外文文獻(xiàn)翻譯ZigBee：無(wú)線技術(shù)-低功耗傳感器網(wǎng)絡(luò)

1、ZigBee :無(wú)線技術(shù)，低功耗傳感器網(wǎng)絡(luò)加里萊格美國(guó)東部時(shí)間2004年5月6日上午12:00技師（工程師）們?cè)诎l(fā)掘無(wú)線傳感器的潛在應(yīng)用方面從未感到任何困難。 例 如，在家庭安全系統(tǒng)方面，無(wú)線傳感器相對(duì)于有線傳感器更易安裝。 而在有線傳 感器的裝置通常占無(wú)線傳感器安裝的費(fèi)用 80%勺工業(yè)環(huán)境方面同樣正確（適用）。 而且相比于有線傳感器的不切實(shí)際甚至是不肯能而言，無(wú)線傳感器更具應(yīng)用性。雖然，無(wú)線傳感器需要消耗更多能量，也就是說(shuō)所需電池的數(shù)量會(huì)隨之增加或 改變過(guò)于頻繁。再加上對(duì)無(wú)線傳感器由空氣傳送的數(shù)據(jù)可靠性的懷疑論，所以無(wú)線傳感器看起來(lái)并不是那么吸引人。一個(gè)低功率無(wú)線技術(shù)被稱為 ZigBee，它

2、是無(wú)線傳感器方程重寫，但是。一 個(gè)安全的網(wǎng)絡(luò)技術(shù)，對(duì)最近通過(guò)的IEEE 802.15.4無(wú)線標(biāo)準(zhǔn)（圖1）的頂部游戲 機(jī)，ZigBee的承諾，把無(wú)線傳感器的一切從工廠自動(dòng)化系統(tǒng)到家庭安全系統(tǒng)， 消費(fèi)電子產(chǎn)品。與802.15.4的合作下，ZigBee提供具有電池壽命可比普通小型 電池的長(zhǎng)幾年。ZigBee設(shè)備預(yù)計(jì)也便宜，有人估計(jì)銷售價(jià)格最終不到3美元每 節(jié)點(diǎn)，。由于價(jià)格低，他們應(yīng)該是一個(gè)自然適應(yīng)于在光線如無(wú)線交換機(jī)，無(wú)線自動(dòng)調(diào)溫器，煙霧探測(cè)器和家用產(chǎn)品APPJCATIOhl/PROFllESAPPLICATION FRAMEWOKKNETWDRK/SECURTIYLAYERSAppli catio

3、nJ ZigBee Ptatlorm Stack 釧gnMAC LAYERPHY LAYERZigBee4 Alliance PtatformIEEE（圖1）雖然還沒有正式的規(guī)范的ZigBee存在（由ZigBee聯(lián)盟是一個(gè)貿(mào)易集團(tuán)，批 準(zhǔn)應(yīng)該在今年年底），但ZigBee的前景似乎一片光明。技術(shù)研究公司In-Stat/MDR在它所謂的“謹(jǐn)慎進(jìn)取”的預(yù)測(cè)中預(yù)測(cè)，802.15.4節(jié)點(diǎn)和芯片銷售 將從今天基本上為零，增加到 2010年的165萬(wàn)臺(tái)。不是所有這些單位都將與ZigBee結(jié)合，但大多數(shù)可能會(huì)。世界研究公司預(yù)測(cè)的到2010年射頻模塊無(wú)線傳感器出貨量4.65億美量，其中77%是ZigBee的相關(guān)

4、。從某種意義上說(shuō)，ZigBee的光明前途在很大程度上是由于其較低的數(shù)據(jù)速率20 kbps到250 kbps的，用于取決于頻段頻率（圖2），比標(biāo)稱1 Mbps的藍(lán)牙和54的802.11g Mbps的Wi - Fi的技術(shù)。但ZigBee的不能發(fā)送電子郵件和大型文件，如 Wi - Fi功能，或文件和音頻，藍(lán)牙一樣。對(duì)于發(fā)送傳感器的 讀數(shù)，這是典型的數(shù)萬(wàn)字節(jié)數(shù)，高帶寬是沒有必要，ZigBee的低帶寬有助于它實(shí)現(xiàn)其目標(biāo)和魯棒性的低功耗，低成本。WK 嘆;卜;KAI；II 訃怔F MM2U GHz ISM Worldwide 250 khp»U915 MHz 1 ISM Ancricaa 411

