Back at the Huntington gardens, Delin enters a conference room bearing an aluminum briefcase, the kind government agents on TV use to carry top-secret gadgets. He takes out four of his latest sensor pods and pries the cover off of one; underneath are circuit boards holding the pod’s guts, including the microprocessor and the radio transceiver that lets it communicate with its companions. He spreads the pods around the room, and within seconds they locate one another and self-organize into a wireless network that monitors temperature and humidity, among other things. A nearby pod-though any of them would do-forwards information from the network to Delin’s laptop for display. To show how the network reacts to its environment, Delin disconnects one of the devices. The laptop screen shows the remaining pods compensating by routing data around the missing pod. He attaches an electric fan to one pod, then holds another pod in his hand; the network detects Delin’s body heat and switches on the fan.
The pods’ ability to communicate by radio, Delin explains, means that they can be scattered in areas that phone and power lines don’t reach and moved around at will. But to get data flowing, nodes must find their neighbors automatically and set up radio connections. Those connections can change rapidly, says Delin, so sharing data over the network is a juggling act. Software running on all of the pods coordinates which of them talk to one another and when. The sensor nodes “listen” for one another and set up times to share data, while a network clock keeps the nodes in sync. The network resembles a mesh rather than the hub-and-spoke arrangement used for cell phones; instead of linking each sensor directly to a central communication point, the nodes send data only to neighbors within radio range, saving power.
It sounds complicated, and it is. But decentralized wireless networks like Delin’s are already cost effective for heavy industry: Ember in Boston, MA, has sold similar technology to customers frustrated with the conventional wired sensors in their manufacturing or heating and ventilation equipment. One customer used to line the pipes of its treatment plant-where oil and gas are separated from wastewater-with expensive wired temperature sensors, attached to heaters that keep the fluid inside from becoming too thick. If a sensor malfunctioned, a tank could burst, forcing the plant to shut down at a cost of $100,000 per hour, says Robert Poor, Ember’s cofounder and chief technology officer. With a wireless network, more sensors can be installed at an affordable price, offering redundancy and yielding more reliable information. “Silicon is cheap. Wiring is not,” Poor says. (For more of Poor’s thoughts on the technology, see Sensors of the World, Unite!).
Several remaining problems, however, obstruct broad commercial application of the technology. The first is its high power consumption. The periodic talk back and forth between the nodes, in particular, is a drain on batteries. “Every bit transmitted brings a sensor node one moment closer to death,” says Greg Pottie, a Sensoria cofounder.
A related issue is that sensor nodes’ radios have a limited range, usually in the tens of meters. So networking a bigger space-say, a large factory-takes a lot of nodes. Numerous nodes sending lots of data create opportunities for localized failures that could leave parts of the network isolated, says Rick Kriss, CEO of San Diego-based Xsilogy. “There’s no such thing as a reliable network, unless you do very aggressive network management,” Kriss says. So Xsilogy’s nodes periodically broadcast their status, letting the network know if their batteries are running low or their reception is fading. Then the network can compensate by routing around the failure points and alerting the user to impending problems.
But there’s another problem that is harder to work around, and that’s price. In a process that is the very opposite of mass production, most sensor-net makers still cobble together off-the-shelf parts by hand, raising the cost of each node into the $80 to $100 range. That price needs to drop below $20 in order for sensor nets to take off commercially, says David Tennenhouse, director of research at Intel.
Standardization could help. “Having open standards and many disinterested groups testing competing approaches will absolutely make or break whether this becomes widely used,” says UC Berkeley’s Culler. But with so many companies and university labs developing their own prototypes, design standards for wireless sensors and networking protocols are only beginning to emerge. One potentially dominant design is called a “mote”; its operating system, TinyOS, was developed by Culler’s group at Berkeley and is undergoing further refinements at Intel and Crossbow Technology in San Jose, CA. The Berkeley motes, which have been tested by hundreds of research groups around the world, are smaller and use less power than most commercial wireless sensors. The trade-off is that they can’t process as much data. But many researchers say their adaptability-it’s easy to snap on sensors for light, sound, temperature, or movement, say-makes them the networked-sensor world’s equivalent of a Windows PC.
In fact, the eventual choice of a wireless-sensor platform could be just as consequential as the emergence of Windows as the dominant consumer operating system-or even, in the eyes of one expert, as the standardization of electricity. “It is sort of like the historic battle between AC and DC,” says Larry Smarr, director of the California Institute for Telecommunications and Information Technology in San Diego. “Until there was a ubiquitous winner, the electrical-appliance industry couldn’t take off.”