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Wireless sensor networks have found their way into a myriad of applications from grape vineyards to factories to office buildings. Because no single wireless technology currently fits all situations, numerous radios and protocols have appeared, muddying the waters for engineers who need to develop these networks. Standards-based technologies including WiFi, ZigBee, WiMAX, and Bluetooth, plus a host of proprietary protocols, are all in use today.

You might think that engineers would opt for a well-defined standard technology such as WiFi rather than turn to a proprietary method. “WiFi ICs and systems are practically commodities because they're easy to implement,” said Dan Piroli, senior VP at Point Six Wireless. “Proprietary technologies are more difficult to deploy, and WiFi-based sensor networks work within a building's infrastructure.” He also noted that IEEE 802.11b/g-based networks can use Internet Protocol.

While Piroli may be right about WiFi being a near commodity, it may not be the best technology at locations where AC power isn't readily available. Farms, vineyards, groves, and environmental monitoring sites, for example, often lack AC power.

“WiFi is a high-power user,” said Robert Robinson, VP of wireless sales and marketing at Crossbow Technology. The client (sensor) and host (hub) need constant contact, and WiFi nodes need constant power, making them less than ideal for battery-powered operations. “We would like to use WiFi,” said Robinson, “but it won't work in areas without AC power. We don't fully implement the current ZigBee description because it calls for powered routers.”

Wireless sensor networks can record measurements in locations without AC power. Courtesy of Point Six Wireless.

As a result of the deficiencies of WiFi and ZigBee, companies such as Crossbow and Digi have developed their own wireless protocols and the hardware to run them. Crossbow's wireless network runs over an IEEE 802.15.4 physical layer, the same as ZigBee, but it's designed to let all nodes go to sleep when not in use. The nodes, therefore, can run for years on a single battery.

Recognizing the power problem in WiFi networks, companies such as GainSpan and G2 Microsystems have begun developing low-power WiFi-compatible ICs. These devices should run for years in sensor networks and other applications such as asset tracking. The figure shows a WiFi-based wireless network.

If ultra-low-power WiFi nodes work as promised, they will solve some of the issues regarding a lack of AC power—as long as the signal can reliably reach a powered access point. That should make WiFi a strong competitor to ZigBee, but it won't solve the problem in large unpowered areas such as agricultural fields. They need low-power, battery-powered hubs that can “sleep” when not in use. They also must let the nodes form their own mesh network to pass data to and from the host without a network hub.