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1、ZigBee: Wireless Technology forLow-Power Sensor NetworksGary Legg5/6/2004 12:00 AM EDTTechnologists have never had trouble coming up with potential applications for wireless sensors. In a home security system, for example, wireless sensors would be much easier to install than sensors that need wirin
2、g. The same is true in industrial environments, where wiring typically accounts for 80% of the cost of sensor installations. And then there are applications for sensors where wiring isnt practical or even possible.The problem, though, is that most wireless sensors use too much power, which means tha
3、t 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 thats sent through the air, and wireless sensors simply havent looked very appealing.A low-power wireless technology called ZigBee is rewriting the wireless s
4、ensor equation, however. A secure network technology that rides on top of the recently ratified IEEE 802.15.4 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 802.15.
5、4, ZigBee offers battery life of up to several years for common 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, w
6、ireless thermostats, and smoke detectors.APPLICAT ION/ PROFILESAPPLICATION FRAMEWORKNETWORK/SECURTtVLAYERSMAG LAYERPHY LAYER一Albsnce-PlatformEEAppl (eatEoni-! WigB 睥 Platfam Stack SHIcsnFigure 1: ZigBee adds network, security, and application-services layers to the PHY and MAC layers of the IEEE 811
7、.15.4 radioAlthough no formal specification for ZigBee yet exists (approval by the ZigBee Alliance, a trade group, should come late this year), the outlook for ZigBee appears bright. Technology research firm In-Stat/MDR, in what it calls a cautious aggressive forecast, predicts that sales of 802.15.
8、4 nodes and chipsets will increase from essentially zero today to 165 million units by 2010. 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 2010, with 77% of them being ZigBee-relat
9、ed.In a sense, ZigBees bright future is largely due to its low data rates20 kbps to 250 kbps, depending on the frequency band used (Figure 2)compared to a nominal 1 Mbps for Bluetooth and 54 Mbps for Wi-Fis 802.11g technology. But ZigBee wont be sending email and large documents, as Wi-Fi does, or d
10、ocuments and audio, as Bluetooth does. For sending sensor readings, which are typically a few tens of bytes, high bandwidth isnt necessary, and ZigBees low bandwidth helps it fulfill its goals of low power, low cost, and robustness.(3ETA U TI Eof(、H 心 msi2.4 GHzISMWorld wide250 kbpsS6S MHzKu rope加91
11、5 MEI 胃ISMAmcnciB4U kbps1IFigure 2: ZigBees data rates range from 20 kbps to 250 kbps, depending on the frequency usedBecause of ZigBee applications low bandwidth requirements, a ZigBee node can sleep most of the time, thus saving battery power, and then wake up, send data quickly, and go back to sl
12、eep. And, because ZigBee can transition from sleep mode to active mode in 15 msec or less, even a sleeping node can achieve suitably low latency. Someone flipping a ZigBee-enabled wireless light switch, for example, would not be aware of a wake-up delay before the light turns on. In contrast, wake-u
13、p delays for Bluetooth are typically around three seconds.A big part of ZigBees power savings come from the radio technology of 802.15.4, which itself was designed for low power. 802.15.4 uses DSSS (direct-sequence spread spectrum) technology, for example, because the alternative FHSS (frequency-hop
14、ping spread spectrum) would have used too much power just in keeping its frequency hops synchronized.ZigBee nodes, using 802.15.4, can communicate in any of several different ways, however, and some ways use more power than others. Consequently, ZigBee users cant necessarily implement a sensor netwo
15、rk any way they choose and still expect the multiple-year battery life that is ZigBees hallmark. In fact, some technologists who are planning very large networks of very small wireless sensors say that even ZigBee is too power hungry for their uses.A ZigBee network node can consume extra power, for
16、example, if it tries to keep its transmissions from overlapping with other nodes transmissions or with transmissions from other radio sources. The 802.15.4 radio used by ZigBee implements CSMA/CA (carrier sense multiple access collision avoidance) technology, and a ZigBee node that uses CSMA/CA is e
17、ssentially taking a listen-before-talk approach to see if any radio traffic is already underway. But, as noted by Venkat Bahl, marketing vice president for sensor company Ember Corp. and vice chairman of the ZigBee Alliance, thats not a preferred approach. Having to listen burns power, says Bahl, an
