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1、畢業(yè)外文翻譯無線局域網(wǎng)技術(shù)最近幾年，無線局域網(wǎng)開始在市場中獨(dú)霸一方。越來越多的機(jī)構(gòu)發(fā)現(xiàn)無線局域網(wǎng)是傳統(tǒng)有線局域網(wǎng)不可缺少的好幫手，它可以滿足人們對移動、布局變動和自組網(wǎng)絡(luò)的需求，并能覆蓋難以鋪設(shè)有線網(wǎng)絡(luò)的地域。無線局域網(wǎng)是利用無線傳輸媒體的局域網(wǎng)。就在前幾年，人們還很少使用無線局域網(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，無線局域網(wǎng)很快就開始流行起來了。無線局域網(wǎng)的應(yīng)用局域網(wǎng)的擴(kuò)展在20世紀(jì)80年代后期出現(xiàn)的無線局域網(wǎng)早期產(chǎn)品都是作為傳統(tǒng)有線局域網(wǎng)替代品而問世的。無線局域網(wǎng)可以節(jié)省局域網(wǎng)纜線的安裝費(fèi)用，簡化重新布局和其他對網(wǎng)絡(luò)結(jié)構(gòu)改動的任務(wù)。但是，

2、無線局域網(wǎng)的這個動機(jī)被以下一系列的事件打消。首先，隨著人們越來越清楚地認(rèn)識到局域網(wǎng)的重要性，建筑師在設(shè)計新建筑時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，人們越來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5類電纜。因此，用無線局域網(wǎng)取代有線局域網(wǎng)的事情從來沒有發(fā)生過。但是，在有些環(huán)境中無線局域網(wǎng)確實起著有線局域網(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庫這樣有大型開闊場地的建筑；沒有足夠雙絞線對，但又禁止打洞鋪設(shè)新線路的有歷史價值的建筑；從經(jīng)濟(jì)角度考慮，安裝和維護(hù)有線

3、局域網(wǎng)劃不來的小型辦公室。在以上這些情況下，無線局域網(wǎng)向人們提供了一個有效且更具吸引力的選擇。其中大多數(shù)情況下，擁有無線局域網(wǎng)的機(jī)構(gòu)同時也擁有支持服務(wù)器和某些固定工作站的有線局域網(wǎng)。因此，無線局域網(wǎng)通常會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。建筑物的互連無線局域網(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)可以是無線的也可以是有線的。在這種情況下，兩個樓之間采用點(diǎn)對點(diǎn)的無線鏈接。被鏈接的設(shè)備通常是網(wǎng)橋或路由器。這種點(diǎn)對點(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)用算作無線局域網(wǎng)。漫游接入漫游接入提供局域網(wǎng)和帶有天線的移動數(shù)據(jù)終端之間的無線鏈接，

4、如膝上型電腦和筆記本電腦。這種應(yīng)用的一個例子是從外地出差回來的職員將數(shù)據(jù)從個人移動電腦傳送到辦公室的服務(wù)器上。漫游接入在某種延伸的環(huán)境下也是十分有用的，如在建筑群之外操作的一臺電腦或一次商務(wù)行為。在以上兩種情況下，用戶會帶著自己的電腦隨意走動，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。自組網(wǎng)絡(luò)自組網(wǎng)絡(luò)（ad hoc network）是為了滿足某些即時需求而臨時而建立的一種對等網(wǎng)絡(luò)（沒有中央服務(wù)器）例如，有一群職員，每人帶著一臺膝上電腦或掌上電腦，會聚在商務(wù)會議室或課堂上。這些職員會將他們的電腦鏈接起來，形成一個臨時性的、僅僅在會議期間存在的網(wǎng)絡(luò)。無線局域網(wǎng)的要求無線局域網(wǎng)必須滿足所有局域

5、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，無線局域網(wǎng)環(huán)境還有一些特殊的要求。以下是一些無線局域網(wǎng)最終要的要求：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線媒體以達(dá)到最大的容量。節(jié)點(diǎn)數(shù)量：無線局域網(wǎng)可能需要支持分布在多個蜂窩中的上百個節(jié)點(diǎn)。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。對于有基礎(chǔ)設(shè)施的無線局域網(wǎng)，很容易通過利用控制模塊完成這個任務(wù)，控制模塊本身就連接著這兩種類型的局域網(wǎng)。對于移動用戶和自組無線網(wǎng)絡(luò)來說，可能需要滿足這個要求。電池能量消耗：移動工作人員用的是由電池供電的工作站，它需要在使用無線適配器的情況下

