無線局域網(wǎng)畢業(yè)論文外文翻譯

1、<p>　　畢業(yè)設計(論文)外文資料翻譯</p><p><b>　　二〇〇九年六月</b></p><p>　　WIRELESS LAN</p><p>　　In just the past few years, wireless LANs have come to occupy a significant niche in t

2、he 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, a

3、nd 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</p><p>　　In

4、this section, we first look at the requirements for and advantages of wireless LANs, and then preview the key approaches to wireless LAN implementation.</p><p>　　Wireless LANs Applications</p><p&g

5、t;　　There are four application areas for wireless LANs: LAN extension, crossbuilding interconnect, nomadic access, and ad hoc networks. Let us consider each of these in turn.</p><p>　　LAN Extension</p>

6、<p>　　Early wireless LAN products, introduced in the late 1980s, 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 relo

7、cation 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 includ

8、e extensive prewiring for data applications. Second, with advances 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 includ

9、e buildings with large open areas, such as manufacturing plants, stock exchange trading floors, and warehouses; historical buildings with insufficient twisted pair and in which drilling holes for new wiring is prohibited

10、; 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</p><p>　　Cross-Building Interconnect</p>&l

11、t;p>　　Another use of wireless LAN technology is to connect LANs 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

12、 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.</p><p>　　Nomadic Access</p><p

13、>　　Nomadic access provides a wireless link 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 en

14、able an employee returning from a trip to transfer data from a personal 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 o

15、f a cluster of buildings. In both of these cases, users may move a</p><p>　　Ad Hoc Networking</p><p>　　An ad hoc network is a peer-to-peer network (no centralized server) set up temporarily to m

16、eet 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

17、network just for the duration of the meeting.</p><p>　　Wireless LAN Requirements</p><p>　　A wireless LAN must meet the same sort of requirements typical of any LAN, including high capacity, abil

18、ity to cover short distances, full connectivity among attached stations, and broadcast capability. In addition, there are a number of requirements specific to the wireless LAN environment. The following are among the mos

19、t important requirements for wireless LANs:</p><p>　　Throughput. The medium access control protocol should make as efficient use as possible of the wireless medium to maximize capacity.</p><p>　

20、　Number of nodes. Wireless LANs may need to support hundreds of nodes across multiple cells.</p><p>　　Connection to backbone LAN. In most cases, interconnection with stations on a wired backbone LAN is requi

21、red. For infrastructure wireless LANs, this is easily accomplished 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 netwo

22、rks.</p><p>　　Service area. A typical coverage area for a wireless LAN may be up to a 300 to 1000 foot diameter.</p><p>　　Battery power consumption. Mobile workers use battery-powered workstatio

23、ns that need to have a long battery life 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 s

24、tationis inappropriate.</p><p>　　Transmission robustness and security. Unless properly designed, a wireless LAN may be interference-prone and easily eavesdropped upon. The design of a wireless LAN must permi

25、t reliable transmission even in a noisy environment and should provide some level of security from eavesdropping.</p><p>　　Collocated network operation. As wireless LANs become more popular, it is quite like

26、ly for two of them to operate 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

27、particular LAN.</p><p>　　License-free operation. Users 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>　　H

28、andoWroaming. The MAC protocol used in the wireless LAN should enable mobile stations to move from one cell to another.</p><p>　　Dynamic configuration. The MAC addressing and network management aspects of th

29、e LAN should permit dynamic and automated addition, deletion, and relocation of end systems without disruption to other users.</p><p>　　Physical Medium Specification</p><p>　　Three physical medi

30、a are defined in the current 802.11 standard:</p><p>　　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

31、.4-GHz ISM band. Up to 7 channels, each with a data rate of 1 Mbps 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 fur

32、ther study.</p><p>　　Wireless LAN Technology</p><p>　　Wireless LANs are generally categorized according to the transmission techniquethat is used. All current wireless LAN products fall into one

33、 of the following categories:</p><p>　　Infrared (IR) LANs. An individual cell of an IR LAN is limited to a single room, as infrared light does not penetrate opaque walls.</p><p>　　Spread Spectru

34、m LANs. This type of LAN makes use of spread 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

