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1、<p><b>  畢業(yè)外文翻譯 </b></p><p><b>  無(wú)線局域網(wǎng)技術(shù)</b></p><p>  最近幾年,無(wú)線局域網(wǎng)開始在市場(chǎng)中獨(dú)霸一方。越來(lái)越多的機(jī)構(gòu)發(fā)現(xiàn)無(wú)線局域網(wǎng)是傳統(tǒng)有線局域網(wǎng)不可缺少的好幫手,它可以滿足人們對(duì)移動(dòng)、布局變動(dòng)和自組網(wǎng)絡(luò)的需求,并能覆蓋難以鋪設(shè)有線網(wǎng)絡(luò)的地域。無(wú)線局域網(wǎng)是利用無(wú)線傳輸媒體的局域網(wǎng)。

2、就在前幾年,人們還很少使用無(wú)線局域網(wǎng)。原因包括成本高、數(shù)據(jù)率低、職業(yè)安全方面的顧慮以及需要許可證。隨著這些問題的逐步解決,無(wú)線局域網(wǎng)很快就開始流行起來(lái)了。</p><p><b>  無(wú)線局域網(wǎng)的應(yīng)用</b></p><p><b>  局域網(wǎng)的擴(kuò)展</b></p><p>  在20世紀(jì)80年代后期出現(xiàn)的無(wú)線局域網(wǎng)早期產(chǎn)

3、品都是作為傳統(tǒng)有線局域網(wǎng)替代品而問世的。無(wú)線局域網(wǎng)可以節(jié)省局域網(wǎng)纜線的安裝費(fèi)用,簡(jiǎn)化重新布局和其他對(duì)網(wǎng)絡(luò)結(jié)構(gòu)改動(dòng)的任務(wù)。但是,無(wú)線局域網(wǎng)的這個(gè)動(dòng)機(jī)被以下一系列的事件打消。首先,隨著人們?cè)絹?lái)越清楚地認(rèn)識(shí)到局域網(wǎng)的重要性,建筑師在設(shè)計(jì)新建筑時(shí)就包括了大量用于數(shù)據(jù)應(yīng)用的預(yù)先埋設(shè)好的線路。其次,隨著數(shù)據(jù)傳輸技術(shù)的發(fā)展,人們?cè)絹?lái)越依賴于雙絞線連接的局域網(wǎng)。特別是3類和5類非屏蔽雙絞線。大多數(shù)老建筑中已經(jīng)鋪設(shè)了足夠的3類電纜,而許多新建筑里則預(yù)埋了

4、5類電纜。因此,用無(wú)線局域網(wǎng)取代有線局域網(wǎng)的事情從來(lái)沒有發(fā)生過。</p><p>  但是,在有些環(huán)境中無(wú)線局域網(wǎng)確實(shí)起著有線局域網(wǎng)替代品的作用。例如,象生產(chǎn)車間、股票交易所的交易大廳以及倉(cāng)庫(kù)這樣有大型開闊場(chǎng)地的建筑;沒有足夠雙絞線對(duì),但又禁止打洞鋪設(shè)新線路的有歷史價(jià)值的建筑;從經(jīng)濟(jì)角度考慮,安裝和維護(hù)有線局域網(wǎng)劃不來(lái)的小型辦公室。在以上這些情況下,無(wú)線局域網(wǎng)向人們提供了一個(gè)有效且更具吸引力的選擇。其中大多數(shù)情況

5、下,擁有無(wú)線局域網(wǎng)的機(jī)構(gòu)同時(shí)也擁有支持服務(wù)器和某些固定工作站的有線局域網(wǎng)。因此,無(wú)線局域網(wǎng)通常會(huì)鏈接到同樣建筑群內(nèi)的有線局域網(wǎng)上。所以我們將此類應(yīng)用領(lǐng)域成為局域網(wǎng)的擴(kuò)展。</p><p><b>  建筑物的互連</b></p><p>  無(wú)線局域網(wǎng)技術(shù)的另一種用途是鄰樓局域網(wǎng)之間的連接,這些局域網(wǎng)可以是無(wú)線的也可以是有線的。在這種情況下,兩個(gè)樓之間采用點(diǎn)對(duì)點(diǎn)的無(wú)線

6、鏈接。被鏈接的設(shè)備通常是網(wǎng)橋或路由器。這種點(diǎn)對(duì)點(diǎn)的單鏈路從本質(zhì)上看不是局域網(wǎng),但通常我們也把這種應(yīng)用算作無(wú)線局域網(wǎng)。</p><p><b>  漫游接入</b></p><p>  漫游接入提供局域網(wǎng)和帶有天線的移動(dòng)數(shù)據(jù)終端之間的無(wú)線鏈接,如膝上型電腦和筆記本電腦。這種應(yīng)用的一個(gè)例子是從外地出差回來(lái)的職員將數(shù)據(jù)從個(gè)人移動(dòng)電腦傳送到辦公室的服務(wù)器上。漫游接入在某種延伸

7、的環(huán)境下也是十分有用的,如在建筑群之外操作的一臺(tái)電腦或一次商務(wù)行為。在以上兩種情況下,用戶會(huì)帶著自己的電腦隨意走動(dòng),并希望可以從不同的位置訪問有線局域網(wǎng)上的服務(wù)器。</p><p><b>  自組網(wǎng)絡(luò)</b></p><p>  自組網(wǎng)絡(luò)(ad hoc network)是為了滿足某些即時(shí)需求而臨時(shí)而建立的一種對(duì)等網(wǎng)絡(luò)(沒有中央服務(wù)器)例如,有一群職員,每人帶著一臺(tái)膝

