計(jì)算機(jī)專業(yè)外文翻譯-----無(wú)線局域網(wǎng)技術(shù)

1、<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 finding that wireless LA

2、Ns 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 wireless LAN is one

3、 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 of wireless LANs, a

4、nd 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 extension, crossbuilding i

5、nterconnect, 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 1980s, were marketed

6、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 motivation for wire

7、less 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 in dat</p>&

8、lt;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, stock exchange tradin

9、g 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 LANs is not economical

10、. 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 in nearby buildings

11、, 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 is not a LAN per se

12、, 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 a mobile data term

13、inal 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 compu

14、ter 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</p><p>

15、;　　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 with a laptop or palm

16、top 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>　　Wireless LAN Requi

17、rements</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 stations, and broadcast

18、 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>　　Throughput. The medi

19、um 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 nodes across multiple

20、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 through the use of

21、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 for a wireless LAN ma

22、y 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 adapters. This sugges

23、ts 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 robustness and security. Un

24、less 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 provide some level of s

25、ecurity 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 where interference betw

26、een 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 would prefer to bu

27、y 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 enable mobile stations

28、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 relocation of end sy

29、stems 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>　　Infrared at 1 Mbps

30、 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 or 2 Mbps, can be u

31、sed.</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><p>　　Wirele

32、ss 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) LANs. An individual

33、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 technology. In most

34、 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 operate at microwav

35、e 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 wireless LAN standa

36、rds 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, it is useful to be f

37、amiliar 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), which consists of some

38、 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 an access point. Th

39、e 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>　　No-transition.

40、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 station movement f

41、rom 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><p>　　ESS-transi

42、tion. 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 supported by 802.1

43、1 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 for a MAC algorith

44、m: 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 transmission by

45、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</p><p

46、>　　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 on top of that. Fi

47、gure 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. Ordinary asynchron

48、ous 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. If a station has a M

49、AC 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 does not include a

50、 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 statio</p><

51、;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 access are as follo

52、ws:</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, the station may i

53、mmediately 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 defers transmissio

54、n 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 for this period, th

55、en 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</p><p>

56、;　　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 when issuing polls.

57、 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, consider the followi

58、ng 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 access.</p>

59、<p>　　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 r

60、esponse, it issues another poll using PIFS. If no response is received during the expected turnaround time, the coordinator issues a poll. If the discipline of the preceding paragraph were implemented, the point coordina

61、tor would lock out all asynchronous traffic by repeatedly issuing polls.</p><p>　　At the beginning of a superframe, the point coordinator may optionally seize control and issue polls fora give period of time

62、. 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 coordina

63、tor 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</p><p><b>　　無(wú)線局域網(wǎng)技術(shù)</b></p><

64、p>　　最近幾年，無(wú)線局域網(wǎng)開(kāi)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)。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(wèn)題的逐步解決，無(wú)線局域網(wǎng)很快就開(kāi)始流行起來(lái)了。</p><p><

65、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)品都是作為傳統(tǒng)有線局域網(wǎng)替代品而問(wèn)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

66、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(méi)有發(fā)生過(guò)。</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

67、大廳以及倉(cāng)庫(kù)這樣有大型開(kāi)闊場(chǎng)地的建筑；沒(méi)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

68、><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&g

69、t;</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(wèn)有線局域網(wǎng)上的服務(wù)器。</p><p>

70、;<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(méi)有中央服務(wù)器）例如，有一群職員，每人帶著一臺(tái)膝上電腦或掌上電腦，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&l

71、t;/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

72、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(guò)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

73、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(tīng)。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(tīng)。</p><p>　

74、　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><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

75、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。</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

76、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(guò)不透明的墻。</p><p>　　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(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&

77、gt;　　窄帶微波：這些局域網(wǎng)運(yùn)行在微波頻率是，但沒(méi)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(lè)意接受的，雖然不是必要的，那就是不需要通過(guò)麻煩的授權(quán)過(guò)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

78、功率為0.5瓦的低功率系統(tǒng)。自從FCC開(kāi)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(kāi)發(fā)MAC協(xié)議以及物理媒體規(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?；?/p>

79、服務(wù)集可以是孤立的，也可以通過(guò)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(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

80、。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><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

81、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)

82、聽(tīng)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>　　IEEE80

83、2.11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)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 Coordinati

84、on 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子層使用一種簡(jiǎn)單的CSMA（載波監(jiān)聽(tīng)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(tīng)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)?/p>

85、在無(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)?/p>

86、站點(diǎn)先監(jiān)聽(tīng)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(kāi)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，并繼續(xù)監(jiān)聽(tīng)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)

87、聽(tīng)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><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

88、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(kāi)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(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(kāi)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