35、in the U.S.</p><p>　　Narrowband Microwave. These LANs operate at microwave frequencies but do not use spread spectrum. Some of these products operate at frequencies that require FCC licensing, while others u

36、se one of the unlicensed ISM bands.</p><p>　　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 st

37、andard and are not reflected in all commercial products. However, it is useful to be familiar with the standard as its features are representative of required wireless LAN capabilities.</p><p>　　The smallest

38、 building block of a wireless LAN is a basic service set (BSS), which 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 iso

39、lated, 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 function house

40、d in the access point. The basic service set gen</p><p>　　No-transition. A station of this type is either stationary or moves only within the direct communication range of the communicating stations of a sin

41、gle BSS.</p><p>　　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

42、be able to recognize the new location of the station.</p><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 sens

43、e 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 further study.</p>&l

44、t;p>　　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 mec

45、hanism; and centralized access protocols, which involve regulation 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 attr

46、active in other wireless LAN configurations that consist primarily of</p><p>　　The end result of the 802.11 is a MAC algorithm called DFWMAC (distributed foundation wireless MAC) that provides a distributed

47、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)

48、. DCF uses a contention algorithm to provide access to all traffic. Ordinary asynchronous traffic directly uses DCF. The point coordination function (PC</p><p>　　Distributed Coordination Function</p>

49、<p>　　The 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 unti

50、l the current transmission is complete before 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

51、 the signals on the medium is very large, so that a transmitting statio</p><p>　　delay known as an interframe space (IFS). In fact, there are three different IFS values, but the algorithm is best explained b

52、y initially ignoring this detail. Using an IFS, the rules for CSMA 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

53、the medium remains idle for a time equal to IFS, and, if this is so, the station may immediately transmit.</p><p>　　2. If the medium is busy (either because the station initially finds the medium busy or bec

54、ause 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.</p><p>　　3. Once the current transmission i

55、s 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 stati

56、on may transmit.</p><p>　　Point Coordination Function</p><p>　　PCF is an alternative access method implemented on top of the DCF. The operation consists of polling with the centralized polling m

57、aster (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 out all asynchronous traffic while it issues p

58、olls and receives responses.</p><p>　　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

59、point coordinator while remaining traffic, using CSMA, contends for access.</p><p>　　The point coordinator could issue polls in a round-robin fashion to all stations configured for polling. When a poll is is

60、sued, 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 during the expected turnaround time, the coordinator issues a p

61、oll. If the discipline of the preceding paragraph were implemented, the point coordinator would lock out all asynchronous traffic by repeatedly issuing polls.</p><p>　　At the beginning of a superframe, the p

62、oint 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

63、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 supe

64、rframe period follows</p><p><b>　　無線局域網(wǎng)技術</b></p><p>　　最近幾年，無線局域網(wǎng)開始在市場中獨霸一方。越來越多的機構發(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)絡的需求，并能覆蓋難以鋪設有線網(wǎng)絡的地域。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

65、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。</p><p><b>　　無線局域網(wǎng)的應用</b></p><p><b>　　局域網(wǎng)的擴展</b></p><p>　　在20世紀80年代后期出現(xiàn)的無線局域網(wǎng)早期產品都是作為傳統(tǒng)有線局域網(wǎng)替代品而

66、問世的。無線局域網(wǎng)可以節(jié)省局域網(wǎng)纜線的安裝費用，簡化重新布局和其他對網(wǎng)絡結構改動的任務。但是，無線局域網(wǎng)的這個動機被以下一系列的事件打消。首先，隨著人們越來越清楚地認識到局域網(wǎng)的重要性，建筑師在設計新建筑時就包括了大量用于數(shù)據(jù)應用的預先埋設好的線路。其次，隨著數(shù)據(jù)傳輸技術的發(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)老建筑中已經鋪設了足夠的3類電纜，而許多新建筑里則預埋了5類電纜。因此，用無線局域網(wǎng)取代

67、有線局域網(wǎng)的事情從來沒有發(fā)生過。</p><p>　　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，但又禁止打洞鋪設新線路的有歷史價值的建筑；從經濟角度考慮，安裝和維護有線局域網(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)的機構同時也擁