8、上電腦或掌上電腦,會(huì)聚在商務(wù)會(huì)議室或課堂上。這些職員會(huì)將他們的電腦鏈接起來(lái),形成一個(gè)臨時(shí)性的、僅僅在會(huì)議期間存在的網(wǎng)絡(luò)。</p><p><b>  無(wú)線局域網(wǎng)的要求</b></p><p>  無(wú)線局域網(wǎng)必須滿足所有局域網(wǎng)的典型要求,包括大容量、近距離的覆蓋能力、相連站點(diǎn)間的完全連接性以及廣播能力。另外,無(wú)線局域網(wǎng)環(huán)境還有一些特殊的要求。以下是一些無(wú)線局域網(wǎng)最終要的

9、要求:</p><p>  吞吐量:媒體接入控制協(xié)議應(yīng)當(dāng)盡可能地有效利用無(wú)線媒體以達(dá)到最大的容量。</p><p>  節(jié)點(diǎn)數(shù)量:無(wú)線局域網(wǎng)可能需要支持分布在多個(gè)蜂窩中的上百個(gè)節(jié)點(diǎn)。</p><p>  連接到主干局域網(wǎng):在大多數(shù)情況下,要求能夠與主干有線局域網(wǎng)的站點(diǎn)相互連接。對(duì)于有基礎(chǔ)設(shè)施的無(wú)線局域網(wǎng),很容易通過利用控制模塊完成這個(gè)任務(wù),控制模塊本身就連接著這兩種

10、類型的局域網(wǎng)。對(duì)于移動(dòng)用戶和自組無(wú)線網(wǎng)絡(luò)來(lái)說(shuō),可能需要滿足這個(gè)要求。</p><p>  電池能量消耗:移動(dòng)工作人員用的是由電池供電的工作站,它需要在使用無(wú)線適配器的情況下,電池供電時(shí)間足夠長(zhǎng)。這就是說(shuō),要求移動(dòng)節(jié)點(diǎn)不停地監(jiān)視接入點(diǎn)或者經(jīng)常要與基站握手的MAC協(xié)議是不適用的。通常,無(wú)線局域網(wǎng)的實(shí)現(xiàn)都具有在不使用網(wǎng)絡(luò)時(shí)減少能量消耗的特殊性能,如睡眠模式。</p><p>  傳輸健壯性和安全

11、性:除非涉及合理,無(wú)線局域網(wǎng)很容易受到干擾并且容易被竊聽。無(wú)線局域網(wǎng)的設(shè)計(jì)必須做到即使在噪音較大的環(huán)境中也能可靠傳輸,并且為應(yīng)用提供某種程度的安全性,以防竊聽。</p><p>  并列的網(wǎng)絡(luò)操作:隨著無(wú)線局域網(wǎng)變得越來(lái)越流行,很可能有兩個(gè)或者更多無(wú)線局域網(wǎng)同時(shí)存在于一個(gè)區(qū)域內(nèi),或在局域網(wǎng)之間可能存在干擾的某些區(qū)域內(nèi)運(yùn)行。這種干擾可能會(huì)阻礙MAC算法的正常運(yùn)行,還可能造成對(duì)特定局域網(wǎng)的非法接入。</p>

12、;<p>  不需要許可證的操作:用戶希望購(gòu)買和運(yùn)行的是這樣的無(wú)線局域網(wǎng)產(chǎn)品,它們不需要專門為局域網(wǎng)所使用的頻帶而申請(qǐng)?jiān)S可證。</p><p>  切換和漫游:無(wú)線局域網(wǎng)中使用的MAC協(xié)議應(yīng)當(dāng)讓移動(dòng)站點(diǎn)能夠從一個(gè)蜂窩移動(dòng)到另一個(gè)蜂窩。</p><p>  動(dòng)態(tài)配置:局域網(wǎng)在MAC地址機(jī)制和網(wǎng)絡(luò)管理方面應(yīng)當(dāng)允許端系統(tǒng)能夠動(dòng)態(tài)且自動(dòng)地增加、刪除和移動(dòng)位置,并且不打擾到其他用戶。&

13、lt;/p><p><b>  無(wú)線局域網(wǎng)技術(shù)</b></p><p>  無(wú)線局域網(wǎng)通常根據(jù)它所采用的傳輸技術(shù)進(jìn)行分類。目前所有無(wú)線局域網(wǎng)產(chǎn)品都可歸為以下三個(gè)大類之一:</p><p>  紅外線(IR)局域網(wǎng):紅外線局域網(wǎng)的一個(gè)蜂窩只能限制在一個(gè)房間里,因?yàn)榧t外線無(wú)法穿過不透明的墻。</p><p>  擴(kuò)頻局域網(wǎng):這種

14、類型的局域網(wǎng)利用了擴(kuò)頻傳輸技術(shù)。在大多數(shù)情況下,這些局域網(wǎng)運(yùn)行在ISM(個(gè)人、科學(xué)和醫(yī)學(xué))波段內(nèi),因此,在美國(guó)使用這些局域網(wǎng)不需要聯(lián)邦通信委員會(huì)(FCC)發(fā)放的許可證。</p><p>  窄帶微波:這些局域網(wǎng)運(yùn)行在微波頻率是,但沒有使用擴(kuò)頻技術(shù)。其中有些產(chǎn)品運(yùn)行的頻率需要FCC的許可證,而其他一些產(chǎn)品則使用了不需要許可的波段。</p><p>  無(wú)線局域網(wǎng)有一個(gè)特性是人們樂意接受的,雖

