Features

French to authorise "magic" cellphone jamming technology - "It won't work" say experts

Back in October, we noticed a strange report that said France was going to allow cinemas and theatres to block signals to cellular phones. We couldn't help wondering: how will this work? What about emergency calls (for example) to the fire brigade?

Cellular jammers are widespread. Illegal in most areas, they have been used in hotels  to prevent guests from making unprofitable (for the hotel) calls on their own handsets, and they have been widely asked for in theatres.

But back last year, the BBC discovered that "The French government has backed a move to install equipment to block signals in cinemas, concert halls and theatres," and it quoted Jean Labbe, president of the National Federation of French Cinemas, as saying that the measure followed "a long-standing request" by cinemas.

The idea certainly pleased some elements of the public! As one blogger put it: "Holy shit: France is doing something right. They're pushing to allow movie theaters to block cell phone signals. Now it's time for the U.S. to follow suit. Next up? A portable, personal cell phone jammer for TOM SHERMAN to use on annoying ass clowns who talk on the train. Yessss." - a not atypical reaction, in fact.

Labbe, or at least one of his colleagues, was aware of the problem of emergency calls, however: "Emergency phone calls and calls outside the performance area will still be permitted."

Oh, yes? And how, exactly?

A jammer isn't magic. It works quite simply by producing a signal on the same frequency as the one you're trying to jam - a bit like getting all the cars in a drive-in cinema to turn their headlights on and point them at the screen.

According to Computency CEO  Bill Pechey, "It's dead easy to make a GSM jammer but, as you suggest, it's virtually impossible to allow emergency calls through."

Pechey explained: "When a GSM phone registers with the network there is an authentication handshake which is proprietary to each network. At the same time the encryption is set up. Both traffic and signalling channels are encrypted. Only the network operator could block non-emergency traffic and yet allow emergency calls through."

In theory, of course, it would be possible for each cellphone network to install a pico-cell - a local transmitter/receiver - inside the cinema. The cell could restrict access to emergency calls. The cost, however, could be discouraging.

Another technical possibility is one analogous to the trick used by  Airbus Industries. That is, set up a picocell which is proprietary to the area, and link it through roaming agreements to all the other networks.

The trick  works in a plane - but a plane has a few technical advantages over a cinema.

First, most cinemas aren't metal cages into which a phone signal has trouble penetrating.

Next, most cinemas are not 40,000 feet up in the air where the ground cells are relatively faint and far away, and easily drowned out by the local transmitter: they're in the city, surrounded by powerful cells. To drown them out, the picocell would have to be the same sort of power as the ones in the street. The Mast Sanity people would go simian.

Finally, the Airbus technique actually uses a jammer. You aren't allowed to call it that, because Airbus gets very exasperated with you if you do, but that's what it is: a device which produces a very low-level signal just strong enough to drown out any signal from the ground without screening the local picocell.

Try that in a cinema, and you'll blot out all local phone signals for several hundred metres in each direction.

To work, it seems the new device would have to be specially installed in the cinema or theatre when it was built. You'd have to build a Faraday cage  - a wire mesh into the bricks and mortar (or concrete and clay) of the building. You'd also have to build a similar mesh into the glass of the windows, and across ventilator ducts - any open port bigger than the wavelength of a cellphone is enough to let the Orange sun shine in.

And then you could use the Airbus technology, and it would work.

"If France has legalised jammers then the law has no effect if no one can make anything that can fit within it - typical French approach!" summarised Pechey. "You may remember that way back, when they were worried about foreign VCRs flooding the French market, they made a regulation that they all had to come through the customs office in Perpignan which was manned by a couple of blokes and a dog..."

The BBC report appears to have been compiled by the cinema industry reporters. Time to turn the high-tech IT staff on it!