5、 kbpsIQE2： ZigBee的數(shù)掘隹扁速率為20kbps到的，這職決于使屈頻率由于ZigBee應(yīng)用的是低帶寬要求，ZigBee節(jié)點(diǎn)大部分時(shí)間可以睡眠模式， 從而節(jié)省電池電源，然后醒來(lái)，快速發(fā)送數(shù)據(jù)，回去睡眠模式。而且，由于ZigBee可以從睡眠模式過(guò)渡到15毫秒或更少主動(dòng)模式下，即使是睡眠節(jié)點(diǎn)也可 以達(dá)到適當(dāng)?shù)牡脱舆t。有人扳動(dòng)支持ZigBee的無(wú)線光開關(guān)，例如，將不會(huì)是一個(gè)喚醒延遲知道前燈亮起。與此相反，支持藍(lán)牙喚醒延遲通常大約三秒鐘。一個(gè)ZigBee的功耗節(jié)省很大一部分來(lái)自802.15.4無(wú)線電技術(shù)，它本身是為 低功耗設(shè)計(jì)的。802.15.4采用DSSS（直接序列擴(kuò)頻）技術(shù)，例如，因?yàn)?/p>

6、（跳頻擴(kuò)頻）另類醫(yī)療及社會(huì)科學(xué)院將在保持一樣使用它的頻率過(guò)大的權(quán)力同步。ZigBee節(jié)點(diǎn)，使用802.15.4，是幾個(gè)不同的溝通方式之一，然而，某些方 面比別人擁有更多的使用權(quán)力。因此，ZigBee的用戶不一定能夠?qū)崿F(xiàn)傳感器網(wǎng)絡(luò)上的任何方式選擇和他們?nèi)匀黄谕嗄甑碾姵貕勖荶igBee的標(biāo)志。事實(shí)上，一些技術(shù)專家打算用小型無(wú)線傳感器創(chuàng)建大的網(wǎng)絡(luò)，即使功率ZigBee的電池需求很大。一個(gè)ZigBee網(wǎng)絡(luò)節(jié)點(diǎn)可以消耗額外的功率，例如，如果它試圖避免與其他 節(jié)點(diǎn)的傳輸或與其他無(wú)線電源傳輸重疊的傳輸。那么在ZigBee 802.15.4無(wú)線電的使用實(shí)現(xiàn)CSMA / CA （載波偵聽多址接入沖突避免）技

7、術(shù)，與 ZigBee節(jié)點(diǎn) 使用CSMA CA是基本上采取了聽先于談話的方式，看是否有無(wú)線電通信已經(jīng)展 開。 但是，正如所指出的Venkat Bahl，傳感器營(yíng)銷公司恩貝爾公司副總裁兼 ZigBee聯(lián)盟的副主席，這不是一個(gè)首選的方法?！坝新犚庖姷臋?quán)力，” Bahl說(shuō)，“我們不喜歡這樣做。”ZigBee和802.15.4通訊的另一個(gè)選擇是指路明燈模式，通常睡覺模式醒來(lái) 網(wǎng)絡(luò)節(jié)點(diǎn)定期接收同步“燈塔”從網(wǎng)絡(luò)的控制節(jié)點(diǎn)。但是，對(duì)于一個(gè)燈塔聽廢物力量，也因?yàn)闀r(shí)間的不確定性，特別是支配節(jié)點(diǎn)打開，以免錯(cuò)過(guò)早期一盞明燈。爭(zhēng)議中的通信為了盡可能節(jié)省電力ZigBee采用一種簡(jiǎn)單交際策略，talk-when-read