15、然不是必要的,那就是不需要通過麻煩的授權(quán)過程就能使用。每個(gè)國(guó)家的許可證發(fā)放制度都不一樣,這就使事情變得更加復(fù)雜。在美國(guó),F(xiàn)CC在ISM波段內(nèi)特許了兩個(gè)不需要許可證的應(yīng)用:最大功率為1瓦的擴(kuò)頻系統(tǒng)合最大運(yùn)行功率為0.5瓦的低功率系統(tǒng)。自從FCC開放了這個(gè)波段以來(lái),在擴(kuò)頻無(wú)線局域網(wǎng)中的應(yīng)用就越來(lái)越普遍。</p><p>  1990年IEEE802.11工作組成立,它的憲章就是要為無(wú)線局域網(wǎng)開發(fā)MAC協(xié)議以及物理媒體

16、規(guī)約。</p><p>  無(wú)線局域網(wǎng)中最小的模塊是基本服務(wù)集(Basic Service Set, BSS),它由一些執(zhí)行相同MAC協(xié)議并爭(zhēng)用同一共享媒體完成接入的站點(diǎn)組成?;痉?wù)集可以是孤立的,也可以通過接入點(diǎn)(Access Point, AP)連到主干分發(fā)系統(tǒng)(Distribution System, DS)上。接入點(diǎn)的功能相當(dāng)于網(wǎng)橋。MAC協(xié)議可以是完全分布式的,也可以由位于接入點(diǎn)的中央?yún)f(xié)調(diào)功能控制。B

17、BS通常與文獻(xiàn)中的蜂窩相對(duì)應(yīng),而DS則有可能是交換機(jī)或有線網(wǎng)絡(luò),也可以是無(wú)線網(wǎng)絡(luò)。</p><p>  MAC層的主要任務(wù)是在MAC實(shí)體之間傳送MSDU,這個(gè)任務(wù)是由分發(fā)服務(wù)實(shí)現(xiàn)的。分發(fā)服務(wù)的正常運(yùn)行需要該ESS內(nèi)所有站點(diǎn)的信息,而這個(gè)信息是由與關(guān)聯(lián)(association)相關(guān)的服務(wù)提供的。在分發(fā)服務(wù)向站點(diǎn)交付數(shù)據(jù)或者接收來(lái)自站點(diǎn)的數(shù)據(jù)之前,該站點(diǎn)必須要建立關(guān)聯(lián)。標(biāo)準(zhǔn)基于移動(dòng)性定義了三種轉(zhuǎn)移類型:</p&

18、gt;<p>  無(wú)轉(zhuǎn)移:這種類型的站點(diǎn)或者是固定的,或者只在一個(gè)BSS的直接通信范圍內(nèi)移動(dòng)。</p><p>  BSS轉(zhuǎn)移:這種類型的站點(diǎn)移動(dòng)是在同一ESS內(nèi)從一個(gè)BSS移動(dòng)到另一個(gè)BSS。在這種情況下,該站點(diǎn)的數(shù)據(jù)交付需要尋址功能,能識(shí)別出該站點(diǎn)的新位置。</p><p>  ESS轉(zhuǎn)移:它的定義是指站點(diǎn)從一個(gè)ESS的BSS到另一個(gè)ESS的BSS移動(dòng)。只有從某種意義上看

19、該站點(diǎn)是能夠移動(dòng)的,才能支持這種類型的轉(zhuǎn)移。</p><p>  802.11工作組考慮了兩類MAC算法建議:分布式接入?yún)f(xié)議和集中式接入?yún)f(xié)議。分布式接入?yún)f(xié)議類似于以太網(wǎng),采用載波監(jiān)聽機(jī)制把傳輸?shù)臎Q定權(quán)分布到所有節(jié)點(diǎn)。集中式接入?yún)f(xié)議由一個(gè)集中的決策模塊來(lái)控制發(fā)送。分布式接入?yún)f(xié)議對(duì)于對(duì)等工作站形式的自組網(wǎng)絡(luò)是有意義的,同時(shí)也可能對(duì)主要是突發(fā)性通信量的其他一些無(wú)線局域網(wǎng)頗具吸引力。如果一個(gè)局域網(wǎng)的配置是由許多互連的無(wú)線

20、站點(diǎn)和以某種形式連接到主干有線局域網(wǎng)的基站組成,則采用集中式接入控制是自然而然的事情。當(dāng)某些數(shù)據(jù)是時(shí)間敏感的或者是高優(yōu)先級(jí)的時(shí),這種方法特別有用。</p><p>  IEEE802.11的最終結(jié)果是一個(gè)稱為分布式基礎(chǔ)無(wú)線MAC(Distributed Foundation Wireless MAC,DFWMAC)的算法,它提供了一個(gè)分布式接入控制機(jī)制,并在頂端具有可選的集中式控制。MAC層的低端子層是分布式協(xié)調(diào)

21、功能(Distributed Coordination Function , DCF).DCF采用爭(zhēng)用算法向所有通信量提供接入。正常的異步通信量直接使用DCF。點(diǎn)協(xié)調(diào)功能(Point Coordination Function, PCF)是一個(gè)集中式MAC算法,用于提供無(wú)爭(zhēng)用服務(wù)。</p><p><b>  分布式協(xié)調(diào)功能</b></p><p>  DCF子層使用

