无线技术低功耗传感器网络—中英文翻译.doc

ZigBee: Wireless Technology for Low-Power Sensor NetworksTechnologists 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 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 isnt 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 thats sent through the air, and wireless sensors simply havent looked very appealing.A low-power wireless technology called ZigBee is rewriting the wireless 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 wireless 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 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 network, security, and application-services layers to the PHY and MAC layers of the IEEE 811.15.4 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 802.15.4 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-related.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 documents 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.Figure 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 sleep. 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-up 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-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 cant necessarily implement a sensor network 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 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 essentially 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, and 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 timing 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. 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 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 discovered 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-your-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.ZigBees 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 transmissions 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-limited, 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 ZigBee 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) 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 thats more complex and less 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 less 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 configurationsZigBee conserves still more power by reducing 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 kbytes 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. Often, 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 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, theres no configuration involved, says the 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 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, 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 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 cant. For example, says Embers 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.Needless to say, many of the major semiconductor 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 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 wireless sensors, however. According to market research firm ON World, it will take five to seven years to convince industrial 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 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 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 example, 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, 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。而有线传感器装置布线的花费通常占无线传感器安装费用的80%的工业环境方面也是同样适用。而且在有线传感器的不适合甚至是不能使用的领域，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充电过于频繁。再加上对无线传感器经空气传送数据可靠性的怀疑，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，它是无线传感器技术的革新。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承诺把无线传感器应用到从工厂自动化系统、家庭安全系统到消费电子产品的一切领域。与802.15.4的合作下，ZigBee使用的电池寿命可比普通小型电池的长几年。 ZigBee设备的价格预计也比较便宜，有人估计销售价格最终不到3美元每节点。由于价格低，他们自然适应用在于如无线光线开关，无线自动调温器，烟雾探测器等家用产品。图1 ZigBee在IEEE811.15.4规范定义的物理层和MAC层的基础上添加了网络/安全层和应用服务层虽然还没有正式的ZigBee规范存在（由于ZigBee联盟是一个贸易集团，规范应该在年底推出），但ZigBee的前景一片光明。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节点和芯片的销售量将从现在基本上为零，增加到2010年的165万个。不是所有这些节点都将与ZigBee结合，但大多数可能会。世界研究公司预测，到2010年射频模块无线传感器出货量达4.65亿，其中77是ZigBee相关的。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，取决于使用的频率（图2）， 比1 Mbps的蓝牙和54Mbps的802.11g的Wi-Fi的技术。但ZigBee不能像Wi-Fi那样发送电子邮件和大型文件，也不能像蓝牙那样发送文件和音频。对于发送传感器的数据，它只有几十字节数，高带宽是没有必要的，ZigBee的低带宽有助于它实现低功耗，低成本和鲁棒性。图2 ZigBee的数据传输速率为20kbps到250kbps，其取决于使用的频率大小 由于ZigBee应用的是低带宽要求，ZigBee节点大部分时间可以采用睡眠模式，从而节省电池电源，然后醒来，快速发送数据，回到睡眠模式。而且，由于ZigBee可以用15毫秒或更少的时间从睡眠模式醒来，即使是睡眠节点也可以达到适当的低延迟。有人反对基于ZigBee的无线光开关，例如，将不会在灯亮起前知道唤醒延迟。与此相反，蓝牙唤醒延迟通常大约三秒钟。ZigBee的功耗节省中的很大一部分来自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FHSS（跳频扩频）会消耗很多能量用于保持它的频率跳变同步。使用802.15.4的ZigBee节点，可以用几种不同的方式通信，然而，某些方式比其他的消耗更多的电能。因此，ZigBee的用户不一定能够实现他们想要的任何传感器网络，他们仍然期望ZigBee的特点多年的电池寿命。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ZigBee的电池满足不了他们的需求。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的传输产生冲突。那么采用802.15.4标准的ZigBee使用CSMA / CA（载波侦听多址接入冲突避免）技术，ZigBee节点使用CSMA / CA本质上采取的是监听先于通话的方式，看是否有无线电通信已经进行。但是，正如传感器营销公司恩贝尔公司副总裁兼ZigBee联盟的副主席Venkat Bahl所指出的，这不是一个首选的方法。“监听消耗更多的电能”，Bahl说，“我们不喜欢这样做。”当面通信ZigBee和802.15.4通讯的另一个选择是同步模式，通常从睡眠模式醒来的网络从节点定期从网络的控制节点接收同步帧。但是，监听同步帧信号浪费电能，同时也因为时间的不确定性，强制节点提前打开，以免错过同步帧信号。为了尽可能节省电力，ZigBee采用一种“就绪时通话”的通信策略，当数据准备好后发送数据，然后等待自动确认。根据ZigBee联盟兼802.15协会主席Bob Heile说，“就绪时通话”是一种“当面通信”的计划，但电力效率很高。“我们广泛地分析了在各种从安静到嘈杂的环境下最省电的策略，”Heile说，“我们发现，我们容易实现在发送数据包和承认它。如果你没收到确认信息，表示你发送失败了，所以进行重发。这种方式比监听并确认通话前是否安静的方式具有更好的电源管理性能。”幸运的是，这种当面策略导致的RF干扰非常小。这主要是因为ZigBee节点具有非常低的占空比，只是偶尔传输并且每次发送少量的数据。其他ZigBee节点，以及Wi-Fi和蓝牙模块，可以轻松应付这么小，不频繁的数据量。但是，ZigBee的就绪时通话计划并不适合所有的目的。例如，在战区中部署由成千上万的微型传感器组成的传感器网络用于监视敌方部队的调动，提供的电能储备仍可能是不够的。每个网络节点周期性地发送，并通过网状网络中的其他邻近节点多次重发以使数据包到达网络控制器，大量的数据包碰撞和重发数据包会浪费电能，并显著缩短传感器节点的电池寿命。如果传感器电池量非常小并且功率有限，这特别成问题。虽然电波信道竞争对于ZigBee不是一个普遍的问题，但它有成为问题的可能。传感器网络公司Dust网络表示，竞争问题使该公司转向ZigBee，至少在现在，即使Dust公司仍是ZigBee联盟的成员。“每个ZigBee设备需要与相关领域争夺信道，” Dust公司产品管理总监Robert Shear 说，“所以有一些不可避免的竞争和低效率。”为了避免ZigBee信道的争夺，Dust公司使用免竞争的TDMA（时分多址）技术。ZigBee通过802.15.4的MAC层提保证时隙，有点类似于TDMA的时隙，但只是作为一个可选的“超帧”，比TDMA更复杂，更省电。但是ZigBee仍然有更省电的方式。例如，它在除了性能更好的全功能设备(FFDs)之外还提供了更省电的半功能设备(RFDs)。每个ZigBee网络至少需要一个全功能设备FFDs作为一个控制器，但大多数网络节点可以是半功能设备RFDs。RFDs只能和FFDs通信，而不能和其他RFDs通信，它们比FFDs电