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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换机，无线自动调温器，烟雾探测器和家用产品。
（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，增加到2010年的165万台。不是所有这些单位都将与ZigBee结合，但大多数可能会。世界研究公司预测的到2010年射频模块无线传感器出货量4.65亿美量，其中77％是ZigBee的相关。
从某种意义上说，ZigBee的光明前途在很大程度上是由于其较低的数据速率20 kbps到250 kbps的，用于取决于频段频率（图2），比标称1 Mbps的蓝牙和54的802.11g Mbps的WiFi功能，或文件和音频，蓝牙一样。对于发送传感器的读数，这是典型的数万字节数，高带宽是没有必要，ZigBee的低带宽有助于它实现其目标和鲁棒性的低功耗，低成本。
由于ZigBee应用的是低带宽要求，ZigBee节点大部分时间可以睡眠模式，从而节省电池电源，然后醒来，快速发送数据，回去睡眠模式。而且，由于ZigBee可以从睡眠模式过渡到15毫秒或更少主动模式下，即使是睡眠节点也可以达到适当的低延迟。有人扳动支持ZigBee的无线光开关，例如，将不会是一个唤醒延迟知道前灯亮起。与此相反，支持蓝牙唤醒延迟通常大约三秒钟。
一个ZigBee的功耗节省很大一部分来自802.15.4无线电技术，它本身是为低功耗设计的。802.15.4采用DSSS（直接序列扩频）技术，例如，因为（跳频扩频）另类医疗及社会科学院将在保持一样使用它的频率过大的权力同步。
ZigBee节点，使用802.15.4，是几个不同的沟通方式之一，然而，某些方面比别人拥有更多的使用权力。因此，ZigBee的用户不一定能够实现传感器网络上的任何方式选择和他们仍然期望多年的电池寿命是ZigBee的标志。事实上，一些技术专家打算用小型无线传感器创建大的网络，即使功率ZigBee的电池需求很大。
一个ZigBee网络节点可以消耗额外的功率，例如，如果它试图避免与其他节点的传输或与其他无线电源传输重叠的传输。那么在ZigBee 802.15.4无线电的使用实现CSMA / CA（载波侦听多址接入冲突避免）技术，与ZigBee节点使用CSMA / CA是基本上采取了听先于谈话的方式，看是否有无线电通信已经展开。但是，正如所指出的Venkat Bahl，传感器营销公司恩贝尔公司副总裁兼ZigBee联盟的副主席，这不是一个首选的方法。“有听意见的权力，”Bahl说，“我们不喜欢这样做。”
ZigBee和802.15.4通讯的另一个选择是指路明灯模式，通常睡觉模式醒来网络节点定期接收同步“灯塔”从网络的控制节点。但是，对于一个灯塔听废物力量，也因为时间的不确定性，特别是支配节点打开，以免错过早期一盏明灯。
争议中的通信
为了尽可能节省电力ZigBee采用一种简单交际策略,talk-when-ready发送数据时,数据准备派遣然后就等着自动确认。根据鲍勃Heile,两ZigBee联盟主席和电子802.15,talk-when-ready是“开门见山地”计划,但却是一种很电力有效率。“我们在广泛的分析,导致了最好的节能策略从各种环境安静喧闹的,”Heile说。“我们发现,手了,好,我们在发送才离开那包东西和承认它。如果你不想让他ack讯息,它只表示你惨败,所以重发给你。你有更好的电源管理，并确定它是否安静，然后再谈谈。”
幸运的是，这种当面策略导致RF干扰非常小。这主要是因为ZigBee节点具有非常低的占空比，只偶尔传输发送少量的数据。其他ZigBee节点，以及WiWave的，而ZigBee系统保持在至少数个月。
通过提供互操作性，但ZigBee的补充能力，专利产品不能。举例说，Ember的义巴尔，互操作性允许照明系统的ZigBee节点的工作，在一个空调系统的ZigBee网络，反之亦然。“飞利浦照明是真的对这个很兴奋，”义巴尔说，“因为原来从一到建筑物的自动化系统的基础设施骨干镇流器生产厂家他们。”
不用说，主要的半导体公司很多，尤其是那些在嵌入式系统公司中大都热切期待ZigBee的投入并且大规模进入市场。飞思卡尔半导体（直到最近，摩托罗拉半导体产品部称）已经提供ZigBee-ready技术来选择客户。其他半导体公司，包括AMI，爱特梅尔，微芯片，飞利浦，瑞萨，都是ZigBee联盟的成员。
ZigBee可能是缓慢渗透到无线传感器的工业市场，但是。据对世界市场研究公司，它会需要五至七年来说服客户在工业上的可靠性，耐用性，以及无线传感器系统的安全。并显著预测在整个世界中ZigBee将长期在工业制造上有增长，因此。到2010年，公司项目，射频模块，应用于工业监控和控制得将达到1.65亿台，同比增长190万元，在世界性预测中，在2004年。大约75％的将基于ZigBee和802.15.4。
最终，ZigBee的可进入各种广泛的应用。家用电器，它可以帮助监测和控制能源消耗。在汽车应用中，它可以提供轮胎压力监测和远程无钥匙进入系统。也可用于ZigBee的医疗设备中，甚至在计算机外围设备，如无线键盘或鼠标。
值得关注的是越来越多，虽然，ZigBee的可能变成一种适合所有的技术的尺寸，并不很适合任何应用程序。一些持怀疑态度，例如，企图使ZigBee无所不包的可能使ZigBee协议栈太大，ZigBee的双重目标是非常低功耗和非常低的成本。如果出现这种情况，那么ZigBee的低功耗，低数据速率利基窄，如果它是，将被证明是过于宽泛的。然后，也许我们会需要另一种无线标准，以配合我们已经有的蓬勃发展的人数。
加里莱格是一家位于波士顿的自由撰稿人。他拥有电子工程学士学位，曾任编辑，EDN杂志执行编辑
ZigBee: Wirele Technology for Low-Power Sensor Networks Gary Legg
5/6/2004 12:00 AM EDT
Technologists have never had trouble coming up with potential applications for wirele sensors.In a home security system, for example, wirele sensors would be much easier to install than sensors that need wiring.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 isn't practical or even poible.The problem, though, is that most wirele 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 that's sent through the air, and wirele sensors simply haven't looked very appealing.A low-power wirele technology called ZigBee is rewriting the wirele sensor 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 wirele sensors in everything from factory automation systems to home security systems to consumer electronics.In conjunction