22、一種簡(jiǎn)單的CSMA(載波監(jiān)聽多點(diǎn)接入)算法。如果站點(diǎn)有一個(gè)MAC幀要發(fā)送,則先監(jiān)聽媒體。如果媒體空閑,站點(diǎn)可以發(fā)送。否則,該站點(diǎn)必須等待直到當(dāng)前的發(fā)送結(jié)束。DCF不包括沖突檢測(cè)功能(CSMA/CD),因?yàn)樵跓o(wú)線網(wǎng)絡(luò)中進(jìn)行沖突檢測(cè)是不實(shí)際的。媒體上信號(hào)變動(dòng)范圍很大,所以如果正在傳輸?shù)恼军c(diǎn)接收到微弱信號(hào),它無(wú)法區(qū)分這是噪聲還是因?yàn)樽约旱膫鬏敹鴰?lái)的影響。</p><p>  為了保證算法的平穩(wěn)和公平運(yùn)行,DCF包含了

23、一組等價(jià)于優(yōu)先級(jí)策略的時(shí)延。我們首先考慮一個(gè)稱為幀間間隔(InterFrame Space,IFS)時(shí)延。采用IFS后CSMA的接入規(guī)則如下:</p><p>  1。有幀要傳輸?shù)恼军c(diǎn)先監(jiān)聽媒體。如果媒體是空閑的,等待IFS長(zhǎng)的一段時(shí)間,再看媒體是否空閑,如果是空閑,立即發(fā)送。</p><p>  2。如果媒體是忙的(或是一開始就發(fā)現(xiàn)忙,或是在IFS空閑時(shí)間內(nèi)發(fā)現(xiàn)媒體忙),則推遲傳輸,并繼

24、續(xù)監(jiān)聽媒體直到當(dāng)前的傳輸結(jié)束。</p><p>  3。一旦當(dāng)前的傳輸結(jié)束,站點(diǎn)再延遲IFS一段時(shí)間。如果媒體在這段時(shí)間內(nèi)都是空閑的,則站點(diǎn)采用二進(jìn)制指數(shù)退避策略等待一段時(shí)間后再監(jiān)聽媒體,如果媒體依然是空閑的,則可以傳輸。在退避期間,如果媒體又變忙了,退避定時(shí)器暫停,并在媒體變空閑后恢復(fù)計(jì)時(shí)。</p><p><b>  點(diǎn)協(xié)調(diào)功能</b></p>&l

25、t;p>  PCF是在DCF之上實(shí)現(xiàn)的另一種接入方式。其操作由中央輪詢主控器(點(diǎn)協(xié)調(diào)器)的輪詢構(gòu)成。點(diǎn)協(xié)調(diào)在發(fā)布輪詢時(shí)采用PIFS。因?yàn)镻IFS比DIFS小,所以點(diǎn)協(xié)調(diào)器在發(fā)布輪詢和接收響應(yīng)時(shí)能獲取媒體并封鎖所有的異步通信量。</p><p>  點(diǎn)協(xié)調(diào)器不斷地發(fā)布輪詢,并永遠(yuǎn)封鎖所有異步通信量。為了避免這種情況,定義了一個(gè)稱為超幀(superframe)的時(shí)間間隔。在超幀時(shí)間的開始部分,點(diǎn)協(xié)調(diào)器以循環(huán)方式

26、向所有配置成輪詢的站點(diǎn)發(fā)布輪詢。然后,在余下的超幀時(shí)間里,點(diǎn)協(xié)調(diào)器空閑,允許異步通信量有一段爭(zhēng)用接入的時(shí)間。</p><p>  在超幀開始時(shí),點(diǎn)協(xié)調(diào)器可以在給定時(shí)間內(nèi)獲得控制權(quán)和發(fā)布輪詢,這由選項(xiàng)決定。由于響應(yīng)站點(diǎn)發(fā)出的幀的長(zhǎng)度是變化的,所以這個(gè)時(shí)間間隔也是變化的。超幀剩余的時(shí)間用于基于爭(zhēng)用的接入。在超幀末尾,點(diǎn)協(xié)調(diào)器泳PIFS時(shí)間爭(zhēng)用媒體接入權(quán)。如果媒體是空閑的,點(diǎn)協(xié)調(diào)器可以立刻接入,然后又是一個(gè)全超幀期。不

27、過,媒體在超幀末尾有可能是忙的。在這種情況下,點(diǎn)協(xié)調(diào)器必須等待直到媒體空閑并獲得接入。其結(jié)果是下一個(gè)循環(huán)中相應(yīng)縮短的超幀期。</p><p>  超寬帶技術(shù)與其它短距離無(wú)線通信技術(shù)的比較</p><p>  隨著個(gè)人通信消費(fèi)電子產(chǎn)業(yè)的迅猛發(fā)展,短距離無(wú)線通信領(lǐng)域的各種新技術(shù)、新方法層出不窮,朝著更快、更方便、更安全有效等方面進(jìn)行發(fā)展。新的技術(shù)在 Intel 接入、信息家電、移動(dòng)辦公、工業(yè)化

