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 packets of data 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.