with 802.15.4, ZigBee offers battery life of up to several years for common small batteries.ZigBee devices are also expected to be cheap, eventually selling for le than $3 per node by some estimates.With prices that low, they should be a natural fit even in household products like wirele light switches, wirele thermostats, and smoke detectors.Figure 1: ZigBee adds network, security, and application-services layers to the PHY and MAC layers of the IEEE 811.15.4 radio Although 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 aggreive” forecast, predicts that sales of 802.15.4 nodes and chipsets will increase from eentially 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 wirele sensor RF modules by 2010, with 77% of them being ZigBee-related.In a sense, ZigBee's bright future is largely due to its low data rates—20 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-Fi's 802.11g technology.But ZigBee won't be sending email and large documents, as Wi-Fi does, or documents and audio, as Bluetooth does.For sending sensor readings, which are typically a few tens of bytes, high bandwidth isn't neceary, and ZigBee's low bandwidth helps it fulfill its goals of low power, low cost, and robustne.Figure 2: ZigBee's data rates range from 20 kbps to 250 kbps, depending on the frequency used Because 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 sleep.And, because ZigBee can transition from sleep mode to active mode in 15 msec or le, even a sleeping node can achieve suitably low latency.Someone flipping a ZigBee-enabled wirele light switch, for example, would not be aware of a wake-up delay before the light turns on.In contrast, wake-up delays for Bluetooth are typically around three seconds.A big part of ZigBee's 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-hopping 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 can't necearily implement a sensor network any way they choose and still expect the multiple-year battery life that is ZigBee's hallmark.In fact, some technologists who are planning very large networks of very small wirele sensors say that even ZigBee is too power hungry for their uses.A ZigBee network node can consume extra power, for example, if it tries to keep its transmiions from overlapping with other nodes' transmiions or with transmiions from other radio sources.The 802.15.4 radio used by ZigBee implements CSMA/CA(carrier sense multiple acce collision avoidance)technology, and a ZigBee node that uses CSMA/CA is eentially 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, that's not a preferred approach.“Having to listen burns power,” says Bahl, “and we don't 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 network's control node.But listening for a beacon wastes power, too, particularly because timing uncertainties force nodes to turn on early to avoid miing a beacon.In-Your-Face Communication To save as much power as poible, 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.According 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 that's 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 discovered that, hands down, we were better off just sending the packet and acknowledging it.If you don'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 it's quiet before you talk.”