6、，電池供電時間足夠長。這就是說，要求移動節(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，無線局域網(wǎng)的實現(xiàn)都具有在不使用網(wǎng)絡(luò)時減少能量消耗的特殊性能，如睡眠模式。傳輸健壯性和安全性：除非涉及合理，無線局域網(wǎng)很容易受到干擾并且容易被竊聽。無線局域網(wǎng)的設(shè)計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。并列的網(wǎng)絡(luò)操作：隨著無線局域網(wǎng)變得越來越流行，很可能有兩個或者更多無線局域網(wǎng)同時存在于一個區(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。這種干擾可能會阻礙MAC算法的正常運(yùn)行，還可能造成對特定局域網(wǎng)的非法接入。不需要許可證的操作：用戶希

7、望購買和運(yùn)行的是這樣的無線局域網(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許可證。切換和漫游：無線局域網(wǎng)中使用的MAC協(xié)議應(yīng)當(dāng)讓移動站點(diǎn)能夠從一個蜂窩移動到另一個蜂窩。動態(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)能夠動態(tài)且自動地增加、刪除和移動位置，并且不打擾到其他用戶。無線局域網(wǎng)技術(shù)無線局域網(wǎng)通常根據(jù)它所采用的傳輸技術(shù)進(jìn)行分類。目前所有無線局域網(wǎng)產(chǎn)品都可歸為以下三個大類之一：紅外線（IR）局域網(wǎng)：紅外線局域網(wǎng)的一個蜂窩只能限制在一個房間里，因為紅外線無法穿過不透明的墻。擴(kuò)頻局域網(wǎng)：這種類型的局域網(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(個人、科學(xué)和醫(yī)學(xué)

8、)波段內(nèi)，因此，在美國使用這些局域網(wǎng)不需要聯(lián)邦通信委員會（FCC）發(fā)放的許可證。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。 無線局域網(wǎng)有一個特性是人們樂意接受的，雖然不是必要的，那就是不需要通過麻煩的授權(quán)過程就能使用。每個國家的許可證發(fā)放制度都不一樣，這就使事情變得更加復(fù)雜。在美國，F(xiàn)CC在ISM波段內(nèi)特許了兩個不需要許可證的應(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開放了這個波段以來，在擴(kuò)頻無線局域網(wǎng)中的應(yīng)用就越來越普遍。1990年IEEE802.11工作組成

9、立，它的憲章就是要為無線局域網(wǎng)開發(fā)MAC協(xié)議以及物理媒體規(guī)約。無線局域網(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)議并爭用同一共享媒體完成接入的站點(diǎn)組成。基本服務(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。BBS通常與文獻(xiàn)中的蜂窩相對應(yīng)，而DS則有可能是交換機(jī)或有線網(wǎng)絡(luò)，也可以是無線網(wǎng)絡(luò)。MAC層的主要任務(wù)是在MAC實體之間傳送MSDU，這個任務(wù)是由分發(fā)服

10、務(wù)實現(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)基于移動性定義了三種轉(zhuǎn)移類型：無轉(zhuǎn)移：這種類型的站點(diǎn)或者是固定的，或者只在一個BSS的直接通信范圍內(nèi)移動。BSS轉(zhuǎn)移：這種類型的站點(diǎn)移動是在同一ESS內(nèi)從一個BSS移動到另一個BSS。在這種情況下，該站點(diǎn)的數(shù)據(jù)交付需要尋址功能，能識別出該站點(diǎn)的新位置。ESS轉(zhuǎn)移：它的定義是指站點(diǎn)從一個ESS的BSS到另一個ESS的BSS移動。只有從某種意義上看該站點(diǎn)是能夠移動的，才能支持這種類型的轉(zhuǎn)移。802.1