8、y 發(fā)送 數(shù)據(jù)時(shí),數(shù)據(jù)準(zhǔn)備派遣然后就等著自動(dòng)確認(rèn)。根據(jù)鮑勃Heile,兩ZigBee聯(lián)盟主席和電子802.15,talk-when-ready 是“開門見山地”計(jì)劃，但卻是一種很電力 有效率?！拔覀?cè)趶V泛的分析，導(dǎo)致了最好的節(jié)能策略從各種環(huán)境安靜喧鬧 的,” Heile說(shuō)?！拔覀儼l(fā)現(xiàn)，手了，好,我們?cè)诎l(fā)送才離開那包東西和承認(rèn)它。如 果你不想讓他ack訊息,它只表示你慘敗，所以重發(fā)給你。你有更好的電源管理， 并確定它是否安靜，然后再談?wù)?。”幸運(yùn)的是，這種當(dāng)面策略導(dǎo)致 RF干擾非常小。 這主要是因?yàn)閆igBee節(jié)點(diǎn) 具有非常低的占空比，只偶爾傳輸發(fā)送少量的數(shù)據(jù)。其他ZigBee節(jié)點(diǎn)，以及Wi -

9、Fi和藍(lán)牙模塊，可以輕松應(yīng)付這么小，頻繁爆發(fā)。ZigBee的通話時(shí)就緒計(jì)劃并不適合所有的目的，但是。 例如，在成千上萬(wàn)的 微型傳感器網(wǎng)絡(luò)進(jìn)入戰(zhàn)區(qū)下降到監(jiān)視敵方部隊(duì)調(diào)動(dòng)，積蓄力量提供的仍可能是不 夠的。每個(gè)網(wǎng)絡(luò)節(jié)點(diǎn)周期性地發(fā)送和反復(fù)通過(guò)網(wǎng)狀網(wǎng)絡(luò)配置中的其他節(jié)點(diǎn)附近多 次以達(dá)到網(wǎng)絡(luò)控制器的大碰撞和重發(fā)的數(shù)據(jù)包數(shù)量可能會(huì)浪費(fèi)功率，并顯著縮短傳感器節(jié)點(diǎn)的電池壽命傳輸數(shù)據(jù)。如果傳感器電池非常小，功率有限，這特別成問(wèn)題。雖然大氣電波訪問(wèn)爭(zhēng)不是一般意義上的 ZigBee問(wèn)題，都可以。傳感器網(wǎng)絡(luò)公司塵埃網(wǎng)絡(luò)，其實(shí)，說(shuō)是保持競(jìng)爭(zhēng)問(wèn)題，從該公司的ZigBee轉(zhuǎn)向?yàn)楝F(xiàn)在，至少，甚至塵土縱然仍是ZigBee聯(lián)盟的成員

10、。 “每個(gè)ZigBee設(shè)備需要與鄰國(guó) 爭(zhēng)奪領(lǐng)空，說(shuō)：” Dust產(chǎn)品管理總監(jiān)羅伯特剪“，所以有一些爭(zhēng)論，一些不可 避免的低效率?！睘榱吮苊鈀igBee的訪問(wèn)的爭(zhēng)奪，爭(zhēng)奪使用免費(fèi)的 TDM（時(shí)分多址）技術(shù)。ZigBee的802.15.4 MAC層通過(guò)提供擔(dān)保的計(jì)劃，有點(diǎn)類似于TDMA勺時(shí)隙，但只是作為一個(gè)可選的“超碼”那更復(fù)雜，更省電，比TDMA勺有效組成部分。ZigBee的已注冊(cè)的袖子更省電的技巧，但是。 例如，它減少了對(duì)節(jié)能減功 能設(shè)備，除了更強(qiáng)大的全功能設(shè)備（FFDS （ RFDS在ZigBee元件提供電力的 消耗。每個(gè)ZigBee網(wǎng)絡(luò)至少需要一個(gè)控制器作為一個(gè)發(fā)展籌資，但大多數(shù)網(wǎng) 絡(luò)節(jié)點(diǎn)

11、可以RFDs（圖3）。RFDs只有FFDs可以談，而不是其他RFDs但它們含有較少的電路比FFDs很少或沒有功率消耗內(nèi)存。ZigBeeFl可以包含在羞種配置如許蓋節(jié)點(diǎn)65&35ZigBee的節(jié)省，減少了相關(guān)處理單仍然需要更多的權(quán)力。簡(jiǎn)單的8位像8051處理器可以處理家務(wù)容易的 ZigBee和ZigBee協(xié)議棧占用很少的內(nèi)存。發(fā)展籌資的一個(gè)堆棧，例如，大概需要32字節(jié)，一個(gè)RFD的堆棧只需要4字節(jié)。這 些數(shù)字比較遠(yuǎn)約250藍(lán)牙技術(shù)更復(fù)雜的字節(jié)。從ZigBee的比較簡(jiǎn)單的實(shí)現(xiàn)，節(jié)約了成本，自然產(chǎn)生。RFDs當(dāng)然，減少漏報(bào)ZigBee的內(nèi)存和其他電路元件成本，以及簡(jiǎn)單的 8位處理器和小協(xié)議棧

