信息工程外文翻譯----無(wú)線局域網(wǎng)dcf接入機(jī)制的opnet性能仿真（節(jié)選）

1、<p>　　中文3815字，2700單詞，13000英文字符</p><p>　　畢業(yè)設(shè)計(jì)（論文）外文文獻(xiàn)翻譯</p><p>　　9-MAC子層的功能描述</p><p>　　在這一條章中提出了MAC的功能描述。MAC子層的體系結(jié)構(gòu)，包括分布式協(xié)調(diào)功能（DCF），點(diǎn)協(xié)調(diào)功能（PCF），混合協(xié)調(diào)功能（HCF），并介紹了在9.1在IEEE 802.11局域網(wǎng)

2、共存。</p><p>　　9.1 MAC 體系結(jié)構(gòu)</p><p>　　MAC架構(gòu)可以描述成如圖9-1所示，提供了PCF和HCF通過(guò)DCF的服務(wù)。但是值得注意的是，在一個(gè)非QOS STA，HCF是不存在的。在QOS標(biāo)準(zhǔn)的實(shí)施中，DCF和HCF都是存在的。而在所有的STAs中PCF都是可選的。如圖9-1MAC結(jié)構(gòu)圖所示：</p><p><b>　　9.

3、1.1 DCF</b></p><p>　　在IEEE 802.11 MAC中的最基本的訪問(wèn)方法是DCF，被稱為載波偵聽(tīng)多路訪問(wèn)沖突避免（CSMA/CA）。DCF應(yīng)在所有STA中實(shí)現(xiàn)，用于在IBSS和基礎(chǔ)設(shè)施網(wǎng)絡(luò)配置中。</p><p>　　一個(gè)STA請(qǐng)求的發(fā)送，應(yīng)識(shí)別媒介中是否有另一個(gè)STA在發(fā)送。如果確認(rèn)介質(zhì)是不忙碌的，則可以傳輸。CSMA/CA分布式算法允許在連續(xù) 幀序列

4、之間存在最小指定時(shí)間間隔。 一個(gè)發(fā)射站應(yīng)保證在所需的時(shí)間之前，傳輸介質(zhì)是空閑的。 如果介質(zhì)當(dāng)前狀態(tài)是忙碌的，那么STA將推遲到當(dāng)前的傳輸結(jié)束后執(zhí)行。 在延時(shí)之后，在成功的發(fā)送后，立馬優(yōu)先嘗試再發(fā)射， 當(dāng)通道是空閑的，STA將選擇一個(gè)隨機(jī)退避間隔 并且消減退避間隔的計(jì)數(shù)器。一個(gè)傳輸成功時(shí)，ACK幀被接收，從STA尋址通過(guò)RA領(lǐng)域解決所傳送的幀，或當(dāng)一個(gè)幀與一組地址在RA領(lǐng)域中被發(fā)送完全時(shí)。一個(gè)細(xì)化的方法也許可以在各種不同的情況下，在數(shù)據(jù)傳

5、輸之前，進(jìn)一步減少碰撞，在測(cè)定發(fā)送和接收站交換短控制幀（RTS和CTS幀）后，確定媒體是否是空閑，存在任何延時(shí)或補(bǔ)償。</p><p><b>　　9.2 DCF</b></p><p>　　基本的介質(zhì)訪問(wèn)協(xié)議DCF 允許自動(dòng)介質(zhì)之間在兼容的PHYs中共享，一個(gè)繁忙的信道狀態(tài)之后，通過(guò)使用CSMA/CA和一個(gè)隨機(jī)退避時(shí)間機(jī)制。除此之外，當(dāng)發(fā)送方?jīng)]有收到ACK，所有單獨(dú)

6、的尋址方式使用即時(shí)確認(rèn)（ACK幀）機(jī)制進(jìn)行重發(fā)。</p><p>　　CSMA/CA協(xié)議的目的是在碰撞最有可能發(fā)生的地方，減少信道中多個(gè)STA訪問(wèn)之間的碰撞概率。在信道由空閑變成繁忙的時(shí)候，是當(dāng)發(fā)生碰撞的概率最高的時(shí)候。這是因?yàn)槎鄠€(gè)STA從一直在等待中再次變?yōu)榭捎?。這種情況，就需要一個(gè)隨機(jī)退避過(guò)程去解決信道中的沖突問(wèn)題。</p><p>　　CS的實(shí)行需要通過(guò)物理和虛擬的機(jī)制來(lái)進(jìn)行。<

