Busy as a ZigBee

Now that you've finally installed Wi-Fi, get ready for a second home network--one that will control the house itself

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.

A ZigBee node is a simple thing. At its heart is a multichannel two-way radio and microcontroller all on a single piece of silicon, tucked inside a plastic package about the size of your pinkie fingernail. Finished products certainly will contain more than just the ZigBee chip. Some, like the light switch, will have little more than a toggle and a faceplate; but others, such as a thermostat, will have additional relays, LCDs, and other features.

ZigBee networks form autonomously, so that when the homeowner installs a second device, it seeks out the first, which is likely to be an initial controlling switch for the network. The third device, when it is installed, will find whichever of the first two nodes is closer, and so on. By default, all the devices after the first one must be authenticated before they are allowed to use the existing devices as a communications ”backbone” to relay messages back and forth.

Authentication might involve simply the push of a button, although it can take a more sophisticated form when the device has a display and a keyboard. In practice, the network is usually configured to require the homeowner’s approval before adding a device, if only to keep a neighbor’s light from joining the network by mistake. After the device has been added, the network is able, on its own, to ask it what it is and what services it can deliver.

Is a ZigBee network something that you, the homeowner, can set up? It depends on how much of a do-it-yourselfer you are. To add a peel-and-stick light switch, you simply replace the old wall switch with a ZigBee-enabled one, and then slap a peel-and-stick switch controller onto the wall. Upgrading the old wired wall switch to a ZigBee version is easy to do using only a screwdriver (remember to shut off the power first!).

Next, you ”introduce” the two by pushing a button on one and then quickly pushing the corresponding button on the other. The two switches work together from that point on. (There eventually will be lighting fixtures that use ZigBee natively, but initially it will be done in the replacement way.) If, however, you want to install new ZigBee-enabled latches on your home’s windows and exterior doors, you probably will want to call in a locksmith.

ZigBee is designed for low-duty-cycle, terse messaging between devices that might spend much of their life asleep. The theoretical maximum data rate, only 250 kilobits per second, is still fast enough to minimize the ”on” time of either the transmitter or the receiver; a typical conversation between two devices generally takes a few milliseconds, allowing the transceiver to go back to sleep quickly.

Using the same spectral band as Wi-Fi, ZigBee was developed by many of the same people involved with the other IEEE 802 standards, such as 802.11 for Wi-Fi and 802.16 for WiMax. There’s only so much license-free spectrum available, and the 802 standards take advantage of much of it, as do many other technologies. Although cost-effectiveness was high on the list for ZigBee’s architects, just as important were robustness, reliability, and noninterference in noisy RF environments.

The designers of the radio portion of the standard looked for the most robust modulation method available, eventually choosing Quadrature PSK, the same technique that NASA has used for the past two decades to communicate with the deep-space crafts Voyager and Galileo, as well as the Mars rovers. The technique is based on the idea of phase-shift keying (PSK) modulation, in which the regularly oscillating peaks and troughs of a radio wave are suddenly jumped half a cycle, so that a peak comes when a trough is expected. The direction of the ”discontinuous jump in phase” indicates the reception of either a 1 or a 0, a discontinuity so sharp that it can be detected even in inhospitable environments.

Quadrature PSK goes a step further. Instead of each phase transition carrying a single 1 or 0, it can carry two bits of information: 00, 01, 10, or 11, allowing twice as much information to arrive in the same amount of time. QPSK is remarkably resilient, but as a modulation method, it was rarely used in inexpensive systems due to its high complexity and cost. Indeed, only advances in RF silicon technology during the past decade have made it practical for consumer systems.

The ZigBee protocol names devices in the network with a system employing 64 bits—more than enough to provide a unique address to each grain of sand on California’s beaches. Once a device is installed, the address gets mapped into a local 16-bit network address, so a ZigBee network can have more than 64 000 devices active. That’s certainly enough for your home and garden.

Even in a network with thousands of devices, all connected at once to the network, most will have sensors that are asleep for much of the time, awakening only to report in or to indicate an event. The underlying packet-radio technology ensures that the message to be transmitted is received successfully, no matter how busy the channel may be. Because a ZigBee radio can check the channel more than a thousand times per second, it’s able to step around bandwidth congestion from Wi-Fi, cordless phones, and microwave ovens. Once it sends its packets, it goes back to sleep.

Unlike many existing systems in the wired and wireless worlds, ZigBee was designed from the start to resist hackers and other threats—whether from the neighbor’s precocious teenager or from a burglar driving down the street trying to pick the electronic locks of any unprotected houses. The IEEE standard uses the Advanced Encryption Standard, a tried-and-true form of public-key cryptography. The system takes advantage of proven security key distribution and management solutions from commercial vendors, who will add an entire suite of security functions. The technique involves not just sending keys through the network securely, but also letting the homeowner or a trusted vendor log into the network, add new sensors, and update the network’s software.