28、等各個(gè)領(lǐng)域得到了廣泛的運(yùn)用。其中,超寬帶(Ultra Wide Band,UWB)技術(shù)是在 20世紀(jì)90 年代以后發(fā)展起來(lái)的一種具有巨大發(fā)展?jié)摿Φ男滦蜔o(wú)線通信技術(shù),被列為未來(lái)通信的十大技術(shù)之一 。</p><p>  1 超寬帶無(wú)線通信 </p><p>  1.1 超寬帶技術(shù)簡(jiǎn)介 </p><p>  UWB(Ultra Wideband,超寬帶)技術(shù)是目前正被廣

29、泛研究的一種新興無(wú)線通信技術(shù),現(xiàn)在已經(jīng)成為高速無(wú)線個(gè)人網(wǎng) (WPAN)的首選技術(shù)。</p><p>  UWB是指信號(hào)帶寬大于 500MHz或者是信號(hào)帶寬與中心頻率之比大于25% 的通信技術(shù)。與常見的通信方式使用連續(xù)的載波不同,UWB 中使用的無(wú)線信號(hào)中心頻率為 4.1GHz,帶寬為 1.4GHz,頻譜范圍很寬,但是發(fā)射功率非常低。通信速度在250Kbit~10Mbit/秒之間。在 250Kbit/秒的傳輸速度下

30、可確保 30m的通信距離。在短距離(13m 以下)有很大優(yōu)勢(shì),最高傳輸速度可達(dá) 1Gb/S。而傳統(tǒng)的窄帶技術(shù)在長(zhǎng)距離、低速傳輸具有優(yōu)勢(shì)。 </p><p>  超寬帶(UWB)技術(shù)最初是面向雷達(dá)應(yīng)用來(lái)開發(fā)的,一般認(rèn)為它屬于一種無(wú)載波通信技術(shù)。2002 年 2 月,美國(guó)聯(lián)邦通信委員會(huì)(FCC)正式將其解禁。目前超寬帶(UWB)技術(shù)正被整合進(jìn)家庭影院和便攜式產(chǎn)品,主要用于視頻和音頻信號(hào)的無(wú)線發(fā)送。寬帶(UWB)

31、自問世后一直被看作是藍(lán)牙技術(shù)的替代品,與其他無(wú)線技術(shù)如 WLAN 、藍(lán)牙等相比,超寬帶(UWB)具有低功耗、高帶寬、低復(fù)雜度、低成本的優(yōu)點(diǎn),完全可以滿足短距離家庭娛樂應(yīng)用需求。 1.2 超寬帶性能特點(diǎn)。</p><p>  超寬帶無(wú)線通信是一種與傳統(tǒng)技術(shù)有很大不同的無(wú)線通信技術(shù)。它能夠?qū)崿F(xiàn)無(wú)線局域網(wǎng)LAN 和個(gè)人區(qū)域網(wǎng)PAN 中無(wú)線接口的互聯(lián)和接入。UWB具有以下特點(diǎn): </p><p&g

32、t;<b>  1)抗干擾性能強(qiáng) </b></p><p>  UWB信號(hào),在發(fā)射時(shí)將微弱的無(wú)線電脈沖信號(hào)分散在寬闊的頻帶中,輸出功率甚至低于普通設(shè)備產(chǎn)生的噪聲。接收時(shí)將信號(hào)能量還原出來(lái),在解擴(kuò)過程中產(chǎn)生擴(kuò)頻增益。因此,與 IEEE 802.11a、IEEE 802.11b 和藍(lán)牙相比,在同等碼速條件下,UWB具有更強(qiáng)的抗干擾性。 </p><p><b>

33、  2)傳輸速率高 </b></p><p>  UWB 以非常寬的頻率帶寬來(lái)?yè)Q取高速的數(shù)據(jù)傳輸,并且不單獨(dú)占用現(xiàn)在已經(jīng)擁擠不堪的頻率資源,而是共享其他無(wú)線技術(shù)使用的頻帶。其數(shù)據(jù)速率可以達(dá)到幾十兆比特每秒到幾百兆比特每秒,有望高于藍(lán)牙100倍,也可以高于IEEE 802.11a和 IEEE 802.11b。 </p><p><b>  3)帶寬極寬 </b&g

34、t;</p><p>  UWB 使用的帶寬在 1GHz 以上,高達(dá)幾吉赫茲,并且可以和目前的窄帶通信系統(tǒng)同時(shí)工作而互不干擾。這在頻率資源日益緊張的今天開辟了一種新的時(shí)域無(wú)線電資源。 </p><p>  4)頻譜利用率高,系統(tǒng)容量大 </p><p>  因?yàn)椴恍枰a(chǎn)生正弦載波信號(hào),可以直接發(fā)射沖激序列,因而 UWB系統(tǒng)具有很寬的頻譜和很低的平均功率,有利于與其他

35、系統(tǒng)共存,從而提高頻譜利用率,帶來(lái)了極大的系統(tǒng)容量。 </p><p><b>  5)功率低 </b></p><p>  UWB系統(tǒng)使用間歇的脈沖來(lái)發(fā)送數(shù)據(jù),脈沖持續(xù)時(shí)間很短,一般在0.20ns~1.5ns 之間,有很低的占空因數(shù),系統(tǒng)耗電可以做到很低,在高速通信時(shí)系統(tǒng)的耗電量?jī)H為幾百μ W~幾十 mw。同時(shí)由于 UWB系統(tǒng)信號(hào)的擴(kuò)頻處理增益比較大,即使采用低增益