7、;/p><p>　　根據(jù)DCF發(fā)送數(shù)據(jù)幀時(shí)必須使用的幀類型的數(shù)據(jù)和子類型的數(shù)據(jù)，或者NULL功能。當(dāng)子類型為NULL功能，那么STA接收數(shù)據(jù)類型的幀將不能指明一個(gè)數(shù)據(jù)的幀到LLC，但應(yīng)當(dāng)指明一個(gè)數(shù)據(jù)幀到LLC當(dāng)替補(bǔ)類型是數(shù)據(jù)，即使幀的結(jié)構(gòu)只包含零字節(jié)。</p><p>　　9.2.4 隨機(jī)退避機(jī)制</p><p>　　一個(gè)STA需要發(fā)起數(shù)據(jù)轉(zhuǎn)讓時(shí)，MPDU和MMPDIS

8、應(yīng)調(diào)用CS機(jī)制來(lái)確定介質(zhì)的狀態(tài)是處于忙碌還是空閑。如果介質(zhì)是繁忙的，STA將推遲到媒介被確認(rèn)為空閑的，沒(méi)有中斷一段時(shí)間，等價(jià)于當(dāng)最后一幀被確認(rèn)在通道中接收完成，或者在媒介被確認(rèn)為空閑之后，沒(méi)有中斷的時(shí)間，等價(jià)于EIFS當(dāng)最后一幀被確認(rèn)在媒介中未被正確的接收。 在DIFS或者EIFS信道空閑的時(shí)間之后，STA將生成一個(gè)隨機(jī)的退避時(shí)間，在傳播之前有一個(gè)額外的延時(shí)，除非退避時(shí)間已經(jīng)包含了一個(gè)非零的值，在不需要隨機(jī)數(shù)的情況下不執(zhí)行。這個(gè)過(guò)程可以

9、最大限度地減少 多個(gè)STA之間存在的延時(shí)競(jìng)爭(zhēng)類似事件之間的沖突碰撞。</p><p>　　Back off Time = Random() × a Slot Time</p><p>　　Back off Time為實(shí)際的退避時(shí)間，Random()為均勻分布在[0，CW]內(nèi)的偽隨機(jī)數(shù)，CW為物理層中最小競(jìng)爭(zhēng)窗口特征值與最大競(jìng)爭(zhēng)窗口特征值之間的數(shù)，a Slot Time（時(shí)隙時(shí)間

10、）是物理層上的值，其大小均由物理層決定。</p><p>　　競(jìng)爭(zhēng)窗口（CW）的參數(shù)應(yīng)當(dāng)取 aCWmin 的初始值。每一階段都應(yīng)保持STA短重試計(jì)數(shù)（SSRC）以及一個(gè)STA長(zhǎng)重試計(jì)數(shù)（SLRC），均應(yīng)采取一個(gè)初始的零值。當(dāng)任何短重試計(jì)數(shù)SRC聯(lián)合任何一個(gè)MPDU的數(shù)據(jù)類型被遞增，SSRC將會(huì)被遞增。 當(dāng)任何長(zhǎng)重試計(jì)數(shù)聯(lián)合任何一個(gè)MPDU的數(shù)據(jù)類型被遞增，SLRC也將遞增。 在連續(xù)的任何時(shí)間內(nèi)有一個(gè)不成功，CW應(yīng)

11、當(dāng)取接下來(lái)的值，并嘗試去傳輸一個(gè)MPDU原因，STA也將再次相反的去遞增，直到連續(xù)波到達(dá)aCWmax的值。一個(gè)重新定義為整個(gè)幀序列發(fā)送的間隔，被SIFS間隔時(shí)間分開(kāi)，在試圖提供一個(gè)MPDU。一旦達(dá)到aCWmax，CW應(yīng)保持在值aCWmax直到連續(xù)復(fù)位。這提高了在高負(fù)載下的訪問(wèn)協(xié)議的穩(wěn)定性。如圖9-2：</p><p><b>　　圖9-2</b></p><p>　　

