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Ad Hoc Hops


December 3, 2003 By

Remember the cell phone owners who lined up at pay phones throughout the Northeast during last summer's blackout? TV images of anxious subscribers who couldn't call home made one thing clear: Traditional wireless networks can fail in an emergency.

When a power outage, earthquake or malicious attack knocks out service at cellular towers, it renders handsets useless. The same goes for wireless devices that communicate with wired transceivers, whether they're voice radios, mobile e-mail platforms or notebook computers with Wi-Fi cards. While a breakdown in commercial service triggers frustration, a similar failure in public-safety communications can have dangerous consequences.

An alternative wireless model might help first responders and other government employees exchange information without a fixed infrastructure. It could also extend communications to places that can't be reached by signals from radio towers.

Called a mobile ad hoc network (MANET), this technology establishes high-speed, peer-to-peer communications among mobile devices, each of which doubles as a router. When one device needs to send information to another, low-powered radio signals relay the data in short "hops" from one node to the next until it reaches its destination.

The system calculates the most efficient route for each data packet, changing that route on the fly as the devices move about. MANETs are designed to carry any sort of digital information, including text, voice, graphics and video.

The MANET concept grew out of research funded in the 1970s and 1980s by the U.S. Defense Advanced Research Projects Agency (DARPA). The goal then was to allow military troops to instantly establish voice and data communications where there was no fixed radio infrastructure.

In the mid-1990s, researchers also started to explore nonmilitary uses in emergency response and disaster relief, said Nader Moayeri, manager of the Wireless Communication Technologies Group at the National Institute of Standards and Technology (NIST).

Better Coverage in Buildings

NIST is one of numerous organizations, including government agencies, universities and corporations, that have conducted research on MANETs. NIST worked for a while on developing an ad hoc network for outdoor use, which could be used, for example, in the event of a hurricane, said Moayeri. "After September 11, we concentrated on doing that type of thing inside a building."

As the world saw during rescue efforts at the World Trade Center, conventional radio systems can't reach the higher floors of tall buildings, he said. "But if you have this natural relaying mechanism, you can penetrate. You can have coverage in larger buildings, and you extend the range of your radio."

Any mobile computing device that accepts the required communications card can serve as a node on a MANET. NIST has developed a pilot network using HP iPAQ Pocket PCs with 802.11b WLAN cards, with added software to handle MANET-specific functions. One protocol, for instance, determines the best route for a given transmission. A voice over IP establishes voice communications.

NIST also included location software, using triangulation techniques to determine the position of each device on the network. A commander at an emergency scene could use this to track responders within a building, Moayeri said. "This would be useful when it comes to saving lives, and also in terms of coordinating the activities of these people," he said.

Along with the mobile nodes, NIST's pilot network includes a fixed communications station to provide a connection to the outside world. In real-world use, this might be a computer in a command vehicle, linked to the Internet via satellite. Responders inside the building could place phone calls by routing communications through this link, Moayeri said.

NIST successfully tested a MANET composed of 30 devices, with 10 to 15 of them communicating at once. It's not yet clear whether MANETs can be built to support larger numbers, Moayeri said.


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