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.