Spectrum and transmission range. ZigBee uses the same unlicensed 2.4-GHz (worldwide), 868-MHz (in Europe), and 915-MHz (in the Americas) radio bands as many cordless telephones. These bands work well for low-cost sensor networks because users don't have to obtain licenses and the spectrum is widely available.
The technology's transmission ranges are 10 meters indoors and 200 meters outdoors for devices using the 2.4-GHz band, and 30 meters indoors and 1,000 meters outdoors for devices using the other bands, according to Dutton.
Data rates. ZigBee transmits data at a relatively slow 250 Kbits per second at 2.4 GHz, 40 Kbps at 915 MHz, and 20 Kbps at 868 MHz, which works well for simple sensor systems that transmit only small amounts of data occasionally, said Orlik. This is considerably slower than other wireless connectivity technologies such as Bluetooth and far too slow for other wireless purposes such as transmitting real-time video.
Security. For security, ZigBee uses 128-bit Advanced Encryption Standard cryptography and trust-center-based authentication, according to David McCartney, vice president of sales and marketing for software vendor Airbee Wireless. A trust center admits devices to a network and hands out ZigBee device and network authentication keys, he explained.
Nodes. Theoretically, ZigBee can support up to 65,536 nodes. However, supporting this many nodes makes network administration unmanageable. To avoid this problem, Dutton said, ZigBee networks should have no more than 3,000 nodes. For situations requiring more nodes, he explained, multiple subnets would work better than trying to make a single large network even bigger.
Low latency. ZigBee is optimized for time-critical applications that won't tolerate latency, such as lighting and power-management controls. ZigBee devices don't have to synchronize with other nodes on a network before joining and beginning communications. Thus, devices require only 30 milliseconds to join a network and 15 milliseconds to access and start communicating with other nodes.
Energy efficiency. Because ZigBee performs just a few specific, simple tasks, the protocol stack can be as small as 28 Kbytes. Thus, system controllers need less memory and consume less power.
In addition, ZigBee devices transmit small amounts of simple data, like those from sensors, which minimizes the amount of energy the systems use, Dutton noted.
Also, he said, ZigBee activates devices only when they're needed to pass or act on data, which also contributes to power efficiency.
ZigBee can thus let batteries in devices run for a long time, which is critical for install-and-forget sensor-related tasks such as wireless monitoring and control systems.