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A Wireless Sensor City

A wireless-sensor network to report pollution and traffic comes to Cambridge, MA.
April 13, 2007

Engineers at Harvard and BBN Technologies are working on a project that will cover the city of Cambridge, MA, with wireless-sensor nodes mounted to telephone poles that could allow researchers to see the specific locations and times of day when pollution peaks. The researchers could also track the city’s weather with more precision and help test new wireless technology for better Wi-Fi. The network, called CitySense, will be an open test bed on which anyone can run experiments, says Matt Welsh, a professor of computer science at Harvard.

Sensor City: This image shows hypothetical locations for wireless sensor nodes in the city of Cambridge, MA. Ultimately, 100 sensor nodes mounted on telephone poles could collect data on pollutants, weather, and traffic. Researchers have already installed such nodes on Harvard’s campus and near BBN Technologies’ corporate offices, but they plan to add about 90 more throughout Cambridge. One of the first projects to use the network will monitor airborne pollution near industrial sites.

Sensor Node: This sensor node, on a rooftop at BBN Technologies, is composed of sensors (left) and computer hardware–which includes a PC that runs the Linux operating system and two Wi-Fi radios–housed in a weatherproof case (white box on right).

The plan is to install 100 general-purpose nodes onto the streetlights of Cambridge, drawing power from the city’s infrastructure. Already there are five installed on Harvard’s campus and five at BBN’s facilities. Each node will be relatively large–about the size of a Mac Mini computer. A node will include a PC that runs the Linux operating system and a couple of gigabytes of flash memory as a hard drive. And instead of using a common low-power wireless-sensor protocol called Zigbee, CitySense nodes will use standard Wi-Fi radios; two radios will be in each node, one for management and control of the network, and the other for experiments. And, Welsh says, virtually any type of sensor will be able to connect to the nodes.

A first batch of sensors will collect weather data such as rainfall, wind speed, and barometric pressure. Another set of sensors will measure pollution such as the amount of particles in the air. Researchers could use the weather data to understand how temperature or wind speed vary throughout the city, and doctors could use the pollution data to advise patients with asthma to stay away from certain areas at certain times of day. Eventually, more sensors could be incorporated: for example, motion sensors could measure traffic flow, and light sensors scattered throughout the city could monitor parking spaces; the data would be uploaded to the CitySense network. “With something like CitySense,” Welsh says, “we’re going to be able to blanket the city with sensors and get a much more complete sense of what’s going on.”

Welsh expects that CitySense will, in addition to collecting and transmitting sensor data, be employed by computer scientists to test new network software and protocols, which could be used to help make Wi-Fi connections more robust. Currently, the only way to test new wireless protocols, says Welsh, is to run them on a computer simulation. But CitySense could be thought of as “an open laboratory,” he says, where researchers can upload and run their programs, collect data, and write papers.

The payoff of having an openly available wireless network like CitySense could be great, says Thomas Little, a professor of electrical and computer engineering at Boston University. “The existence of a wireless backbone like CitySense becomes an enormous asset,” he says. “There are very interesting opportunities to exploit,” he adds, including business opportunities. He envisions being able to integrate all sorts of sensors into the CitySense backbone, such as those that track the position of public transportation–which could help people know when the next bus is coming–and even video cameras that could monitor anything from traffic to urban wildlife.

For now, says Welsh, his team has no plans to integrate video cameras into the network. However, he believes that the project is a great opportunity to explore the social ramifications of collecting data on an entire city. “CitySense is a good way to come face-to-face with the questions of what it means to outfit a city like this,” he says.

In recent years, wireless-sensor networks have gained more prominence as a commercially viable technology thanks to companies such as Dust Networks and Arch Rock, both University of California, Berkeley, research spinoffs. The most common types of sensor networks are often found in industrial settings, where they monitor manufacturing equipment in hard-to-reach places. For the most part, these sensor networks employ tiny, battery-powered sensor “motes” designed to use little power and collect specific types of data.

Building a wireless backbone across an entire city that can accommodate numerous, simultaneous research problems will be tough, says Joshua Bers, a researcher who leads BBN’s effort on the CitySense project. One challenge will be to make the network reliable enough by keeping the hardware from failing. “Clearly, if researchers are going to be using it, we don’t want this thing to crash and have to have someone go up a light pole 30 feet off the ground to fix it,” Bers says. There’s one way to safeguard against this, he says: there will be software that will monitor the health of the node, and if something goes wrong, it will automatically reboot.

In addition, CitySense will use “mesh” networking to send data from one point in the network to another point far away. In such a networking scheme, information is transferred to its final destination by hopping from node to node instead of being transmitted directly. BBN has developed a number of mesh-networking protocols that adjust when nodes fail.

Another issue that will need to be addressed, says Welsh, is hardware resource allocations. Since numerous projects and various software applications will be running on the same hardware simultaneously, the researchers will need to find a way to make sure that the processors and memory are divvied up appropriately.

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