who duplicated the effect, said this might be the case even with the wire plugged in. ‘The wire is not designed to do what it is called upon to do and there will be a lot of attenuation of the signal passing along it. I suspect that in some circumstances the data is being transmitted more by the RF than by the wire.’
Homeplug avoids frequencies used by radio amateurs but can interfere with other broadcasts.

New technologies about to hit the market offer speeds of up to 200Mbits/sec; Panasonic has talked of installing its PLC technology into all its home devices, so they can be networked simply by plugging them into the mains.

Britain’s Ofcom watchdog says devices sold here have to conform to the European EMC directive that says interference must ‘allow radio equipment to operate as intended’. A spokesman said: ‘Anyone who sells devices that do cause interference knows that we would soon put a stop to their activities.’

But Stott says the directive, and proposals for governing DSL and PLC interference, do not go far enough. Individual installations, might pass EMC tests but he has fears about the cumulative effect of hundreds or even thousands of installations. ‘It’s not so much interference on the ground because there are buildings in the way to block it. But there is nothing to stop interference reaching aircraft.’

Aircraft cannot escape by gaining height because though the interference from any one source gets weaker, more of them become visible. Many of the frequencies used by aircraft are in the sub-30MHz range used by PLC systems.