12、CW應(yīng)復(fù)位到aCWmin，在每成功發(fā)送MPDU或MMPDU后，在SLRC到達(dá)dot11LongRetryLimit階段，或者當(dāng)SSRC到達(dá)dot11LongRetryLimit階段。 SSRC應(yīng)當(dāng)被重置為0當(dāng)CTS幀來(lái)響應(yīng)一個(gè)RTS幀接收，當(dāng)ACK幀響應(yīng)接收一個(gè)MPDU或者M(jìn)MPDU的傳輸，或者當(dāng)一個(gè)幀和一組地址在地址1場(chǎng)內(nèi)被傳輸。SLRC應(yīng)復(fù)位到0當(dāng)ACK幀響應(yīng)一個(gè)MPDU發(fā)送或接收MMPDU長(zhǎng)度大于dot11rtsthreshold

13、，或當(dāng)一幀中的地址1組地址字段被傳輸。</p><p>　　連續(xù)值的設(shè)定應(yīng)依次上升2，整數(shù)次冪減1，開(kāi)始與物理層明確的aCWmin的值，繼續(xù)直至并包括物理層具體的aCWmax價(jià)值。</p><p>　　9.2.5 DCF訪問(wèn)過(guò)程</p><p>　　CSMA/CA接入方法基礎(chǔ)是DCF。執(zhí)行方式規(guī)則在DCF和PCF之間有細(xì)微的不同。</p><p&

14、gt;　　9.2.5.1 基礎(chǔ)訪問(wèn)</p><p>　　基本的訪問(wèn)是指 STA的使用，確定它是否可以成為傳遞的核心機(jī)制。一般來(lái)說(shuō)，一個(gè)站可以發(fā)送一個(gè)掛起的MPDU 當(dāng)它是DCF接入方式下的訪問(wèn)方式，無(wú)論是否缺少PC，或者在PCF的CP接入方式，當(dāng)STA確定介質(zhì)是空閑的，大于或等于DIFS時(shí)間，或EIFS期間如果立刻在先前的介質(zhì)繁忙的事件是人為偵查的幀沒(méi)有被接收，在這個(gè)階段有一個(gè)正確的MAC FCS值。如果，在這些

15、條件下，媒介是由CS機(jī)制確定的繁忙的，當(dāng)一個(gè)STA請(qǐng)求開(kāi)始最初的幀的一個(gè)幀交換，專用的CF階段，隨機(jī)退避程序應(yīng)遵循9.2.5.2的描述。在這樣的條件下，在9.2.5.2和9.2.5.5指定的地方，隨機(jī)退避程序應(yīng)當(dāng)遵循甚至嘗試去發(fā)起一個(gè)幀去交換序列。</p><p>　　在STA中具有跳頻PHY，對(duì)信道的控制實(shí)在丟失的保留時(shí)間邊界和STA將競(jìng)爭(zhēng)信道，在居住邊界之后。它要求有一個(gè)跳頻PHY完成STA整個(gè)層傳輸和相關(guān)的

16、確認(rèn)（如果需要）前停留時(shí)間的邊界。如果當(dāng)發(fā)送或者程序存在一個(gè)MPDU，沒(méi)有足夠的剩余時(shí)間去允許傳送MPDU的確認(rèn)信息（是否所需的），STA將會(huì)通過(guò)選擇一個(gè)隨機(jī)退避時(shí)間，推遲傳送，利用現(xiàn)有的CW（沒(méi)有推進(jìn)到系列中的一個(gè)值）。 這個(gè)短和長(zhǎng)的重試計(jì)數(shù)器的MSDU計(jì)數(shù)不受影響。</p><p>　　基本訪問(wèn)機(jī)制如圖9-3所示。</p><p>　　圖9-3 基本訪問(wèn)機(jī)制圖</p>&

17、lt;p>　　9.5.2.2 DCF退避過(guò)程</p><p>　　本小節(jié)介紹了退避過(guò)程，使用時(shí)要調(diào)用的DCF。</p><p>　　退避機(jī)制為STA傳送一個(gè)幀時(shí)，當(dāng)發(fā)現(xiàn)信道繁忙時(shí)，表明無(wú)論是物理的或虛擬的CS的機(jī)制，退避程序也應(yīng)當(dāng)調(diào)用一個(gè)發(fā)射站推斷傳輸失敗。如圖9-6所示：</p><p>　　開(kāi)始退避機(jī)制時(shí)，STA將設(shè)置他的退避時(shí)間器區(qū)一個(gè)隨機(jī)退避時(shí)間使用