ZigBee radios and processors have to be small in size and low in wattage to be successful, but that’s not enough. Like other networking standards, they will have to be compatible with other devices that adhere to the standard. Otherwise, ZigBee will never achieve the widespread use that allows manufacturers to enjoy economies of scale. So an industry consortium, the ZigBee Alliance, hopes to come up with requirements for compliance and a test methodology with the standard and for RF compatibility—just as IEEE 802.11 has the Wi-Fi Alliance and 802.16 has the WiMax Forum.

The not-for-profit ZigBee Alliance, in San Ramon, Calif., was incorporated in 2002 by many of the same people involved in the underlying IEEE standard, and it now has about 200 member companies. They include traditional makers of control systems (such as Honeywell, Johnson Controls, and Siemens), new control systems companies (Control4 Corp. of Salt Lake City), industry-specific companies (Danish HVAC manufacturer Danfoss Group Global and Swedish lock maker Assa Abloy), ZigBee-specific start-ups (Daintree Networks in Australia, Orange Logic in South Korea, and San Juan Software, located near Seattle), and semiconductor companies (Freescale Semiconductor, STMicroelectronics, and Texas Instruments). A number of consumer electronics manufacturers and telecommunications carriers are members as well.

The retail price of radio transceivers compliant with the standard is rapidly approaching $1, and single-package radio/applications processor/memory products are currently about $3, prices that will continue to fall as volume increases.

ZigBee is not the only option for home control, but the alternatives all fall short of it in one way or another. X10 is probably the most commonly used, but it’s primarily a wire line–based technology with a few wireless capabilities and a small number of available addresses to identify each object—a drawback that generally limits the size of an X10 network to 16 devices.

Then there’s Bluetooth, an increasingly popular short-range wireless networking technology that, at least in theory, could be recrafted to support hundreds of devices in a typical residential environment. Unfortunately, it was never designed to support small, battery-operated devices over years of operation, so the modifications needed to use Bluetooth for home control would amount to designing an entirely new radio technology.

Bluetooth also suffers from the same lack of addressing space as X10—in fact, all existing wireless technologies that can be used for home control have this limitation. So on a Bluetooth home network, devices would have to constantly drop out and rejoin, yielding a several-second delay between, say, flipping a light switch and the light coming on.

The first ZigBee products are expected to show up at electronics stores and home improvement centers next year. We estimate that the peel-and-stick light switch, if it’s among the first wave, will cost about $20 for the controlling switch unit and $15 for the slap-on unit. Right now, that’s about $5 less than comparable units based on X10 technology—a healthy discount, in percentage terms, but not a huge difference.

Those prices should come down quickly, though, because of the economies of scale and competition from lots of vendors. And the ZigBee unit will be much more functional than its alternatives. For example, the motion sensor that notices when someone walks into a room can both start up the air conditioning and turn on the lamp, no matter who manufactured the two appliances. In the same vein, smoke detectors can turn on the lights and turn off the air-handling unit, improving your chances of escaping a burning building. Until now, such integrated home control was difficult, expensive, or both.

Will ZigBee technology transform markets such as the one for home security, which until now has relied on systems that are proprietary and self-contained? Will professional installers soon be able to choose latches from Assa Abloy, alarms from Siemens, or controllers from Honeywell? It’s hard to say, but ZigBee at least makes it all possible.

While we’ve mainly discussed home use of ZigBee, the technology was designed to have the robustness demanded by commercial and industrial users as well. Such users, facing a small fraction of the costs of installing a new wire-line network, are likely to find that ZigBee can pay for itself many times over simply by lowering energy costs through more precise control of HVAC systems.

Nonetheless, commercial and industrial customers will need some time to adopt the ZigBee systems. Because business networks will have thousands of nodes, their systems will be planned, installed, and maintained by professionals, much as computer networks are installed and run by a dedicated information technology staff. So, ZigBee probably will find its way into homes first, with products that the average homeowner, locksmith, or plumber can install.

Your ZigBee network eventually will be able to talk to you while you’re out of town, either by connecting to your home’s Internet line or by sending text messages to your cellphone. If cold weather moves into town while you’re away, ZigBee-enabled water-pipe sensors in the crawl space can let you know; perhaps you’ve left a key with a next-door neighbor whom you can call to turn up the heat a notch and rescue your plumbing from a terrible fate. Your cellphone will itself have ZigBee technology inside, allowing you to interact directly with your home when you’re inside the house or out in the yard, using the phone as a flexible and powerful remote control.

The ZigBee-enabled home will spend less on heating and air conditioning, and will be more secure and comfortable. And that’s no small thing.

About the Authors

JON ADAMS is director of radio technology at Freescale Semiconductor in Tempe, Ariz. BOB HEILE is the chairman of both the ZigBee Alliance and of the IEEE 802.15 working group on personal area networks. He is based in Boston.

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