36、的全向天線發(fā)射,也可使用小于 1mW 的發(fā)射功率實(shí)現(xiàn)幾千米的通信。 </p><p><b>  6)安全性好 </b></p><p>  UWB 安全性表現(xiàn)在兩方面:一方面是采用跳時(shí)擴(kuò)頻,接收機(jī)只有已知發(fā)送端擴(kuò)頻碼時(shí)才能解出發(fā)射數(shù)據(jù);另一方面是系統(tǒng)的發(fā)射功率譜密度極低。有用信息完全淹沒在噪聲中,被截獲概率很小,被檢測(cè)的概率也很低,用傳統(tǒng)的接收機(jī)無(wú)法接收。 <

37、/p><p><b>  2 Wi-Fi </b></p><p>  2.1 Wi-Fi技術(shù)簡(jiǎn)介 </p><p>  Wi-Fi(Wireless Fidelity,無(wú)線高保真)也是一種無(wú)線通信協(xié)議,正式名稱是IEEE802.11b。 </p><p>  Wi-Fi 是以太網(wǎng)的一種無(wú)線擴(kuò)展,Wi-Fi 網(wǎng)絡(luò)可以使用來(lái)

38、互連電腦鏈接上互連網(wǎng)。Wi-Fi 網(wǎng)絡(luò)在無(wú)執(zhí)照的 2.4 和 5 千兆 Hz 的無(wú)線電頻帶經(jīng)營(yíng),數(shù)據(jù)速率可達(dá) 11Mbps(802.11b)~54Mbps(802.11a),或包含以上兩條頻帶的產(chǎn)品。理論上只要用戶位于一個(gè)接入點(diǎn)四周的一定區(qū)域內(nèi),就能以最高約 11Mb/s 的速度接入Web。但實(shí)際上,如果有多個(gè)用戶同時(shí)通過一個(gè)點(diǎn)接入,帶寬被多個(gè)用戶分享。 </p><p>  由于 Wi-Fi 使用電波作為傳

39、送媒介, 資料包被截取的可能性高, 這成為用戶所擔(dān)心問題?,F(xiàn)在 Wi-Fi 產(chǎn)品利用WED(Wired Equivalent Privacy)技術(shù)作資料加密之用。然而,其保密的效能卻倍受質(zhì)疑,支援新一代加密方式的 Wi-Fi 產(chǎn)品亦相繼出現(xiàn)。雖然 Wi-Fi 在數(shù)據(jù)安全性方面比藍(lán)牙技術(shù)要差一些,但在電波的覆蓋范圍方面卻略勝一籌,可達(dá) 100 m 左右。 </p><p>  Wi-Fi 可以提供熱點(diǎn)覆蓋、低移

40、動(dòng)性和高數(shù)據(jù)傳輸速率, 無(wú)線接入和高速傳輸是Wi-Fi 技術(shù)的主要特點(diǎn)。國(guó)內(nèi)的電信、網(wǎng)通、移動(dòng)都非常關(guān)注 Wi-Fi 技術(shù)的發(fā)展和應(yīng)用?;跓狳c(diǎn)的接入服務(wù)曾經(jīng)一度被看成是對(duì)3G的巨大沖擊, 但Wi-Fi技術(shù)傳輸距離短和信號(hào)穿透能力差的特點(diǎn)同樣也是不可克服的。 </p><p>  2.2 Wi-Fi技術(shù)的優(yōu)勢(shì) </p><p>  近幾年,WAP 的數(shù)量呈迅猛增長(zhǎng),無(wú)線網(wǎng)絡(luò)的方便與高效

41、使其得到迅速的普及。無(wú)論是無(wú)線城市的建設(shè),還是企事業(yè)單位局域網(wǎng)的開通,還是手機(jī)的 Wi-Fi 功能,都與Wi-Fi 技術(shù)自身的優(yōu)勢(shì)是分不開的: </p><p>  1)較廣的局域網(wǎng)覆蓋范圍 </p><p>  基于藍(lán)牙技術(shù)的電波覆蓋范圍非常小,半徑大約只有 15m,而 Wi-Fi 的半徑則可達(dá)100 m,可以覆蓋整棟辦公大樓; </p><p><b

42、>  2)傳輸速度快 </b></p><p>  Wi-Fi 技術(shù)傳輸速度非???,可以達(dá)到 11Mbps(802.11b)或者 54Mbps (802.11a),適合高速數(shù)據(jù)傳輸?shù)臉I(yè)務(wù); </p><p><b>  3)無(wú)需布線 </b></p><p>  Wi-Fi 最主要的優(yōu)勢(shì)在于不需要布線,可以不受布線條件的限

43、制,因此非常適合移動(dòng)辦公用戶的需要。在機(jī)場(chǎng)、車站、咖啡店、圖書館等人員較密集地方設(shè)置“熱點(diǎn)” ,并通過高速線路將因特網(wǎng)接入上述場(chǎng)所。用戶只要將支持無(wú)線 LAN的筆記本電腦或PDA拿到該區(qū)域內(nèi),即可高速接入因特網(wǎng); </p><p><b>  4)健康安全 </b></p><p>  IEEE802.11規(guī)定的發(fā)射功率不可超過 100 毫瓦,實(shí)際發(fā)射功率約 60