18、的方程式。所有的退避時(shí)隙發(fā)生之后，一個(gè)DIFS 時(shí)間在信道被確認(rèn)閑置的DIFS期間或之后期間，或者跟在一個(gè)EIFS期間之后當(dāng)信道被確認(rèn)為空閑的。</p><p>　　STA執(zhí)行退避程序應(yīng)使用CS機(jī)制以確定是否有每個(gè)退避期間的活動(dòng)。如果沒(méi)有信道活動(dòng)去表明一個(gè)特定的退避時(shí)隙的時(shí)間，然后退避程序?qū)?huì)通過(guò)a Slot Time.消耗退避時(shí)間。</p><p>　　如果信道在所有退避機(jī)制中被確認(rèn)是繁

19、忙的，那么退避機(jī)制將會(huì)被暫停。那么，退避計(jì)時(shí)器將不會(huì)被消耗。當(dāng)信道在持續(xù)的DIFS時(shí)期或者EIFS被確認(rèn)為空閑時(shí)，作為適當(dāng)?shù)?，在退避過(guò)程允許恢復(fù)之前。當(dāng)退避計(jì)時(shí)器達(dá)到零時(shí)開(kāi)始傳輸。</p><p>　　退避程序應(yīng)當(dāng)在每個(gè)傳輸?shù)慕Y(jié)束后立即執(zhí)行，并且將MPDU的數(shù)據(jù)類型的更多的零碎比特設(shè)置為0，管理，或者控制PS-Poll的圖表類型，即使沒(méi)有額外的傳送在當(dāng)前的隊(duì)列等待。在傳輸成功的情況下，退避過(guò)程應(yīng)當(dāng)在收到ACK幀結(jié)

20、束后開(kāi)始。在不成功傳輸?shù)那闆r下</p><p>　　需要確認(rèn)，這種退避程序應(yīng)在確認(rèn)的超時(shí)時(shí)間結(jié)束后開(kāi)始。一個(gè)不成功的傳輸是一個(gè)ACK幀沒(méi)有接收從STA尋址的一個(gè)單獨(dú)的地址。如果發(fā)送成功，CW值恢復(fù)到之前aCWmin隨機(jī)退避間隔選擇，SSRC和SLRC被更新。這確保了發(fā)送幀從STA總是由至少一個(gè)退避間隔。</p><p>　　這個(gè)程序的作用是，當(dāng)多個(gè)站點(diǎn)正在推遲進(jìn)入隨機(jī)退避，然后STA選擇最

21、小的退避時(shí)間使用隨機(jī)函數(shù)將贏得競(jìng)爭(zhēng) （假設(shè)所有的STAs檢測(cè)WM活動(dòng)相同的實(shí)例在各自的接收機(jī)）。</p><p>　　在IBSS中，時(shí)間退避機(jī)制。在IBSS，一個(gè)單獨(dú)的退避間隔應(yīng)產(chǎn)生Beacon幀的傳輸。</p><p>　　9.2.5.4 設(shè)置和重置 NAV</p><p>　　STA接收到有效的幀應(yīng)該更新他的NAV，在持續(xù)的收到的所有幀的信息，當(dāng)新的NAV的值

22、大于當(dāng)前NAV的值，排除NAV不被更新，當(dāng)RA等于接收到的STA的MAC地址時(shí)。在收入一個(gè)PS-Poll幀，一個(gè)STA應(yīng)被更新他的NAV，適當(dāng)設(shè)置，在數(shù)據(jù)的選擇規(guī)則下使用一個(gè)持續(xù)的值相同的時(shí)間，在微秒內(nèi)，需要發(fā)送一個(gè)ACK幀加一個(gè)SIFS間隔，但是只有當(dāng)新的NAV的值大于當(dāng)前NAV的值。如果計(jì)算時(shí)間包括小數(shù)微秒，值圍捕高一個(gè)整數(shù)。各種附加的條件可以被設(shè)置或重置NAV。</p><p>　　圖9-7表明STA可能收

