【正文】 enready is an inyourface scheme, but one that39。t get an ack, it just means you got clobbered, so send it again. You wind up having much better power management than if you listen and determine if it39。s largely because ZigBee nodes have very low duty cycles, transmitting only occasionally and sending only small amounts of data. Other ZigBee nodes, as well as WiFi and Bluetooth modules, can easily deal with such small, infrequent bursts. ZigBee39。t suit all purposes, however. For example, in a work of thousands of tiny sensors dropped into a war zone to monitor enemy troop movements, the power savings provided still might not be enough. With each work node sending data periodically—and with transmissions repeated numerous times through other nearby nodes of a mesh work configuration in order to reach a work controller—large numbers of packet collisions and retransmissions could waste power and significantly shorten sensor node battery life. If the sensor batteries are very small and powerlimited, that39。t generally a problem for ZigBee, it can be. Sensorwork pany Dust Networks, in fact, says contention issues are keeping the pany from turning to ZigBee—for now, at least—even though Dust remains a member of the ZigBee Alliance. Each ZigBee device needs to contend for airspace with its neighbors, says Dust director of product management Robert Shear, so there39。s access contention, Dust uses contentionfree TDMA (time division multiple access) technology. ZigBee, through the MAC layer, provides guaranteed time slots in a scheme that somewhat resembles TDMA, but only as part of an optional superframe that39。s relatively simple implementations, cost savings naturally accrue. RFDs, of course, reduce ZigBee ponent costs by omitting memory and other circuitry, and simple 8bit processors and small protocol stacks help keep system costs down. Often, an application39。s no configuration involved, says the ZigBee Alliance39。re done. ZigBee works can also selfform in mercial and industrial settings, but professional installers will have tools that provide additional control, particularly for security. ZigBee security is flexible, says Heile, to give both consumer and professional users what they need. You don39。m in a highrise office plex, that39。m going to have for my fluorescent light fixtures. If you39。t want someone going down the street and turning your lights off. Proprietary Competition Competition for ZigBee es almost entirely from proprietary technologies. Sensor pany Dust, as noted, is sticking with its own technology, and Ember, although pushing strongly into the ZigBee arena, plans to keep offering its proprietary EmberNet as well. In addition, Zensys is providing its ZWave technology to customers. Sylvania, for example, is already using ZWave for lighting control, while ZigBee systems remain at least several months away. By offering interoperability, however, ZigBee adds capabilities that proprietary products can39。s Bahl, interoperability allows the ZigBee nodes of a lighting system to work with the ZigBee work of an HVAC system, or vice versa. Philips Lighting is really excited about this, Bahl, says, because it turns them from a ballast manufacturer into the infrastructure backbone of a buildingautomation system. Needless to say, many of the major semiconductor panies, and especially those that are big in embedded systems, are eagerly anticipating ZigBee39。s Semiconductor Products Sector) is already providing ZigBeeready technology to select customers. Other semiconductor panies, including AMI, Atmel, Microchip, Philips, and Renesas, are members of the ZigBee Alliance. ZigBee will likely be slow to perate the industrial market for wireless sensors, however. According to market research firm ON World, it will take five