July 2003

Casting the Wireless Sensor Net

Smart, networked sensors will soon be all around us, collectively processing vast amounts of previously unrecorded data to help run factories, maintain crops, and even watch for earthquakes.

By Gregory T. Huang

"I didn't know this before, but plants have sex," says Kevin Delin. He's gesturing toward two huge cycads, palmlike fugitives from the Dinosaur Age growing in a corner of the Huntington Botanical Gardens, a sanctuary for 15,000 rare plant species in San Marino, CA. Delin's ignorance of botany is excusable. He's an engineer from NASA's nearby Jet Propulsion Laboratory, and what truly interest him are not the male and female cycads but the pair of "sensor web pods" lodged in the ground under the plants. Each pod is the size of a handheld computer and contains a processor, battery, solar cell, radio, memory, and sensors to monitor heat, humidity, and soil moisture. The pods are the surrogate eyes, ears, and even brains of the garden's curators, keeping track of how much sunlight and rain the plants are getting-critical factors for cycads, which need specific conditions to reproduce.

Sensors are nothing new. A car, for instance, uses dozens of them to monitor factors such as engine conditions. But the sensors in today's automobiles, factories, and office buildings are, for the most part, dumb. They lack the intelligence to analyze or act on their findings; instead, they send measurements back to a central processor. Most current sensors are also stuck in place, with any move requiring expensive rewiring. Delin's pods are different. They talk wirelessly with each other and with 18 other pods in the garden, forming their own intelligent network. Every few minutes, the pods update each other about their latest readings, together process the information into an overall picture of temperature and soil conditions, and send this analysis to the curators. It's as if an autonomous, highly aware computer were spread across 40 hectares of landscape.

"It's all about synthesizing global knowledge from raw data on the fly," says Delin. His pods foretoken a future where smart sensors suck in vast amounts of vital data-say, mechanical stresses on the beams of a bridge, or the rumble of an enemy convoy on a moonless desert night-that currently go unrecorded. Wireless and battery-powered, such sensors will be accessed remotely and put where it would be impractical to string data and power lines. Small and cheap, they will be liberally distributed and closely spaced, yielding fine-grained pictures of phenomena such as climate that are currently charted only on a large scale. And because they will act cooperatively-organizing themselves and sharing computations across the mesh-they will provide people with usable chunks of predigested information rather than a confusing wash of numbers.

Indeed, wireless sensor networks are one of the first real-world examples of "pervasive" computing, the notion that small, smart, and cheap sensing and computing devices will eventually permeate the environment. That notion has been percolating in information technology circles for more than a decade. But now, after several years of research investments by the U.S. Defense Advanced Research Projects Agency, the National Science Foundation, and a handful of high-tech giants like Intel, the hardware and software fundamental to pervasive computing are emerging.

Though the technology is still in its early days, the range of potential applications is mind-boggling (see Sensing the Potential). Scientists at Intel and the University of California, Berkeley, have developed a wireless, pager-sized "chassis" that can be customized with many kinds of sensors. The researchers are using the devices to track microclimates and pests in vineyards, monitor the nesting habits of rare sea birds, and control heating and ventilation systems. And 600 kilometers down the road at the University of California, Los Angeles, other researchers are deploying wireless sensors to gain detailed measurements of the effects of seismic waves on buildings. Still others are working on ways to let businesses monitor and control their work spaces, from local offices to assembly lines half a continent away. "The applications are everywhere," says David Culler, a leading networked-sensing researcher at UC Berkeley.

In the minds of many, it's a technology that could prove as important as the Internet: for just as the Internet allows computers to tap digital information no matter where it's stored, sensor networks will expand people's ability to remotely interact with the physical world. Culler calls the devices "a new class of computer systems," distinguished from the hardware of the past by their ubiquity and their collective analytical skill. Within this decade, he predicts, distributed sensing and computing will creep into every home, building, office, factory, car, street, and farm.

Not surprisingly, there are plenty of challenges before that happens. In many ways, wireless sensor webs are as far along as the Internet was in the 1970s, when the network linked fewer than 200 universities and military labs, and researchers were still experimenting with communications protocols and address schemes. Today, most wireless sensor networks connect fewer than 100 points, or "nodes"; any more and the lines of communication become so tangled that they break down. The cost of the average node is close to $100, while battery life is measured in, at best, months. And no one is exactly sure what application will transform the technology into a commercial bonanza. "Everyone and their aunt and uncle is interested," says Deborah Estrin, director of UCLA's Center for Embedded Networked Sensing. "But it's a struggle to find the business model."

Researchers say none of these problems is likely to be prohibitive. Some wireless sensors are already on the market, and products with intriguing new capabilities could be available within a few years. Sensoria in San Diego, for one, is developing sensors that could turn cars into traveling nodes in urban wireless networks, allowing groups of vehicles to automatically assemble real-time pictures of local traffic or to share communications duties when accessing information about local destinations. William Kaiser, a UCLA electrical engineer and founder of Sensoria, maintains, "The Internet changed how we do business with computers. This will change the way we live our everyday lives."