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QuickStudy: ZigBee

15.05.2006
With its oddball name, you might expect ZigBee to be a carbonated beverage, a child's toy or perhaps some sort of cult. In fact, it's yet another type of wireless network. Any wireless networking technology inevitably represents a compromise among a number of competing and conflicting characteristics, including power consumption, reliability, range of operation, speed of communications, cost, security, and the size and type of network connectivity. Thus, the specific data communications needs of any particular application will dictate which wireless technologies are likely to work most effectively.

ZigBee is designed for relatively short distances, low-cost implementation and extremely low power consumption. It supports connectivity over a mesh network topology, which makes for exceptional reliability and path redundancy and virtually unlimited scalability to any number of connected devices. These specialized characteristics make ZigBee particularly well suited for use in building automation and in industrial, medical and residential control and monitoring applications. If this new protocol achieves the sort of widespread acceptance that Wi-Fi has, in a few years we might see ZigBee-based wireless light switches, smoke and carbon-monoxide detectors, thermostats, home security devices, remote controls for consumer electronics equipment, medical sensing and monitoring units, and even motorized blinds and window shades.

Technical underpinnings

ZigBee starts with the IEEE 802.15.4 packet radio standard, which defines the low-rate physical layer and the media access control portion of the data link layer. It communicates over unlicensed areas of the spectrum that cordless telephones and microwave ovens use. Most chips and modules use the 2.4-GHz band. A few make use of the 900-MHz band, which offers longer range and greater penetration through walls.

To this direct-sequence spread-spectrum radio specification, ZigBee adds a network layer that incorporates ad hoc self-organizing multi-hop routing. ZigBee can be configured in three different network topologies -- star, peer-to-peer and mesh -- but it is the latter that offers the most promise. Just as the Internet does, a mesh network provides multiple pathways from device to device, ensuring that a message will get through. If nodes stop working or are taken out of the network, ZigBee signals can still use other pathways to reach their destinations, which dramatically improves overall reliability by eliminating a single point of failure. ZigBee was designed to coexist with hostile radio frequencies, such as those typically found in industrial and commercial applications, which adds to its reliability.

Depending on the frequency band used, ZigBee devices can pass data and messages at rates of 20K to 250Kbit/sec. and have a range of 10 to 400 meters from one device to the next in the mesh. A security toolbox for ZigBee includes the use of access control lists, packet freshness timers, and encryption based on the AES-128 standard certified by the National Institute of Standards and Technology.