12、 幫助保持系統(tǒng)成本。通常，一個(gè)應(yīng)用程序的主處理器可以很容易地承擔(dān)了ZigBee處理額外的負(fù)載小，使得 ZigBee的功能不必要單獨(dú)的處理器。但是，保持 ZigBee 的低價(jià)格的主要策略是因?yàn)橛泻艽蟮氖袌?chǎng)和高容量。 ZigBee 聯(lián)盟，通過(guò)一個(gè)開放的標(biāo)準(zhǔn)，并通過(guò)大力推進(jìn) ZigBee 設(shè)備之間的互操作 性， ZigBee 的預(yù)期應(yīng)用非常大，如家庭與樓宇自動(dòng)化應(yīng)用。該聯(lián)盟目前正在為這些特殊應(yīng)用努力，它預(yù)計(jì)將在今年較遲時(shí)與 ZigBee 規(guī)范 1.0 的互操作性的 程序完一起完成。一個(gè)有關(guān)的 ZigBee 家庭自動(dòng)化與安全通過(guò)樂觀的原因是它的易用性。 ZigBee 網(wǎng)絡(luò)的自我形成， 使消費(fèi)者更容易對(duì)

13、它們進(jìn)行設(shè)置。 “在居住空間， 有沒有配 置參與，：” ZigBee 聯(lián)盟的 Heile 說(shuō)。 “你從箱子拿一些東西，放電池進(jìn)去， 可能做一些簡(jiǎn)單的按鈕操作， 按下安全帶來(lái)兩個(gè)設(shè)備并攏， 按動(dòng)按鈕， 直到綠色 燈光來(lái)，你就完成了?！盳igBee 網(wǎng)絡(luò)還可以自行在商業(yè)和工業(yè)環(huán)境的形式， 但專業(yè)安裝人員將有特別 的安全工具，提供額外的控制。 ZigBee 是安全靈活的， Heile 說(shuō)，給消費(fèi)者和 專業(yè)用戶他們需要的。 “你不必有 128 位公共密鑰加密的煙霧探測(cè)器， ”他說(shuō)， “但如果我在一幢復(fù)雜的高層辦公樓， 這正是我的安全級(jí)別將有熒光燈。 如果你 在第五大道上的高層建筑里，你不想去的人在街上

14、，把你的燈關(guān)了。“專有比賽ZigBee的比賽幾乎完全來(lái)自主專有技術(shù)。傳感器公司Dust，如上所述，是堅(jiān)持使用自己的技術(shù)，顯然的，雖然強(qiáng)烈的推到 ZigBee 舞臺(tái)上，計(jì)劃繼續(xù)提 供其專有EmberNet設(shè)備添加也。此外，Zensys是其Wave技術(shù)提供給客戶的Z-。西爾韋尼亞，例如，已經(jīng)使用照明控制 Z - Wave的，而ZigBee系統(tǒng)保持在至少數(shù)個(gè)月。通過(guò)提供互操作性， 但 ZigBee 的補(bǔ)充能力， 專利產(chǎn)品不能。 舉例說(shuō)， Ember 的義巴爾，互操作性允許照明系統(tǒng)的 ZigBee節(jié)點(diǎn)的工作，在一個(gè)空調(diào)系統(tǒng)的 ZigBee 網(wǎng)絡(luò)，反之亦然。 “飛利浦照明是真的對(duì)這個(gè)很興奮，”義巴爾說(shuō)，