68、有支持服務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的有線局域網(wǎng)上。所以我們將此類應用領域成為局域網(wǎng)的擴展。</p><p><b>　　建筑物的互連</b></p><p>　　無線局域網(wǎng)技術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，兩個樓之間采用點對點的無線鏈接。被鏈接的設備通常是網(wǎng)橋或路

69、由器。這種點對點的單鏈路從本質上看不是局域網(wǎng)，但通常我們也把這種應用算作無線局域網(wǎng)。</p><p><b>　　漫游接入</b></p><p>　　漫游接入提供局域網(wǎng)和帶有天線的移動數(shù)據(jù)終端之間的無線鏈接，如膝上型電腦和筆記本電腦。這種應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

70、群之外操作的一臺電腦或一次商務行為。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)上的服務器。</p><p><b>　　自組網(wǎng)絡</b></p><p>　　自組網(wǎng)絡（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（沒有中央服務器）例如，有一群職員，每人帶著一臺膝上電腦或掌上電腦，會聚在商務會議

71、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。</p><p><b>　　無線局域網(wǎng)的要求</b></p><p>　　無線局域網(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：</p>&

72、lt;p>　　吞吐量：媒體接入控制協(xié)議應當盡可能地有效利用無線媒體以達到最大的容量。</p><p>　　節(jié)點數(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。</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)的站點相互連接。對于有基礎設施的無線局域網(wǎng)，很容易通過利用控制模塊完成這個任務，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

73、無線網(wǎng)絡來說，可能需要滿足這個要求。</p><p>　　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，電池供電時間足夠長。這就是說，要求移動節(jié)點不停地監(jiān)視接入點或者經常要與基站握手的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)絡時減少能量消耗的特殊性能，如睡眠模式。</p><p>　　傳輸健壯性和安全性：除非涉及合理，無線局域網(wǎng)很容

74、易受到干擾并且容易被竊聽。無線局域網(wǎng)的設計必須做到即使在噪音較大的環(huán)境中也能可靠傳輸，并且為應用提供某種程度的安全性，以防竊聽。</p><p>　　并列的網(wǎng)絡操作：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，或在局域網(wǎng)之間可能存在干擾的某些區(qū)域內運行。這種干擾可能會阻礙MAC算法的正常運行，還可能造成對特定局域網(wǎng)的非法接入。</p><p>　　不需

75、要許可證的操作：用戶希望購買和運行的是這樣的無線局域網(wǎng)產品，它們不需要專門為局域網(wǎng)所使用的頻帶而申請許可證。</p><p>　　切換和漫游：無線局域網(wǎng)中使用的MAC協(xié)議應當讓移動站點能夠從一個蜂窩移動到另一個蜂窩。</p><p>　　動態(tài)配置：局域網(wǎng)在MAC地址機制和網(wǎng)絡管理方面應當允許端系統(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。</p><p

76、><b>　　無線局域網(wǎng)技術</b></p><p>　　無線局域網(wǎng)通常根據(jù)它所采用的傳輸技術進行分類。目前所有無線局域網(wǎng)產品都可歸為以下三個大類之一：</p><p>　　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。</p><p>　　擴頻局域網(wǎng)：這種類型的局域網(wǎng)利用了擴頻傳輸技術。

77、在大多數(shù)情況下，這些局域網(wǎng)運行在ISM(個人、科學和醫(yī)學)波段內，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。</p><p>　　窄帶微波：這些局域網(wǎng)運行在微波頻率是，但沒有使用擴頻技術。其中有些產品運行的頻率需要FCC的許可證，而其他一些產品則使用了不需要許可的波段。</p><p>　　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

78、煩的授權過程就能使用。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雜。在美國，F(xiàn)CC在ISM波段內特許了兩個不需要許可證的應用：最大功率為1瓦的擴頻系統(tǒng)合最大運行功率為0.5瓦的低功率系統(tǒng)。自從FCC開放了這個波段以來，在擴頻無線局域網(wǎng)中的應用就越來越普遍。</p><p>　　1990年IEEE802.11工作組成立，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。</p>&