18、d we dont like to do that.Another ZigBee and 802.15.4 communications option is the beacon mode, in which normally sleeping network slave nodes wake up periodically to receive a synchronizing beacon from the networks control node. But listening for a beacon wastes power, too, particularly because tim
19、ing uncertainties force nodes to turn on early to avoid missing a beacon.In-Your-Face CommunicationTo save as much power as possible, ZigBee employs a talk-when-ready communication strategy, simply sending data when it has data ready to send and then waiting for an automatic acknowledgement. Accordi
20、ng to Bob Heile, who is chairman of both the ZigBee Alliance and IEEE 802.15, talk-when-ready is an in-your-face scheme, but one thats very power efficient. We did an extensive analysis that led to the best power-saving strategy in various kinds of environments from quiet to noisy, Heile says. We di
21、scovered that, hands down, we were better off just sending the packet and acknowledging it. If you dont 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 its quiet before you talk.Fortunately, this in-yo
22、ur-face strategy leads to very little RF interference. Thats 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 Wi-Fi and Bluetooth modules, can easily deal with such small, infrequent bursts.Z
23、igBees talk-when-ready scheme doesnt suit all purposes, however. For example, in a network 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 network node sending data periodicallyand with transmissio
24、ns repeated numerous times through other nearby nodes of a mesh network configuration in order to reach a network controllerlarge numbers of packet collisions and retransmissions could waste power and significantly shorten sensor node battery life. If the sensor batteries are very small and power-li
25、mited, thats especially problematic.Although contention for airwave access isnt generally a problem for ZigBee, it can be. Sensor-network company Dust Networks, in fact, says contention issues are keeping the company from turning to ZigBeefor now, at leasteven though Dust remains a member of the Zig
26、Bee Alliance. Each ZigBee device needs to contend for airspace with its neighbors, says Dust director of product management Robert Shear, so theres inevitably some contention and some inefficiency. To avoid ZigBees access contention, Dust uses contention-free TDMA (time division multiple access) tec
27、hnology. ZigBee, through the 802.15.4 MAC layer, provides guaranteed time slots in a scheme that somewhat resembles TDMA, but only as part of an optional superframe thats more complex and less power-efficient than TDMA.ZigBee has still more power-saving tricks up its sleeve, however. For example, it
28、 reduces power consumption in ZigBee components by providing for power-saving reduced-function devices (RFDs) in addition to more capable full-function devices (FFDs). Each ZigBee network needs at least one FFD as a controller, but most network nodes can be RFDs (Figure 3). RFDs can talk only with F
29、FDs, not to other RFDs, but they contain less circuitry than FFDs, and little or no power-consuming memory.Rcducrd. Funclkm Duvicc心 r TrevPAN eoorJinarorFull l-unLliiwiFigure 3： ZigBee networks can contain as many as 65,536 nodes in a variety of configurationsZigBee conserves still more power by red
30、ucing the need for associated processing. Simple 8-bit processors like an 8051 can handle ZigBee chores easily, and ZigBee protocol stacks occupy very little memory. An FFD stack, for example, needs about 32 kbytes, and an RFD stack needs only about 4 kbytes. Those numbers compare with about 250 kby
31、tes for the far more complex Bluetooth technology.From ZigBees relatively simple implementations, cost savings naturally accrue. RFDs, of course, reduce ZigBee component costs by omitting memory and other circuitry, and simple 8-bit processors and small protocol stacks help keep system costs down. O
32、ften, an applications main processor can easily bear the small additional load of ZigBee processing, making a separate processor for ZigBee functions unnecessary.But the main strategy for keeping ZigBee prices low is to have big markets and high volumes. The ZigBee Alliance, by making ZigBee an open
33、 standard and by vigorously promoting interoperability among ZigBee devices, expects that ZigBee will be very big in applications such as home and building automation. The alliance is currently working on interoperability procedures for those particular applications, which it expects to complete lat
