You’re enjoying a rare bit of peace and quiet, perhaps dozing on a train, when someone begins a loud cellphone conversation nearby. Electromagnetic countermeasures are available to reclaim the silence, but for the most part they are illegal. In the United States, for example, only federal agents can legally use signal jammers to block cellular communications by transmitting interfering signals. Even state prison officials, who want jammers to prevent inmates from talking on contraband cellphones, are barred from using them.
This aspect of U.S. law has created quite a stir in recent years. Things boiled over in January, when lawyers for the CTIA, a wireless-industry association, petitioned the courts to forbid even a 30-minute test of a cellphone-jamming system in a Washington, D.C., prison. On the flip side, legislation proposed in Congress in January would legalize cellphone jamming in prisons.
One person who believes he has a better solution for prisons, concert halls, or wherever cellphone communications are prohibited is J. David Derosier, a founder of Cell Block Technologies, in Fairfax, Va. ”I went out to a nice dinner with my wife—it was supposed to be romantic—and the guy at the table next to us started yelling at his kid on his cellphone,” Derosier says of the incident that inspired him to create the start-up. After briefly considering the jamming of all cellphone frequencies, he decided to find a more discerning way to prevent most cellphone conversations, while allowing emergency calls to get through.
Derosier’s prototype convinces nearby handsets to think it’s the cellular base station with the best signal. But instead of handling most calls, the unit fools phones into operating on a channel with no service. Derosier says his system could be used legally in many parts of the world, including the European Union, although not in the United States.
Aaron Dow, an R&D engineer for Alcatel-Lucent in Wellington, New Zealand, and his colleagues have tried a similar approach to block cellphones at one prison, using a full-blown cellular base station installed nearby—”a honey pot” as he calls it. The idea is to make sure the phones all connect with this station, which then doesn’t service the calls.
But this approach is expensive—up to several hundred thousand U.S. dollars, according to Dow. Costs should plummet, though, with a technology that seems destined to become ubiquitous: femtocells. These are small cellular base stations that residential consumers and businesses will likely begin buying en masse in the next year or so to improve cellular coverage indoors. Any cellphone in the vicinity of a femtocell will connect to it wirelessly, and in turn the femtocell will hook up with the mobile carriers’ networks through a broadband Internet connection. ”If we could adapt the femto solution to do the blocking, it would be much more attractive,” says Dow.
Indeed, someone could employ a femtocell to limit the use of cellphone handsets, not just in prisons but in any building. The gadget itself is just a wireless access point, notes Manish Singh, vice president for product-line management at Continuous Computing, a San Diego–based company that provides, among other things, software for femtocells. A suitably smart server attached to the femtocell could screen calls in all sorts of ways. ”You can block the traffic right then and there,” says Singh.
A business setting up such a system would have the technical ability to permit or deny calls to or from cellphones located within its walls—but would that practice be allowed? In contrast to Wi-Fi routers, femtocells operate in a licensed portion of the radio spectrum, limiting what sorts of functions vendors will be permitted to offer, which is why Singh regards such filtering not as a technological challenge but as ”a regulatory issue.” And it’s a thorny one at that.