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Turning the world into a sensor network

Carnegie Mellon projects show you need more than a battery-powered radio

By , Network World
August 11, 2010 11:16 AM ET
CMU-Firefly

Network World - Try to imagine a "world littered with trillions" of wireless sensors. Now try to imagine the problems getting even a few thousand of them to work together in any kind of intelligible way so you can know if that interstate bridge is near collapse or the natural gas pipe behind a housing development has a crack in it or how dropping your AC temperature by 3 degrees during peak demand will clobber your electric bill.

Wireless sensor net helps UC-Berkeley boost data center efficiencies by 20%

Those are the problems that a new research project at Carnegie Mellon University (CMU) is going to explore. It has, as most such government-industry-academia joint efforts do, the cumbersome name of Pennsylvania Smart Infrastructure Incubator (PSII). The basic idea: Bring together some smart people, give them state of the art facilities and communications, and ask them to wrestle with how to build and run really big sensor networks that can deliver useable information.

CMU already has a lot of practical experience in sensors. It's launched an internal project called Sensor Andrew, which is gradually adding a wireless sensor infrastructure burrowed into every campus building. So far, Sensor Andrew reaches five buildings on the Pittsburgh campus, each using the networks for different purposes, from tracking locations of people to warning that a printer is still using maximum power, due to a low-toner alert, instead of shutting down.

The campus sensor network makes use of homegrown technology: a low-cost wireless mesh node called FireFly, and a real-time operating system specifically designed for such networks. Like other similar products, FireFly uses an IEEE 802.15.4 transceiver, good for 150 to 300 feet. It has a maximum raw data rate of 250Kbps and an 8-bit microcontroller, and SD Flash card slot, to process data from four optional on-board sensors: light, audio, temperature, humidity, acceleration.

What's different is that the FireFly node also has a low-power AM/FM radio receiver. That radio can pick up a periodic time synchronization pulse, from an AM carrier current transmitter that can flood an eight-story building with the signal. This kind of synchronization makes possible very energy-efficient operation, and extends the battery life of each node by a factor of four or five, according to CMU. The pulse enables precise scheduling of data transmits and receives, leaving the nodes "sleeping" the rest of the time.

The use of a real-time OS, called Nano-RK, for FireFly reflects the embedded systems background of Ragunathan Rajkumar, a professor with CMU's Department of Electrical and Computer Engineering, who oversaw FireFly's development. The RTOS creates a "bounded" system, with a high degree of predictability. What's more, tasks can specify their differing resource demands, and Nano-RK creates guaranteed, controlled access to resources like CPU cycles and network packets. Temperature changes slowly; audio requires a much higher sampling rate. "We can deal with multiple sensors at once," Rajkumar says. "Each sensors operates at its own ‘natural’ frequency."

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