44、~70毫瓦,而手機(jī)的發(fā)射功率約 200毫瓦~1瓦間,手持式對(duì)講機(jī)高達(dá)5瓦。與后者相比,Wi-Fi 產(chǎn)品的輻射更小。 </p><p><b>  3 結(jié)論 </b></p><p>  每一種無(wú)線通信方式都有其獨(dú)特之處,超寬帶技術(shù)傳輸速率高,Wi-Fi 技術(shù)覆蓋范圍廣。隨著無(wú)線通信技術(shù)的發(fā)展,人們對(duì)高速短距離無(wú)線通信的要求越來(lái)越高,超寬帶技術(shù)、Wi-Fi 技術(shù)都有著廣

45、泛的發(fā)展前景。</p><p>  WIRELESS LAN</p><p>  In just the past few years, wireless LANs have come to occupy a significant niche in the local area network market. Increasingly, organizations are findi

46、ng that wireless LANs are an indispensable adjunct to traditional wired LANs, as they satisfy requirements for mobility, relocation, ad hoc networking, and coverage of locationsdifficult to wire. As the name suggests, a

47、wireless LAN is one that makes use of a wireless transmission medium. Until relatively recently, wireless LANs were little used</p><p>  In this section, we first look at the requirements for and advantages

48、of wireless LANs, and then preview the key approaches to wireless LAN implementation.</p><p>  Wireless LANs Applications</p><p>  There are four application areas for wireless LANs: LAN extensi

49、on, crossbuilding interconnect, nomadic access, and ad hoc networks. Let us consider each of these in turn.</p><p>  LAN Extension</p><p>  Early wireless LAN products, introduced in the late 19

50、80s, were marketed as substitutes for traditional wired LANs. A wireless LAN saves the cost of the installation of LAN cabling and eases the task of relocation and other modifications to network structure. However, this

51、motivation for wireless LANs was overtaken by events. First, as awareness of the need for LAN became greater, architects designed new buildings to include extensive prewiring for data applications. Second, with advances

52、in dat</p><p>  However, in a number of environments, there is a role for the wireless LAN as an alternative to a wired LAN. Examples include buildings with large open areas, such as manufacturing plants, st

53、ock exchange trading floors, and warehouses; historical buildings with insufficient twisted pair and in which drilling holes for new wiring is prohibited; and small offices where installation and maintenance of wired LAN

54、s is not economical. In all of these cases, a wireless LAN provides an effective and more</p><p>  Cross-Building Interconnect</p><p>  Another use of wireless LAN technology is to connect LANs

55、in nearby buildings, be they wired or wireless LANs. In this case, a point-to-point wireless link is used between two buildings. The devices so connected are typically bridges or routers. This single point-to-point link

56、is not a LAN per se, but it is usual to include this application under the heading of wireless LAN.</p><p>  Nomadic Access</p><p>  Nomadic access provides a wireless link between a LAN hub and

57、 a mobile data terminal equipped with an antenna, such as a laptop computer or notepad computer. One example of the utility of such a connection is to enable an employee returning from a trip to transfer data from a pers

58、onal portable computer to a server in the office. Nomadic access is also useful in an extended environment such as a campus or a business operating out of a cluster of buildings. In both of these cases, users may move a&

59、lt;/p><p>  Ad Hoc Networking</p><p>  An ad hoc network is a peer-to-peer network (no centralized server) set up temporarily to meet some immediate need. For example, a group of employees, each wi

60、th a laptop or palmtop computer, may convene in a conference room for a business or classroom meeting. The employees link their computers in a temporary network just for the duration of the meeting.</p><p> 

61、 Wireless LAN Requirements</p><p>  A wireless LAN must meet the same sort of requirements typical of any LAN, including high capacity, ability to cover short distances, full connectivity among attached stat

62、ions, and broadcast capability. In addition, there are a number of requirements specific to the wireless LAN environment. The following are among the most important requirements for wireless LANs:</p><p>  T

63、hroughput. The medium access control protocol should make as efficient use as possible of the wireless medium to maximize capacity.</p><p>  Number of nodes. Wireless LANs may need to support hundreds of nod

64、es across multiple cells.</p><p>  Connection to backbone LAN. In most cases, interconnection with stations on a wired backbone LAN is required. For infrastructure wireless LANs, this is easily accomplished

65、through the use of control modules that connect to both types of LANs. There may also need to be accommodation for mobile users and ad hoc wireless networks.</p><p>  Service area. A typical coverage area fo

66、r a wireless LAN may be up to a 300 to 1000 foot diameter.</p><p>  Battery power consumption. Mobile workers use battery-powered workstations that need to have a long battery life when used with wireless ad

67、apters. This suggests that a MAC protocol that requires mobile nodes to constantlymonitor access points or to engage in frequent handshakes with a base stationis inappropriate.</p><p>  Transmission robustne

68、ss and security. Unless properly designed, a wireless LAN may be interference-prone and easily eavesdropped upon. The design of a wireless LAN must permit reliable transmission even in a noisy environment and should prov

69、ide some level of security from eavesdropping.</p><p>  Collocated network operation. As wireless LANs become more popular, it is quite likely for two of them to operate in the same area or in some area wher

70、e interference between the LANs is possible. Such interference may thwart the normal operation of a MAC algorithm and may allow unauthorized access to a particular LAN.</p><p>  License-free operation. Users

71、 would prefer to buy and operate wireless LAN products without having to secure a license for the frequency band used by the LAN.</p><p>  HandoWroaming. The MAC protocol used in the wireless LAN should enab

72、le mobile stations to move from one cell to another.</p><p>  Dynamic configuration. The MAC addressing and network management aspects of the LAN should permit dynamic and automated addition, deletion, and r