79、lt;p>　　無線局域網(wǎng)中最小的模塊是基本服務集（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組成?；痉占梢允枪铝⒌?，也可以通過接入點（Access Point, AP）連到主干分發(fā)系統(tǒng)（Distribution System, DS）上。接入點的功能相當于網(wǎng)橋。MAC協(xié)議可以是完全分布式的，也可以由位于接入點的中央?yún)f(xié)調功能控制。BBS通常與文獻中的蜂窩相對應，而

80、DS則有可能是交換機或有線網(wǎng)絡，也可以是無線網(wǎng)絡。</p><p>　　MAC層的主要任務是在MAC實體之間傳送MSDU，這個任務是由分發(fā)服務實現(xiàn)的。分發(fā)服務的正常運行需要該ESS內所有站點的信息，而這個信息是由與關聯(lián)（association）相關的服務提供的。在分發(fā)服務向站點交付數(shù)據(jù)或者接收來自站點的數(shù)據(jù)之前，該站點必須要建立關聯(lián)。標準基于移動性定義了三種轉移類型：</p><p>　　

81、無轉移：這種類型的站點或者是固定的，或者只在一個BSS的直接通信范圍內移動。</p><p>　　BSS轉移：這種類型的站點移動是在同一ESS內從一個BSS移動到另一個BSS。在這種情況下，該站點的數(shù)據(jù)交付需要尋址功能，能識別出該站點的新位置。</p><p>　　ESS轉移：它的定義是指站點從一個ESS的BSS到另一個ESS的BSS移動。只有從某種意義上看該站點是能夠移動的，才能支持這種

82、類型的轉移。</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)聽機制把傳輸?shù)臎Q定權分布到所有節(jié)點。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和以某種形式連接到主干有線局

83、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。</p><p>　　IEEE802.11的最終結果是一個稱為分布式基礎無線MAC（Distributed Foundation Wireless MAC,DFWMAC）的算法，它提供了一個分布式接入控制機制，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)調功能（Distributed C

84、oordination 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。點協(xié)調功能（Point Coordination Function, PCF）是一個集中式MAC算法，用于提供無爭用服務。</p><p><b>　　分布式協(xié)調功能</b></p><p>　　DCF子層使用一種簡單的CSMA（載波監(jiān)聽多點

85、接入）算法。如果站點有一個MAC幀要發(fā)送，則先監(jiān)聽媒體。如果媒體空閑，站點可以發(fā)送。否則，該站點必須等待直到當前的發(fā)送結束。DCF不包括沖突檢測功能（CSMA/CD），因為在無線網(wǎng)絡中進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。</p><p>　　為了保證算法的平穩(wěn)和公平運行，DCF包含了一組等價于優(yōu)先級策略的時延。我們

86、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：</p><p>　　1。有幀要傳輸?shù)恼军c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。</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空閑時間內發(fā)現(xiàn)媒體忙），則推遲傳輸，并繼續(xù)監(jiān)聽媒體直到當前的傳輸結束。&

87、lt;/p><p>　　3。一旦當前的傳輸結束，站點再延遲IFS一段時間。如果媒體在這段時間內都是空閑的，則站點采用二進制指數(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計時。</p><p><b>　　點協(xié)調功能</b></p><p>　　PCF是在DC

88、F之上實現(xiàn)的另一種接入方式。其操作由中央輪詢主控器（點協(xié)調器）的輪詢構成。點協(xié)調在發(fā)布輪詢時采用PIFS。因為PIFS比DIFS小，所以點協(xié)調器在發(fā)布輪詢和接收響應時能獲取媒體并封鎖所有的異步通信量。</p><p>　　點協(xié)調器不斷地發(fā)布輪詢，并永遠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）的時間間隔。在超幀時間的開始部分，點協(xié)調器以循環(huán)方式向所有配置成輪詢的站點發(fā)布輪詢。

89、然后，在余下的超幀時間里，點協(xié)調器空閑，允許異步通信量有一段爭用接入的時間。</p><p>　　在超幀開始時，點協(xié)調器可以在給定時間內獲得控制權和發(fā)布輪詢，這由選項決定。由于響應站點發(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，點協(xié)調器泳PIFS時間爭用媒體接入權。如果媒體是空閑的，點協(xié)調器可以立刻接入，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。