A Network That Builds Itself

Original Article

Mesh together: This prototype
relay node, developed by NIST,
features an LED that automatically
changes from green to red whenever
a new node needs to be set down.
Credit: NIST

Building an on-the-fly wireless communications networks is a vital part of firefighting, handling hostage situations, and dealing with other emergencies. But it is difficult to build such networks quickly and reliably.

Soon these emergency wireless networks could help build themselves. The National Institute of Standards and Technology (NIST) recently presented details of two experimental networks that tell emergency workers when to set down wireless transmitters to ensure a good signal.

Ad hoc wireless networks relay messages between transmitters, or nodes, without requiring any central control. But as things stand, emergency workers simply follow suggested guidelines for building such a wireless network--placing each node 15 or 30 meters apart and at key points, like the corners of a building. Or they periodically check back with the command center to make sure they're still in touch. Neither method is terribly efficient in an emergency, however. The process can also be costly if a large number of nodes are used.

The NIST prototypes, which have been under development for more than three years, use algorithms to monitor the signal-to-noise ratio of transmissions and automatically warn when a new node should be set down.

"We didn't want to have fixed rules, because there can be a lot of metal in walls or cinder block," meaning signal strength varies building to building, says Nader Moayeri, a senior technical advisor in NIST's Advanced Network Technologies Division. "Plus, you don't want to deploy too many, because of the cost factor as well as potential for communication delays."

Moayeri says that NIST considered having nodes ping each other with short messages to see how many messages were lost in transit. The problem with this approach is that the person deploying the network would not detect a weak connection immediately and might have to backtrack. Having an algorithm measure the signal-to-noise ratio instead avoids this problem and provides a clearer picture of connection strength.

NIST built two prototype networks using off-the-shelf hardware. One operates at 900 megahertz and uses Crossbow MICA2 Motes to transmit radio signals. The other, a Wi-Fi network operating at 2.4 gigahertz , uses Linux-based Gumstix transmitters. But Moayeri says that the NIST algorithm should work with any wireless hardware and on any available spectrum.

In the Crossbow system, each node has an LED that automatically changes color, from green to red, when a new node needs to be set down. The Gumstix system issues alerts via a handheld or tablet computer connected to the same wireless network.



Each hardware platform has different strengths and weaknesses. The Crossbow system can be customized easily but has a maximum data transfer speed of 35 kilobits per second, limiting the network to text messaging. The Gumstix system is less flexible but can transfer data at 54 megabits per second, allowing users to talk and send other data over the network. Both types of node measure approximately five by ten centimeters and cost between $200 and $300.

Moayeri's team tested the Crossbow network in an 11-story building on the NIST campus in Gaithersburg, MD, deploying 11 nodes in the stairwell. The Gumstix network was tested throughout another NIST building that goes 40 feet belowground and features winding corridors as well as a number of metal doors. A total of eight nodes were used to cover about 300 meters.

Moayeri says that the maximum transmission power for both prototypes was about 100 milliwatts. Since a typical police or firefighter radio transmits at one to five watts, far fewer nodes would be needed in a real-world scenario. However, it's not clear how much it will cost to make rugged and fireproof nodes.

A potential downside of the NIST prototype is that it does not include the ability to track location, unless it is in a building that already has passive RFID chips installed.

Moayeri and his colleague Michael Souryal presented details of the two prototype networks at the third annual Precision Indoor Personnel Location and Tracking for Emergency Responders technology workshop held at Worcester Polytechnic Institute in early August.

Their presentation caught the interest of one workshop attendee--Alan Kaplan, chief technology officer at Drakontas, a company based in Glenside, PA, that makes communications software for public safety and security operations. His firm's software currently requires users to check connections between nodes as they are deployed. "What I thought was cool is that the technology seemed to help users as they built out this network, telling where they should actually place these nodes," says Kaplan. "Potentially, this is something that anyone who does public safety or security would want."