23、到RTS幀的，而其他的STAs只能接收到CTS幀，導(dǎo)致低的NAV門檻被表現(xiàn)出（除STA的RTS處理）。如圖9-7 RTS/CTS/數(shù)據(jù)/ACK設(shè)置所示：</p><p>　　一個(gè)STA使用從一個(gè)RTS幀作為最新的基礎(chǔ)去更新他的NAV設(shè)置，并允許重置他的NAV如果沒(méi)有PHY-RXSTART。指示被發(fā)現(xiàn)從物理層的時(shí)期有一段持續(xù)的時(shí)間（(2× a SIFS Time)+(CTS Time)+a PHY-RX

24、-START-Delay+(2×a Slot Time)開(kāi)始于PHY-RXEND。在RTS幀檢測(cè)相應(yīng)的指示。“CTS Time” 應(yīng)該被計(jì)算出用于CTS幀的長(zhǎng)度，數(shù)據(jù)的速率在RTS幀用于最新的NAV更新被接收。</p><p>　　9.2.5.6 RTS/CTS 的小部分使用</p><p>　　以下是一個(gè)描述使用RTS/CTS對(duì)小部分 MSDU 或者 MMPDU。RTS/CT

25、S幀定義下列持續(xù)的幀和確認(rèn)的。在數(shù)據(jù)和ACK的時(shí)間/ID字段幀指定的持續(xù)的一個(gè)片段和確認(rèn)的總時(shí)間，這是在圖9-9所示。</p><p>　　每一幀包含定義下一次傳輸?shù)某掷m(xù)時(shí)間的信息。RTS幀的時(shí)間信息將被用于更新NAV去表明繁忙的直到ACK結(jié)束為0。兩片段0和ACK 0應(yīng)包含時(shí)間信息去更新NAV去表示繁忙直到ACK為1結(jié)束。這將通過(guò)使用時(shí)間/ ID字段中的數(shù)據(jù)和ACK幀。這將繼續(xù)下去直到最后一個(gè)片段，這將持續(xù)一個(gè)

26、ACK時(shí)間加上一個(gè)SIFS時(shí)間，及其應(yīng)答，這將它的持續(xù)時(shí)間/標(biāo)識(shí)字段設(shè)置為0。每個(gè)片段和ACK作為一個(gè)虛擬的RTS和CTS；因此，沒(méi)有進(jìn)一步的RTS/CTS幀需要在 RTS/CTS幀序列開(kāi)始交換之后發(fā)生，即使隨后的碎片可能會(huì)比dot11rtsthreshold大。在STA的使用fhphy，當(dāng)沒(méi)有足夠的時(shí)間在存在于分界之前去傳輸后來(lái)的片段，那么STA起始幀交換序列可能設(shè)置時(shí)間/ ID字段結(jié)構(gòu)在數(shù)據(jù)末尾或者管理幀去發(fā)射，在邊界一個(gè)ACK的時(shí)

27、間前加上一個(gè)SIFS時(shí)間。</p><p>　　在這種情況下，確認(rèn)發(fā)送但是不接收源STA。 STA取得片段，或者ACK，將標(biāo)志信道忙碌，下一幀的交換為了NAV從這些幀中被更新。這是最壞的情況，它是在圖9所示。如果確認(rèn)不是由目的站發(fā)送的STA，STA只能聽(tīng)到目的STA，將不會(huì)更新他的NAV并且嘗試去使用通道，當(dāng)他的NAV更新來(lái)自事先接收的幀的層級(jí)為0.所有的STA收聽(tīng)到源將會(huì)最有的使用通道當(dāng)他們NAV從發(fā)射片段被失

28、效之后。</p><p>　　9. MAC sublayer functional description</p><p>　　The MAC functional description is presented in this clause. The architecture of the MAC sublayer, including the distributed coordina

29、tion function (DCF), the point coordination function (PCF), the hybrid coordination function (HCF), and their coexistence in an IEEE 802.11 LAN are introduced in 9.1. </p><p>　　9.1 MAC architecture</p>

30、;<p>　　The MAC architecture can be described as shown in Figure 9-1 as providing the PCF and HCF through the services of the DCF. Note that in a non-QoS STA, HCF is not present. In a QoS STA implementation, both D

31、CF and HCF are present. PCF is optional in all STAs.</p><p><b>　　9.1.1 DCF</b></p><p>　　The fundamental access method of the IEEE 802.11 MAC is a DCF known as carrier sense multiple