15、 “因?yàn)樵瓉?lái)從一到建筑物的自動(dòng)化系統(tǒng)的基礎(chǔ)設(shè)施骨干鎮(zhèn)流器生產(chǎn)廠家他們。 ”不用說(shuō)，主要的半導(dǎo)體公司很多，尤其是那些在嵌入式系統(tǒng)公司中大都熱切 期待 ZigBee 的投入并且大規(guī)模進(jìn)入市場(chǎng)。飛思卡爾半導(dǎo)體（直到最近，摩托羅 拉半導(dǎo)體產(chǎn)品部稱）已經(jīng)提供 ZigBee - ready 技術(shù)來(lái)選擇客戶。其他半導(dǎo)體公 司，包括AMI，愛特梅爾，微芯片，飛利浦，瑞薩，都是 ZigBee聯(lián)盟的成員。ZigBee 可能是緩慢滲透到無(wú)線傳感器的工業(yè)市場(chǎng)， 但是。據(jù)對(duì)世界市場(chǎng)研究 公司，它會(huì)需要五至七年來(lái)說(shuō)服客戶在工業(yè)上的可靠性， 耐用性， 以及無(wú)線傳感 器系統(tǒng)的安全。并顯著預(yù)測(cè)在整個(gè)世界中 ZigBee 將長(zhǎng)期

16、在工業(yè)制造上有增長(zhǎng)， 因此。到 2010 年，公司項(xiàng)目，射頻模塊，應(yīng)用于工業(yè)監(jiān)控和控制得將達(dá)到1.65億臺(tái)，同比增長(zhǎng) 190 萬(wàn)元，在世界性預(yù)測(cè)中，在 2004 年。大約 75的將基于 ZigBee 和 802.15.4 。最終，ZigBee的可進(jìn)入各種廣泛的應(yīng)用。家用電器，它可以幫助監(jiān)測(cè)和控制 能源消耗。 在汽車應(yīng)用中， 它可以提供輪胎壓力監(jiān)測(cè)和遠(yuǎn)程無(wú)鑰匙進(jìn)入系統(tǒng)。 也 可用于 ZigBee 的醫(yī)療設(shè)備中，甚至在計(jì)算機(jī)外圍設(shè)備，如無(wú)線鍵盤或鼠標(biāo)。值得關(guān)注的是越來(lái)越多， 雖然， ZigBee 的可能變成一種適合所有的技術(shù)的尺 寸，并不很適合任何應(yīng)用程序。一些持懷疑態(tài)度，例如，企圖使 ZigBe

17、e 無(wú)所不 包的可能使 ZigBee 協(xié)議棧太大， ZigBee 的雙重目標(biāo)是非常低功耗和非常低的成 本。如果出現(xiàn)這種情況，那么 ZigBee 的低功耗，低數(shù)據(jù)速率利基窄，如果它是， 將被證明是過(guò)于寬泛的。 然后，也許我們會(huì)需要另一種無(wú)線標(biāo)準(zhǔn)， 以配合我們已 經(jīng)有的蓬勃發(fā)展的人數(shù)。加里萊格是一家位于波士頓的自由撰稿人他擁有電子工程學(xué)士學(xué)位，曾任編輯， EDN 雜志執(zhí)行編輯ZigBee: Wireless Technology forLow-Power Sensor NetworksGary Legg5/6/2004 12:00 AM EDTTechnologists have never ha

18、d trouble coming up with potential applications for wireless sensors. In a homesecurity 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 insta

19、llations. And then there are applications for sensors where wiring isn'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

20、 the reliability of sensor data that's sent through the air, and wireless sensors simply haven't 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

21、 802.15.4 radio standard (Figure 1), ZigBee promises to put wireless sensors in everything from factory automation systems to homesecurity 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 a

22、lso expected to be cheap, eventually selling for less than $3 per node by someestimates. With prices that low, they should be a natural fit even in household products like wireless light switches, wireless thermostats, and smoke detectors.APPLICATION PROFILESApplicationI ZigBct Platform SiliconMAC L

23、AYERPHY LAVER* Alanco PlatformIEEEIapplication frameworkNETWORK/SECURTI? LAYERSFigure 1:ZigBee adds n etwork, security, andapplication-services layers to the PHYand MACayers of the IEEE 811.15.4 radioAlthough no formal specificati on for ZigBee yet exists (approval by theZigBee Allianee,a trade grou

24、p, should comelate this year), the outlookfor ZigBee appears bright.Tech no logy research firm In-Stat/MDR, in whatit calls a "cautious aggressive" forecast, predicts that sales of 802.15.4 no des and chipsets will in crease from esse ntially zero today to 165 millionunits by 2010. Not all

25、 of these units will be coupled with ZigBee,but most probably will be. Research firm ON World predicts shipme nts of 465 million wireless 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,