11、1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)議由一個集中的決策模塊來控制發(fā)送。分布式接入?yún)f(xié)議對于對等工作站形式的自組網(wǎng)絡(luò)是有意義的，同時也可能對主要是突發(fā)性通信量的其他一些無線局域網(wǎng)頗具吸引力。如果一個局域網(wǎng)的配置是由許多互連的無線站點(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)是時間敏感的或者是高優(yōu)先級的時，這種方法特別有用。IEEE802.11的最終結(jié)果是一個稱為分布式基礎(chǔ)無線MAC（Distributed Foundation Wir

12、eless MAC,DFWMAC）的算法，它提供了一個分布式接入控制機(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)功能（Distributed Coordination Function , DCF）.DCF采用爭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）是一個集中式MAC算法，用于提供無爭用服務(wù)。分布式協(xié)調(diào)功能DCF子層使用一種簡單的CSMA（載波監(jiān)聽多點(diǎn)接入）算法。如果站點(diǎn)有一個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。

13、DCF不包括沖突檢測功能（CSMA/CD），因為在無線網(wǎng)絡(luò)中進(jìn)行沖突檢測是不實際的。媒體上信號變動范圍很大，所以如果正在傳輸?shù)恼军c(diǎn)接收到微弱信號，它無法區(qū)分這是噪聲還是因為自己的傳輸而帶來的影響。為了保證算法的平穩(wěn)和公平運(yùn)行，DCF包含了一組等價于優(yōu)先級策略的時延。我們首先考慮一個稱為幀間間隔（InterFrame Space,IFS）時延。采用IFS后CSMA的接入規(guī)則如下：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長的一段時間，再看媒體是否空閑，如果是空閑，立即發(fā)送。2。如果媒體是忙的（或是一開始就發(fā)現(xiàn)忙，或是在IFS空閑時間內(nèi)發(fā)現(xiàn)媒體忙），則推遲傳輸，并繼續(xù)監(jiān)聽媒體直到

14、當(dāng)前的傳輸結(jié)束。3。一旦當(dāng)前的傳輸結(jié)束，站點(diǎn)再延遲IFS一段時間。如果媒體在這段時間內(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，退避定時器暫停，并在媒體變空閑后恢復(fù)計時。點(diǎn)協(xié)調(diào)功能PCF是在DCF之上實現(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)布輪詢時采用PIFS。因為PIFS比DIFS小，所以點(diǎn)協(xié)調(diào)器在發(fā)布輪詢和接收響應(yīng)時能獲取媒體并封鎖所有的異步通信量。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，定義了一個稱為超幀（superframe）的時間間

15、隔。在超幀時間的開始部分，點(diǎn)協(xié)調(diào)器以循環(huán)方式向所有配置成輪詢的站點(diǎn)發(fā)布輪詢。然后，在余下的超幀時間里，點(diǎn)協(xié)調(diào)器空閑，允許異步通信量有一段爭用接入的時間。在超幀開始時，點(diǎn)協(xié)調(diào)器可以在給定時間內(nèi)獲得控制權(quán)和發(fā)布輪詢，這由選項決定。由于響應(yīng)站點(diǎn)發(fā)出的幀的長度是變化的，所以這個時間間隔也是變化的。超幀剩余的時間用于基于爭用的接入。在超幀末尾，點(diǎn)協(xié)調(diào)器泳PIFS時間爭用媒體接入權(quán)。如果媒體是空閑的，點(diǎn)協(xié)調(diào)器可以立刻接入，然后又是一個全超幀期。不過，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)果是下一個循環(huán)中相應(yīng)縮短的超幀期。WIRELESS LANIn just

16、 the past few years, wireless LANs have come to occupy a significant niche in the local area network market. Increasingly, organizations are finding that wireless LANs are an indispensable adjunct to traditional wired LANs, as they satisfy requirements for mobility, relocation, ad hoc networking, an

17、d coverage of locationsdifficult to wire. As the name suggests, a wireless LAN is one that makes use of a wireless transmission medium. Until relatively recently, wireless LANs were little used; the reasons for this included high prices, low data rates, occupational safety concerns, and licensing re

18、quirements. As these problems have been addressed, the popularity of wireless LANs has grown rapidly.In this section, we first look at the requirements for and advantages of wireless LANs, and then preview the key approaches to wireless LAN implementation.Wireless LANs ApplicationsThere are four app