73、elocation of end systems without disruption to other users.</p><p>  Physical Medium Specification</p><p>  Three physical media are defined in the current 802.11 standard:</p><p> 

74、 Infrared at 1 Mbps and 2 Mbps operating at a wavelength between 850 and 950 nm.</p><p>  Direct-sequence spread spectrum operating in the 2.4-GHz ISM band. Up to 7 channels, each with a data rate of 1 Mbps

75、or 2 Mbps, can be used.</p><p>  Frequency-hopping spread spectrum operating in the 2.4-GHz ISM band. The details of this option are for further study.</p><p>  Wireless LAN Technology</p>

76、<p>  Wireless LANs are generally categorized according to the transmission techniquethat is used. All current wireless LAN products fall into one of the following categories:</p><p>  Infrared (IR) L

77、ANs. An individual cell of an IR LAN is limited to a single room, as infrared light does not penetrate opaque walls.</p><p>  Spread Spectrum LANs. This type of LAN makes use of spread spectrum transmission

78、technology. In most cases, these LANs operate in the ISM (Industrial, Scientific, and Medical) bands, so that no FCC licensing is required for their use in the U.S.</p><p>  Narrowband Microwave. These LANs

79、operate at microwave frequencies but do not use spread spectrum. Some of these products operate at frequencies that require FCC licensing, while others use one of the unlicensed ISM bands.</p><p>  A set of

80、wireless LAN standards has been developed by the IEEE 802.11 committee. The terminology and some of the specific features of 802.11 are unique to this standard and are not reflected in all commercial products. However, i

81、t is useful to be familiar with the standard as its features are representative of required wireless LAN capabilities.</p><p>  The smallest building block of a wireless LAN is a basic service set (BSS), whi

82、ch consists of some number of stations executing the same MAC protocol and competing for access to the same shared medium. A basic service set may be isolated, or it may connect to a backbone distribution system through

83、an access point. The access point functions as a bridge. The MAC protocol may be fully distributed or controlled by a central coordination function housed in the access point. The basic service set gen</p><p&g

84、t;  No-transition. A station of this type is either stationary or moves only within the direct communication range of the communicating stations of a single BSS.</p><p>  BSS-transition. This is defined as a

85、 station movement from one BSS to another BSS within the same ESS. In this case, delivery of data to the station requires that the addressing capability be able to recognize the new location of the station.</p>&l

86、t;p>  ESS-transition. This is defined as a station movement from a BSS in one ESS to a BSS within another ESS. This case is supported only in the sense that the station can move. Maintenance of upper-layer connections

87、 supported by 802.11 cannot be guaranteed. In fact, disruption of service is likely to occur. details of this option are for further study.</p><p>  The 802.11 working group considered two types of proposals

88、 for a MAC algorithm: distributed-access protocols which, like CSMAICD, distributed the decision to transmit over all the nodes using a carrier-sense mechanism; and centralized access protocols, which involve regulation

89、of transmission by a centralized decision maker. A distributed access protocol makes sense of an ad hoc network of peer workstations and may also be attractive in other wireless LAN configurations that consist primarily

90、of</p><p>  The end result of the 802.11 is a MAC algorithm called DFWMAC (distributed foundation wireless MAC) that provides a distributed access-control mechanism with an optional centralized control built

91、 on top of that. Figure 13.20 illustrates the architecture. The lower sublayer of the MAC layer is the distributed coordination function (DCF). DCF uses a contention algorithm to provide access to all traffic.

92、 Ordinary asynchronous traffic directly uses DCF. The point coordination function (PC</p><p>  Distributed Coordination Function</p><p>  The DCF sublayer makes use of a simple CSMA algorithm. I

93、f a station has a MAC frame to transmit, it listens to the medium. If the medium is idle, the station may transmit; otherwise, the station must wait until the current transmission is complete before transmitting. The DCF

94、 does not include a collision-detection function (i.e., CSMAICD) because collision detection is not practical on a wireless network. The dynamic range of the signals on the medium is very large, so that a transmitting st

95、atio</p><p>  delay known as an interframe space (IFS). In fact, there are three different IFS values, but the algorithm is best explained by initially ignoring this detail. Using an IFS, the rules for CSMA

96、access are as follows:</p><p>  I. A station with a frame to transmit senses the medium. If the medium is idle, the station waits to see if the medium remains idle for a time equal to IFS, and, if this is so

97、, the station may immediately transmit.</p><p>  2. If the medium is busy (either because the station initially finds the medium busy or because the medium becomes busy during the IFS idle time), the station

98、 defers transmission and continues to monitor the medium until the current transmission is over.</p><p>  3. Once the current transmission is over, the station delays another IFS. If the medium remains idle

99、for this period, then the station backs off using a binary exponential backoff scheme and again senses the medium. If the medium is still idle, the station may transmit.</p><p>  Point Coordination Function&

100、lt;/p><p>  PCF is an alternative access method implemented on top of the DCF. The operation consists of polling with the centralized polling master (point coordinator). The point coordinator makes use of PIFS

101、when issuing polls. Because PIFS is smaller than DIFS, the point coordinator can seize the medium and lock out all asynchronous traffic while it issues polls and receives responses.</p><p>  As an extreme, c

102、onsider the following possible scenario. A wireless network is configured so that a number of stations with time-sensitive traffic are controlled by the point coordinator while remaining traffic, using CSMA, contends for

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