26、depe nding on the freque ncy band used(Figure2) compared to a nominal 1 Mbps for Bluetooth and 54 Mbps for Wi-Fi's 802.11g tech no logy.But ZigBee won't be sending email and large docume nts,as Wi-Fi does, or docume nts and audio, as Bluetooth does. For sending sen sor read in gs, which are

27、typically a few tens of bytes, high ban dwidth isn't necessary, and ZigBee's low bandwidth helps it fulfill its goals of low power, low cost, and robust ness.卜:乩w： ix i蟲口E： £21Figure 2:ZigBee's data rates range from 20 kbpsto 250 kbps, depe nding on the freque ncy usedBecause of Zig

28、Bee 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 sui

29、tably 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 ZigBee's power savings come from the radio technolo

30、gy 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 80

31、2.15.4, can communicate in any of several different ways, however, and some ways use more power than others. Consequently, ZigBee users can't necessarily implement a sensor network any way they choose and still expect the multiple-year battery life that is ZigBee's hallmark. In fact, sometec

32、hnologists whoare 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 transmi

33、ssions 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/CAis essentially taking a listen-before-talkapproach to see if anyradio traffic is already underway. But, as noted by

34、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 t

35、he 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 missing

36、 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 an

37、d 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

38、 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 managementthan if you listen and determine if it's quiet before you talk."Fortunately, this in-your-face strate

39、gy 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'

40、;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 transmiss

41、ions repeated numerous times through other nearby nodes of a mesh network configuration in order to reach a network controllerlargenumbers of packet collisions and retransmissions could waste power and significantly shorten sensor node battery life. If the sensor batteries are very small and power-l

42、imited, that's especially problematic.Although contention for airwave access isn't generally a problem for ZigBee, it can be. Sensor-network company Dust Networks, in fact, says contention issues are keeping the companyfrom turning to ZigBee for now, at least even though Dust remains a membe

43、rof the ZigBee Alliance. "Each ZigBee device needs to contend for airspace with its neighbors," says Dust director of product managementRobert Shear, "so there's inevitably some contention and some inefficiency." To avoid ZigBee's access contention, Dust uses contention-f

44、ree TDMA(time division multiple access) technology. ZigBee, through the 802.15.4 MAC layer, provides guaranteed time slots in a scheme that somewhatresembles TDMA,but only as part of an optional "superframe" that's more complex and less power-efficient than TDMA.ZigBee has still more p

45、ower-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 netwo

46、rk 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 n etworks can con tai n as many as65,536 no des in a variety of con figuratio nsZigBee con serves still more power by re

47、duc ing the n eed for associated processing. Simple 8-bit processors like an 8051 can handle ZigBee chores easily, and ZigBee protocol stacks occupy very little memory. An FFDstack, for example, n eeds about 32 kbytes, and an RFD stack n eeds only about 4 kbytes. Those nu mbers compare with about 25

48、0 kbytes for the far more complex Bluetooth tech no logy.From ZigBee's relatively simple implementations,cost savings naturallyaccrue. 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 cos

49、ts dow n. Often, an applicati on's main processor can easily bear the small additi on al load of ZigBee process ing, making a separate processor for ZigBee functions unn ecessary.But the main strategy for keeping ZigBee prices low is to have big markets and high volumes. The ZigBee Allianee,by m

50、aking ZigBee an open standardand by vigorously promoti ng in teroperability among ZigBee devices, expects that ZigBee will be very big in applicati ons such as home and building automation. The alliance is currently working on in teroperabilityprocedures for those particularapplicati ons,which itexp

51、ects to complete later this year along with ZigBee Specification1.0.One reas on for optimism about ZigBee adopti on for home automati on 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

52、 configuration involved," says the ZigBee Allianee's Heile. "You take someth ing out of the box, put the batteries in, and maybe do somethi ngas simple as button-press securitybring two devices close together,push the buttons until the green lights come on, and you're done."Zi

53、gBee 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, saysHeile, to give both consumer and professional users what they need. "You don't hav

54、e 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

55、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

56、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 capabi

57、lities 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 turn

58、s them from a ballast manufacturer into the infrastructure backbone of a building-automation system."Needless to say, manyof the major semiconductor companies, and especially those that are big in embeddedsystems, are eagerly anticipating ZigBee's entry into mass markets. Freescale Semiconductor