19、lication areas for wireless LANs: LAN extension, crossbuilding interconnect, nomadic access, and ad hoc networks. Let us consider each of these in turn.LAN ExtensionEarly wireless LAN products, introduced in the late 1980s, were marketed as substitutes for traditional wired LANs. A wireless LAN save

20、s the cost of the installation of LAN cabling and eases the task of relocation and other modifications to network structure. However, this motivation for wireless LANs was overtaken by events. First, as awareness of the need for LAN became greater, architects designed new buildings to include extens

21、ive prewiring for data applications. Second, with advances in data transmission technology, there has been an increasing reliance on twisted pair cabling for LANs and, in particular, Category 3 unshielded twisted pair. Most older building are already wired with an abundance of Category 3 cable. Thus

22、, the use of a wireless LAN to replace wired LANs has not happened to any great extent.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, stock exchange trading f

23、loors, 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 LANs is not economical. In all of these cases, a wireless LAN provides an effective and more attractive a

24、lternative. In most of these cases, an organization will also have a wired LAN to support servers and some stationary workstations. For example, a manufacturing facility typically has an office area that is separate from the factory floor but which must be linked to it for networking purposes. There

25、fore, typically, a wireless LAN will be linked into a wired LAN on the same premises. Thus, this application area is referred to as LAN extension.Cross-Building InterconnectAnother use of wireless LAN technology is to connect LANs in nearby buildings, be they wired or wireless LANs. In this case, a

26、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 is not a LAN per se, but it is usual to include this application under the heading of wireless LAN.Nomadic AccessNomadic access provides a wireless l

27、ink between a LAN hub and 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 personal portable computer to a server in the office. Nomad

28、ic 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 around with their portable computers and may wish access to the servers on a wired LAN from various locations.Ad Hoc NetworkingAn ad hoc

29、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 with 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

30、 network just for the duration of the meeting.Wireless LAN RequirementsA 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 stations, and broadcast capability. In addition, there are a n

31、umber of requirements specific to the wireless LAN environment. The following are among the most important requirements for wireless LANs:Throughput. The medium access control protocol should make as efficient use as possible of the wireless medium to maximize capacity.Number of nodes. Wireless LANs

32、 may need to support hundreds of nodes across multiple cells.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 through the use of control modules that connect to both types of LA

33、Ns. There may also need to be accommodation for mobile users and ad hoc wireless networks.Service area. A typical coverage area for a wireless LAN may be up to a 300 to 1000 foot diameter.Battery power consumption. Mobile workers use battery-powered workstations that need to have a long battery life

34、 when used with wireless adapters. 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.Transmission robustness and security. Unless properly designed, a wireless LAN may be interferenc

35、e-prone and easily eavesdropped upon. The design of a wireless LAN must permit reliable transmission even in a noisy environment and should provide some level of security from eavesdropping.Collocated network operation. As wireless LANs become more popular, it is quite likely for two of them to oper

36、ate in the same area or in some area where 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.License-free operation. Users would prefer to buy and operate wireless LAN products without

37、 having to secure a license for the frequency band used by the LAN.HandoWroaming. The MAC protocol used in the wireless LAN should enable mobile stations to move from one cell to another.Dynamic configuration. The MAC addressing and network management aspects of the LAN should permit dynamic and aut

38、omated addition, deletion, and relocation of end systems without disruption to other users.Physical Medium SpecificationThree physical media are defined in the current 802.11 standard:Infrared at 1 Mbps and 2 Mbps operating at a wavelength between 850 and 950 nm.Direct-sequence spread spectrum opera

39、ting in the 2.4-GHz ISM band. Up to 7 channels, each with a data rate of 1 Mbps or 2 Mbps, can be used.Frequency-hopping spread spectrum operating in the 2.4-GHz ISM band. The details of this option are for further study.Wireless LAN TechnologyWireless LANs are generally categorized according to the

40、 transmission techniquethat is used. All current wireless LAN products fall into one of the following categories:Infrared (IR) LANs. An individual cell of an IR LAN is limited to a single room, as infrared light does not penetrate opaque walls.Spread Spectrum LANs. This type of LAN makes use of spre