34、er this year along with ZigBee Specification 1.0.One reason for optimism about ZigBee adoption for home automation and security is its ease of use. ZigBee networks are self-forming, making it easy even for consumers to set them up. In the residential space, theres no configuration involved, says the
35、 ZigBee Alliances Heile. You take something out of the box, put the batteries in, and maybe do something as simple as button-press securitybring two devices close together, push the buttons until the green lights come on, and youre done.ZigBee networks can also self-form in commercial and industrial
36、 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 dont have to have 128-bit public-key encryption for a smoke detector, he says,
37、but if Im in a high-rise office complex, thats exactly the level of security Im going to have for my fluorescent light fixtures. If youre in a high-rise building on Fifth Avenue, you dont want someone going down the street and turning your lights off.Proprietary CompetitionCompetition for ZigBee com
38、es almost entirely from proprietary technologies. Sensor company 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 Z-Wave technology to custome
39、rs. Sylvania, for example, is already using Z-Wave for lighting control, while ZigBee systems remain at least several months away.By offering interoperability, however, ZigBee adds capabilities that proprietary products cant. For example, says Embers Bahl, interoperability allows the ZigBee nodes of
40、 a lighting system to work with the ZigBee network 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 building-automation system.Needless to say, many of the major semico
41、nductor companies, and especially those that are big in embedded systems, are eagerly anticipating ZigBees entry into mass markets. Freescale Semiconductor (until recently known as Motorolas Semiconductor Products Sector) is already providing ZigBee-ready technology to select customers. Other semico
42、nductor companies, including AMI, Atmel, Microchip, Philips, and Renesas, are members of the ZigBee Alliance.ZigBee will likely be slow to penetrate the industrial market for wireless sensors, however. According to market research firm ON World, it will take five to seven years to convince industria
43、l customers of the reliability, robustness, and security of wireless-sensor systems. ON World does predict significant long-term growth of ZigBee in industry, though. By 2010, the company projects, RF modules used in industrial monitoring and control will reach 165 million units, up from 1.9 million
44、 in 2004. About 75% of those, ON World predicts, will be based on ZigBee and 802.15.4.Eventually, ZigBee could go into a wide variety of applications. In household appliances, it could help monitor and control energy consumption. In automotive applications, it could provide tire-pressure monitoring
45、and remote keyless entry. ZigBee could also be used in medical devices or even in computer peripherals, such as wireless keyboards or mice.Concern is increasing, though, that ZigBee could turn into a one-size-fits-all technology that doesnt fit any application particularly well. Some skeptics, for e
46、xample, worry that an attempt to make ZigBee all-encompassing could make the ZigBee protocol stack too large for ZigBees twin goals of very low power consumption and very low cost. If that happens, then ZigBees low-power, low-data-rate nichenarrow as it iswill have proven to be too broad. And then,
47、perhaps, well need yet another wireless standard to go with the burgeoning number we already have.ZigBee :无线技术，低功耗传感器网络加里莱格美国东部时间2004年5月6日上午12:00技师（工程师）们在发掘无线传感器的潜在应用方面从未感到任何困难。例 如，在家庭安全系统方面，无线传感器相对于有线传感器更易安装。而在有线传 感器的装置通常占无线传感器安装的费用80%的工业环境方面同样正确（适用）。 而且相比于有线传感器的不切实际甚至是不肯能而言，无线传感器更具应用性。虽然，无线传感器需要消耗
48、更多能量，也就是说所需电池的数量会随之增加或 改变过于频繁。再加上对无线传感器由空气传送的数据可靠性的怀疑论，所以无 线传感器看起来并不是那么吸引人。一个低功率无线技术被称为ZigBee，它是无线传感器方程重写，但是。一个 安全的网络技术，对最近通过的IEEE 802.15.4无线标准（图1）的顶部游戏机， ZigBee的承诺，把无线传感器的一切从工厂自动化系统到家庭安全系统，消费 电子产品。与802.15.4的合作下，ZigBee提供具有电池寿命可比普通小型电池 的长几年。ZigBee设备预计也便宜，有人估计销售价格最终不到3美元每节点，。 由于价格低，他们应该是一个自然适应于在光线如无线交
49、换机，无线自动调温器， 烟雾探测器和家用产品。APPLICATION/PROFILESAppllctEonAPPLICATION FRAMEWORKNETWORK/SECURTIYLAYERSG LAYERPHY LAYER ZigBee Platform Stack SiliconZigBeer Alhanc?PlatformIEEE（图1）虽然还没有正式的规范的ZigBee存在（由ZigBee联盟是一个贸易集团，批 准应该在今年年底），但ZigBee的前景似乎一片光明。技术研究公司 In-Stat/MDR在它所谓的“谨慎进取”的预测中预测，802.15.4节点和芯片销售 将从今天基本上为零，增加
