They tell inventors to build a better mousetrap, but what we really need is a better light switch. Think about how one works: the light itself might be in the middle of the ceiling, but you have to run a wire to the doorway if that’s where you want to be standing when you turn the light on.
It’s all so 20th century! Today, with wireless radio technology, you ought to have a switch that you could slap on the wall with double-sided tape. That way, if you wanted to place it lower so a houseguest in a wheelchair could reach it, you’d just peel it off, move it down, and slap it back on the wall.
Buildings have had wired control networks for decades, and electrical engineers have been turning wired networks into wireless ones ever since the time of Marconi. It wasn’t until 1998, though, that a now-defunct industry organization, the HomeRF Working Group, started designing a system that would allow for such things as a slap-on light switch. For various reasons, the half-dozen or so existing wireless standards were not ideal for the job, so researchers began fashioning a new one.
In 2000, the IEEE started its own project, IEEE 802.15.4, for wireless personal area networks, or WPANs. The resulting standard, which was released in 2003 and updated earlier this year, is more euphonically known as ZigBee 1.0. The name is commonly understood to evoke the zigzag dance by which honeybees guide their hive-mates to flowers—a metaphor for the way devices on the network find and interact with one another. (The truth is somewhat more prosaic: the working group was mainly looking for an available domain name built out of a couple of short, easy-to-remember words.) Compliant products are expected to buzz their way into stores within a year.
Like IEEE 802.11, known more commonly as Wi-Fi, ZigBee is a local area networking technology that blankets a home with wireless coverage [see illustration]. But in other ways, the two standards couldn’t be more different. Wi-Fi uses a ton of power to provide a torrent of data, while ZigBee uses almost none to provide a trickle. Wi-Fi uses a single central router to radiate its coverage, whereas ZigBee builds up coverage out of small nodes that join together into a network.
It all has to do with intended applications. ZigBee is connecting light switches, not multimedia entertainment centers, so it can easily manage with one-fiftieth the Wi-Fi data rate. And with ZigBee, some nodes will be situated under floors and in other hard-to-reach places, so even though they draw little power, they need to have batteries aboard. A single ZigBee control point can run on a pair of alkaline AA batteries for years.
ZigBee’s possibilities go far beyond light switches. For several years there have been sensor technologies that could transform the home. [See the July 2004 IEEE Spectrum special report, ”Sensor Nation.”] What we haven’t had, until now, is a cost-effective way to use them.
For example, sensors on the windows and doors of your house can wirelessly report several times each minute to a central security controller that ”all is well”—a level of security that, until a few years ago, only a museum or a mansion in a James Bond movie might have. Sensors in each room can monitor human presence, temperature, humidity, and light levels, and then send the data to a heating, ventilation, and air conditioning (HVAC) controller. The controller first figures out whether anybody is in a given room, and only then decides whether to raise or lower the window shades, open or close the air dampers, or run the HVAC system at full or reduced power. Rooms that are occupied can be maintained within a narrow comfort zone, while the temperature in vacant rooms is allowed to move up or down through a wider range.
Meanwhile, the cat door will check your pet’s radio-frequency ID tag, letting kitty back into the house while keeping out the neighborhood raccoons. Sensors in the vegetable garden will constantly check soil moisture and temperature, insolation, and air temperature, turning on the sprinkler system as needed.
Control systems doing some of these things have existed for years in office buildings and factories. Cabling was strung throughout the building, connecting costly sensors to central servers. The idea of rewiring a home to do this was absurd, though—workers would have to drill holes and scramble around an attic or wiggle through a crawl space. Often, walls would have to be opened up. After the wire-pulling exercise was completed, extensive cosmetic repairs would begin.
Although the cable itself might cost only about US $0.33 per meter, the work of wiring a control network can easily set you back $22 per square meter of floor space. Building such a system into a new house costs half as much as retrofitting an existing one, but it still isn’t cheap. The average new house in the United States has floor space of 220 square meters, requiring about 300 sensor nodes; the structured wiring alone would cost about $350 per room.
You can try to get around the installation costs by using a house’s existing ac power wiring, and there are systems that do just that. However, you can reach only to those areas that have access to the electrical system, and you can use them only if they are close enough to the control unit to keep the noise on the power wiring from drowning out the signal. In a ZigBee network, messages are reliably received and are unaffected by the vagaries of RF propagation and absorption, including messages from far-flung outposts such as the garage windows and the soil sensors in the yard.