41、ad spectrum transmission 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.Narrowband Microwave. These LANs operate at microwave frequencies but do not use spread spectrum. Some of these

42、 products operate at frequencies that require FCC licensing, while others use one of the unlicensed ISM bands.A set of 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

43、 in all commercial products. However, it is useful to be familiar with the standard as its features are representative of required wireless LAN capabilities.The smallest building block of a wireless LAN is a basic service set (BSS), which consists of some number of stations executing the same MAC pr

44、otocol 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 an access point. The access point functions as a bridge. The MAC protocol may be fully distributed or controlled by a central coordination funct

45、ion housed in the access point. The basic service set generally corresponds to what is referred to as a cell in the literature. An extended service set (ESS) consists of two or more basic service sets interconnected by a distribution system. Typically, the distribution system is a wired backbone LAN

46、. The extended service set appears as a single logical LAN to the logical link control (LLC) level. The standard defines three types of stations, based on mobility:No-transition. A station of this type is either stationary or moves only within the direct communication range of the communicating stat

47、ions of a single BSS.BSS-transition. This is defined as a 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.ESS-transition. This is defined as a

48、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 supported by 802.11 cannot be guaranteed. In fact, disruption of service is likely to occur. details of this option are for f

49、urther study.The 802.11 working group considered two types of proposals 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 of transm

50、ission 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 of bursty traffic. A centralized access protocol is natural for configurations in which a

51、 number of wireless stations are interconnected with each other and with some sort of base station that attaches to a backbone wired LAN; it is especially useful if some of the data is time-sensitive or high priority.The end result of the 802.11 is a MAC algorithm called DFWMAC (distributed foundati

52、on wireless MAC) that provides a distributed access-control mechanism with an optional centralized control built 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 a

53、ccess to all traffic. Ordinary asynchronous traffic directly uses DCF. The point coordination function (PCF) is a centralized MAC algorithm used to provide contention-free service. PCF is built on top of DCF and exploits features of DCF to assure access for its users. Let us consider these two subla

54、yers in turn.Distributed Coordination FunctionThe DCF sublayer makes use of a simple CSMA algorithm. If 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

55、 transmitting. The DCF 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 station cannot effectively distinguish incoming weak signals

56、 from noise and the effects of its own transmission. To ensure the smooth and fair functioning of this algorithm, DCF includes a set of delays that amounts to a priority scheme. Let us start by considering a singledelay known as an interframe space (IFS). In fact, there are three different IFS value

57、s, but the algorithm is best explained by initially ignoring this detail. Using an IFS, the rules for CSMA access are as follows: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 i

58、s so, the station may immediately transmit.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 defers transmission and continues to monitor the medium until the current transmission is over.3.

59、Once the current transmission is over, the station delays another IFS. If the medium remains idle 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.Point Coordination FunctionPCF is

60、 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 when issuing polls. Because PIFS is smaller than DIFS, the point coordinator can seize the medium and lock o

61、ut all asynchronous traffic while it issues polls and receives responses.As an extreme, consider 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, con

62、tends for access.The point coordinator could issue polls in a round-robin fashion to all stations configured for polling. When a poll is issued, the polled station may respond using SIFS. If the point coordinator receives a response, it issues another poll using PIFS. If no response is received duri

63、ng the expected turnaround time, the coordinator issues a poll. If the discipline of the preceding paragraph were implemented, the point coordinator would lock out all asynchronous traffic by repeatedly issuing polls. To prevent this situation, an interval known as the superframe is defined. During

64、the first part of this interval, the point coordinator issues polls in a round-robin fashion to all stations configured for polling. The point coordinator then idles for the remainder of the superframe, allowing a contention period for asynchronous access.At the beginning of a superframe, the point

65、coordinator may optionally seize control and issue polls fora give period of time. This interval varies because of the variable frame size issued by responding stations. The remainder of the superframe is available for contention-based access. At the end of the superframe interval, the point coordinator contends for access to the medium using PIFS. If the medium is idle, the point coordinator gains immediate access, and a full superframe period follows. However, the medium may be busy at the end of a superframe. In this case, the point coordinator must wait until the me