Monday, November 5, 2007

Jamming Cell Phones

Jamming cell phone signals is illegal in the United States but not illegal in other countries. As a result, people in the United States are finding and buying these devices on the Internet. The New York Times ran an interesting piece on November 2 titled Devices Enforce Silence of Cellphones, Illegally. The piece starts with the following:

One afternoon in early September, an architect boarded his commuter train and became a cellphone vigilante. He sat down next to a 20-something woman who he said was “blabbing away” into her phone.

“She was using the word ‘like’ all the time. She sounded like a Valley Girl,” said the architect, Andrew, who declined to give his last name because what he did next was illegal.

Andrew reached into his shirt pocket and pushed a button on a black device the size of a cigarette pack. It sent out a powerful radio signal that cut off the chatterer’s cellphone transmission — and any others in a 30-foot radius. “She kept talking into her phone for about 30 seconds before she realized there was no one listening on the other end,” he said.

I've written about wireless signal jamming in the past - specifically when I wrote about the New England Patriots video taping scandal that included rumors of wireless communications signal jamming. I have to admit, while watching the Colts/Patriots game yesterday and seeing Tom Brady struggle hearing plays using his helmet headset, jamming was on my mind. It got to the point plays were being relayed in the old fashioned way - was it crowd noise or could it have been signal jamming?

Let's take a basic look at the technology. Communications technicians and engineers are constantly aware of something called the signal-to-noise ratio, commonly abbreviated as SNR. The noise we are typically dealing with in unjammed situations is commonly referred to as Johnson or White Noise and it's always there. If systems are designed properly and when in range, communications signals are stronger than the noise and the noise is less significant because the SNR is high (think divide signal strength by noise strength). Basically, if a wanted communications signal is stronger than the noise then communications happens. If the noise is stronger than the communications signal then communications does not happen. We've all experience this in our cars while driving and listening to a radio station. As we get closer to a station signal source (antenna), the signal gets stronger and we hear the station clearer.

Now back to jammers - in simplest terms, jammers work by sending out strong signals that overpower the wanted communications signals between a cell phone and cellular antenna towers. Basically they generate the equivalent of noise at specific cell frequencies. Phones end up not communicating with the towers and users see a "No Service" type message on their phones.

A quick search of the Internet brings up a number of off-shore companies selling signal jammers.
One of the more popular companies is London based sells lots of different jamming devices ranging from a $149 portable jammer that runs on 9V batteries and has a range of 5m to
a $3995 ultra high power phone jammer which is described as the most sophisticated digital cell phone jammer of its class, with tough die-cast aluminum casing and dual inter cooler, ideal for large hall type rooms or outdoor locations. It comes with a high gain base station type antenna.

The New York Times quotes's site operator as follows:

"Victor McCormack, the site’s operator, says he ships roughly 400 jammers a month into the United States, up from 300 a year ago. Orders for holiday gifts, he said, have exceeded 2,000."

I'll write more about different kinds of wireless signal jamming this week. Remember - the devices are illegal in the United States.

1 comment:

Anonymous said...
This comment has been removed by a blog administrator.