【正文】 device needs to contend for airspace with its neighbors, says Dust director of product management Robert Shear, so there39。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。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 9 WiFi and Bluetooth modules, can easily deal with such small, infrequent bursts. ZigBee39。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 control node. But listening for a beacon wastes power, too, particularly because timing uncertainties force nodes to turn on early to avoid missing a beacon. InYourFace Communication To save as much power as possible, ZigBee employs a talkwhenready munication strategy, simply sending data when it has data ready to send and then waiting for an automatic acknowledgement. According to Bob Heile, who is chairman of both the ZigBee Alliance and IEEE , talkwhenready is an inyourface scheme, but one that39。s not a preferred approach. Having to listen burns power, says Bahl, and we don39。s hallmark. In fact, some technologists who are planning very large works of very small wireless sensors say that even ZigBee is too power hungry for their uses. A ZigBee work node can consume extra power, for example, if it tries to keep its transmissions from overlapping with other nodes39。s power savings e from the radio technology of , which itself was designed for low power. uses DSSS (directsequence spread spectrum) technology, for example, because the alternative FHSS (frequencyhopping spread spectrum) would have used too much power just in keeping its frequency hops synchronized. 8 ZigBee nodes, using , can municate in any of several different ways, however, and some ways use more power than others. Consequently, ZigBee users can39。s data rates range from 20 kbps to 250 kbps, depending on the frequency used Because of ZigBee applications39。t necessary, and ZigBee39。s technology. But ZigBee won39。t looked very appealing. A lowpower wireless technology called ZigBee is rewriting the wireless sensor equation, however. A secure work technology that rides on top of the recently ratified IEEE radio standard (Figure 1), ZigBee promises to put wireless sensors in everything from factory automation systems to home security systems to consumer electronics. In conjunction with , ZigBee offers battery life of up to several years for mon small batteries. ZigBee devices are also expected to be cheap, eventually selling for less than $3 per node by some estimates. With prices that low, they should be a natural fit even in household products like wireless light switches, wireless thermostats, and smoke detectors. Figure 1: ZigBee adds work, security, and applicationservices layers to the PHY and MAC layers of the IEEE radio 7 Although no formal specification for ZigBee yet exists (approval by the ZigBee Alliance, a trade group, should e late this year), the outlook for ZigBee appears bright. Technology research firm InStat/MDR, in what it calls a cautious aggressive forecast, predicts that sales of nodes and chipsets will increase from essentially zero today to 165 million units by 2021. Not all of these units will be coupled with ZigBee, but most probably will be. Research firm ON World predicts shipments of 465 million wireless sensor RF modules by 2021, with 77% of them being ZigBeerelated. In a sense, ZigBee39。t practical or even possible. The problem, though, is that most wireless sensors use too much power, which means that their batteries either have to be very large or get changed far too often. Add to that some skepticism about the reliability of sensor data that39。中文 7130 字 第一章 Development of Wireless Communication Introduction Nowadays, our world is full of wireless products. Such as WiFi, cell phone, GPS and so forth are all over the world, which make our life much more convenient. However, most people only know nothing about wireless munication except for cellphone. What’s more, they also have no idea about the history of wireless munication, though they serve for our national defense as well as daily life. In fact, it costs a long period of time in history to gain such achievements in wireless munication. Several talented scientists push this field forwards step by step and their names and work deserve to be remembered by us. In history, the first and the most fundamental discovery that was related to wireless munication was Maxwell39。s equations, which were put forward by Maxwell. Buy solving those equations, he predicted that electromagic wave exist in the universe, further, he demonstrated light also is a kind of electromagic wave. The next great scientist is Hertz. He found electromagic wave in experiment, who also strongly demonstrated Maxwell’s prediction. The discoveries above are the most important and essential events in the history of wireless development. Also, photophone, radio, 3G are all the essential steps in the wireless progress. What’s more, like other industry, wireless munication industry also experienced its own golden and bleak age. Classification There are lots of methods to connect two or more points to sen