外文翻譯英文文獻(xiàn)中英版ZigBee：無(wú)線技術(shù)_低功耗傳感器網(wǎng)絡(luò)2

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%的工業(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(chēng)為ZigBee，它是無(wú)線傳感器方程重

2、寫(xiě)，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(xiāo)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。（圖1）雖然還沒(méi)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-St

3、at/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(xiāo)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)。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(chēng)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

4、 - 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(méi)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。 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(kāi)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)

5、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)椋ㄌl擴(kuò)頻）另類(lèi)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(hù)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(zhuān)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

6、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(tīng)多址接入沖突避免）技術(shù)，與ZigBee節(jié)點(diǎn)使用CSMA / CA是基本上采取了聽(tīng)先于談話的方式，看是否有無(wú)線電通信已經(jīng)展開(kāi)。 但是，正如所指出的Venkat Bahl，傳感器營(yíng)銷(xiāo)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。 “有聽(tīng)意見(jiàn)的權(quán)力，”Bahl說(shuō)，“我們不喜歡這樣做?！盳igBee和802.15.4通訊的另一個(gè)選擇是指路明燈模式，通常睡覺(jué)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

7、，對(duì)于一個(gè)燈塔聽(tīng)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(kāi)，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-ready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(kāi)門(mén)見(jiàn)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)。“我們發(fā)現(xiàn),手了,好,我們?cè)诎l(fā)送才離開(kāi)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

8、談?wù)劇！毙疫\(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 - 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

9、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。 “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?！?#160;為了避免ZigBee的訪問(wèn)的爭(zhēng)奪，爭(zhēng)奪使用免費(fèi)的TDMA（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(lèi)似于TDMA的時(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的有效組成部分。

10、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)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(méi)有功率消耗內(nèi)存。 ZigBee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

11、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(kāi)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)

12、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(lè)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)它們進(jìn)行設(shè)置。 “在居住空間，有沒(méi)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。”ZigBee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(zhuān)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

13、的，Heile說(shuō)，給消費(fèi)者和專(zhuān)業(yè)用戶(hù)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，把你的燈關(guān)了?！皩?zhuān)有比賽ZigBee的比賽幾乎完全來(lái)自主專(zhuān)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(zhuān)有EmberNet設(shè)備添加也。 此外，Zensys是其Wave技術(shù)提供給客戶(hù)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

14、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(zhuān)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)，“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(chēng)）已經(jīng)提供ZigBee - ready技術(shù)來(lái)選擇客戶(hù)。其他半導(dǎo)體公司，包括AMI，愛(ài)特梅爾，微芯片，飛利浦，瑞薩，都

15、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(hù)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(chē)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

16、至在計(jì)算機(jī)外圍設(shè)備，如無(wú)線鍵盤(pán)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ZigBee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 for L

17、ow-Power Sensor NetworksGary Legg 5/6/2004 12:00 AM EDT Technologists 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 industr

18、ial 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 possible.The problem, though, is that most wireless sensors use too much power, which means that their batteries either

19、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 wireless sensors simply haven't looked very appealing.A low-power wireless technology called ZigBee is rewriting the wireless sensor equation,

20、 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 offer

21、s 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 thermos

22、tats, 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 outloo

23、k 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 ZigBe

24、e, 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, ZigBee's bright future is largely due to its low data rates20 kbps to 250 kbps, depending on the frequency band used (Figur

25、e 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

26、't necessary, and ZigBee's low bandwidth helps it fulfill its goals of low power, low cost, and robustness.Figure 2:  ZigBee's data rates range from 20 kbps to 250 kbps, depending on the frequency usedBecause of ZigBee applications' low bandwidth requirements, a ZigBee node can

27、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

28、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 us

29、es 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,

30、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, some technologists who are planning very large networks of very small wire

31、less 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 ZigBe

32、e 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 Embe

33、r 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 node

34、s 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 a beacon.In-Your-Face CommunicationTo save as much power as

35、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"

36、 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 i

37、t. 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 largel

38、y 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, h

39、owever. 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

40、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, that's especially problematic.Although content

41、ion 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 company from turning to ZigBeefor now, at leasteven though Dust remains a member of the ZigBee Alliance. "Each ZigBee device needs to

42、 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 access contention, Dust uses contention-free TDMA (time division multiple access) technology. ZigB

43、ee, 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 less power-efficient than TDMA.ZigBee has still more power-saving tricks up its sleeve, however. For example

44、, 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 wi

45、th 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-

46、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.F

47、rom 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 processors and small protocol stacks help keep system costs down. Often, an application's main processor can

48、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 int

49、eroperability 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 Specificat

50、ion 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 H

51、eile. "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 you're done."ZigBee networks can also self-form in commercial and industrial s

52、ettings, 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 don't have to have 128-bit public-key encryption for a smoke detector,&

53、quot; 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.&qu

54、ot;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,

55、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, say

56、s 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."Needless to say, many of the major semiconductor companies, and especially those that are big in embedded systems, are eagerly anticipating ZigBee's entry into mass markets. Freescale Semiconductor (until recently known as Motorola's S