Catching Intruders in a WiFi Net

Curious as to why the strength of their wireless network varied even when no one was using it, researchers in the Department of Computer

Science decided to test it in the most controlled environment they could think of--a bank vault. The vault's thick metal walls would block any possible sources of interference, theoretically leaving a completely static environment. What they found in that vault may open the door to revolutionary new security and tracking systems using existing wireless networks.

Inside the vault, the wireless fidelity, or WiFi, network emitted a completely constant signal. That is until one of the researchers walked in.

Professor and inventor Ashok Agrawala, graduate research assistant Moustafa Youssef and doctoral candidate Leila Shahamatdar realized that their network was detecting the presence of a person--a quality they could harness to turn a WiFi network into a security system.

Now, Agrawala, Youssef and Shahamatdar have created software that does just that. They call their new security technology "Nuzzer," after the Farsi word for "to look."

WiFi networks, which are finding increasing applications in both commercial and private settings, use radio frequencies (Rf) to transmit packets of information between access and receiving points at a rate of up to 10 times per second. Each packet carries an additional piece of information, which is a measure of the strength of the Rf signal. These Rf signals behave differently in different environments--responding to the presence of building materials, furniture and other objects.

Nuzzer system for different environments by creating a map of what the signal strength is expected to be at any given point in that environment. By comparing the actual signal strength at a receiving point to the map, researchers can identify anomalies caused by the presence of people.

"To a WiFi network, humans all look like big bags of water. And water is not a good conductor of radio frequencies," Agrawala explains.

Nuzzer's current application is as a kind of motion detector, but, by using multiple access and receiving points, researchers envision being able to use the technology as a tracking system as well. "We should be able to determine the position of a person within a few feet," says Agrawala. Though a tracking system is still some time away, Nuzzer's current motion detection application could be commercially available within 12-18 months.

Nuzzer's inventors say it has a number of advantages over current security technology, primarily because it doesn't require any additional hardware. The system can be implemented wherever a WiFi network exists. Even receiving points such as laptops are not a necessary part of the equation because access points can double as receiving points. Other advantages of the system are that it is easy to implement, and, because it is already integrated with the Internet, it is able to provide many options for communications, says Agrawala. "Nuzzer could automatically notify you of an intruder by e-mail, even if you're halfway around the world--not that there's very much you could do about it from there."