Fortunately, this in-your-face strategy leads to very little RF interference.That'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 Wi-Fi and Bluetooth modules, can easily deal with such small, infrequent bursts.ZigBee's talk-when-ready scheme doesn't 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 periodically—and with transmiions repeated numerous times through other nearby nodes of a mesh network configuration in order to reach a network controller—large numbers of packet collisions and retransmiions could waste power and significantly shorten sensor node battery life.If the sensor batteries are very small and power-limited, that's especially problematic.Although contention for airwave acce isn't generally a problem for ZigBee, it can be.Sensor-network company Dust Networks, in fact, says contention iues are keeping the company 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 there's inevitably some contention and some inefficiency.” To avoid ZigBee's acce contention, Dust uses contention-free TDMA(time division multiple acce)technology.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” that's more complex and le power-efficient than TDMA.ZigBee has still more power-saving tricks up its sleeve, however.For example, it 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 FFDs, not to other RFDs, but they contain le circuitry than FFDs, and little or no power-consuming memory.Figure 3: ZigBee networks can contain as many as 65,536 nodes in a variety of configurations ZigBee conserves still more power by reducing the need for aociated proceing.Simple 8-bit proceors 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 kbytes for the far more complex Bluetooth technology.From ZigBee's relatively simple implementations, cost savings naturally accrue.RFDs, of course, reduce ZigBee component costs by omitting memory and other circuitry, and simple 8-bit proceors and small protocol stacks help keep system costs down.Often, an application's main proceor can easily bear the small additional load of ZigBee proceing, making a separate proceor for ZigBee functions unneceary.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 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 later 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, there's no configuration involved,” says the ZigBee Alliance's Heile.“You take something out of the box, put the batteries in, and maybe do something as simple as button-pre security—bring two devices close together, push the buttons until the green lights come on, and you're done.” ZigBee networks can also self-form in commercial and industrial settings, but profeional installers will have tools that provide additional control, particularly for security.ZigBee security is flexible, says Heile, to give both consumer and profeional users what they need.“You don't have to have 128-bit public-key encryption for a smoke detector,” he says, “but if I'm in a high-rise office complex, that's exactly the level of security I'm going to have for my fluorescent light fixtures.If you're in a high-rise building on Fifth Avenue, you don't want someone going down the street and turning your lights off.” Proprietary Competition Competition for ZigBee comes 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 customers.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 can't.For example, says Ember's Bahl, interoperability allows the ZigBee nodes of 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.” Needle to say, many of the major semiconductor companies, and especially those that are big in embedded systems, are eagerly anticipating ZigBee's entry into ma markets.Freescale Semiconductor(until recently known as Motorola's Semiconductor Products Sector)is already providing ZigBee-ready technology to select customers.Other semiconductor 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 wirele sensors, however.According to market research firm ON World, it will take five to seven years to convince industrial customers of the reliability, robustne, and security of wirele-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 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-preure monitoring and remote keyle entry.ZigBee could also be used in medical devices or even in computer peripherals, such as wirele keyboards or mice.Concern is increasing, though, that ZigBee could turn into a one-size-fits-all technology that doesn't fit any application particularly well.Some skeptics, for example, worry that an attempt to make ZigBee all-encompaing could make the ZigBee protocol stack too large for ZigBee's twin goals of very low power consumption and very low cost.If that happens, then ZigBee's low-power, low-data-rate niche—narrow as it is—will have proven to be too broad.And then, perhaps, we'll need yet another wirele standard to go with the burgeoning number we already have.About the Author Gary Legg is a Boston-based freelance writer.He holds a BSEE degree and is a former editor and executive editor of EDN magazine.He can be reached at gary@garylegg.com.