32、access with collision avoidance (CSMA/CA). The DCF shall be implemented in all STAs, for use within both IBSS and infrastructure network configurations.</p><p>　　For a STA to transmit, it shall sense the med

33、ium to determine if another STA is transmitting. If the medium is not determined to be busy (see 9.2.1), the transmission may proceed. The CSMA/CA distributed algorithm mandates that a gap of a minimum specified

34、 duration exist between contiguous frame sequences. A transmitting STA shall ensure that the medium is idle for this required duration before attempting to transmit. If the medium is determined to be busy, t

35、he STA shall </p><p><b>　　9.2 DCF</b></p><p>　　The basic medium access protocol is a DCF that allows for automatic medium sharing between compatible PHYs through the use of CSMA/

36、CA and a random backoff time following a busy medium condition. In addition, all individually addressed traffic uses immediate positive acknowledgment (ACK frame) where retransmission is scheduled by the sender if no

37、ACK is received.</p><p>　　The CSMA/CA protocol is designed to reduce the collision probability between multiple STAs accessing a medium, at the point where collisions would most likely occur. Just after the

38、medium becomes idle following a busy medium (as indicated by the CS function) is when the highest probability of a collision exists. This is because multiple STAs could have been waiting for the medium to become availabl

39、e again. This is the situation that necessitates a random backoff procedure to resolve medium conte</p><p>　　CS shall be performed both through physical and virtual mechanisms.</p><p>　　The virt

40、ual CS mechanism is achieved by distributing reservation information announcing the impending use of the medium. The exchange of RTS and CTS frames prior to the actual data frame is one means of distribution of this

41、medium reservation information. The RTS and CTS frames contain a Duration field that defines the period of time that the medium is to be reserved to transmit the actual data frame and the returning ACK frame. All STAs wi

42、thin the reception range of either the originating S</p><p>　　9.2.4 Random backoff time</p><p>　　A STA desiring to initiate transfer of data MPDUs and/or MMPDUs shall invoke the CS mechanism (

43、see 9.2.1) to determine the busy/idle state of the medium. If the medium is busy, the STA shall defer until the medium is determined to be idle without interruption for a period of time equal to DIFS when the last frame

44、 detected on the medium was received correctly, or after the medium is determined to be idle without interruption for a period of time equal to EIFS when the last frame detect</p><p>　　Backoff Time =

45、 Random() × aSlotTime</p><p><b>　　where</b></p><p>　　Random() = Pseudo-random integer drawn from a uniform distribution over the interval </p><p>　　[0,CW], where

46、 CW is an integer within the range of values of the PHY </p><p>　　characteristics aCWmin and aCW-max, aCWmin ≤ CW ≤ aCWmax. It is </p><p>　　Important that designers recognize the need f

47、or statistical independence </p><p>　　among the random number streams among STAs. </p><p>　　aSlotTime = The value of the correspondingly named PHY characteristic.</p><p>　　The

48、 contention window (CW) parameter shall take an initial value of aCWmin. Every STA shall maintain a STA short retry count (SSRC) as well as a STA long retry count (SLRC), both of which shall take an initial value of zero

49、. The SSRC shall be incremented when any short retry count (SRC) associated with any MPDU of type Data is incremented. The SLRC shall be incremented when any long retry count (LRC) associated with any MPDU of type Da

50、ta is incremented. The CW shall take the next va</p><p>　　The CW shall be reset to aCWmin after every successful attempt to transmit an MPDU or MMPDU, when SLRC reaches dot11LongRetryLimit, or when

51、SSRC reaches dot11ShortRetryLimit. The SSRC shall be reset to 0 when a CTS frame is received in response to an RTS frame, when an ACK frame is received in response to an MPDU or MMPDU transmission, or when a frame with a

52、 group address in the Address1 field is transmitted. The SLRC shall be reset to 0 when an ACK frame is received in response to transmission </p><p>　　The set of CW values shall be sequentially ascending inte

53、ger powers of 2, minus 1, beginning with a PHY-specific aCWmin value, and continuing up to and including a PHY-specific aCWmax value.</p><p>　　9.2.5 DCF access procedure</p><p>　　The CSMA/CA acc

54、ess method is the foundation of the DCF. The operational rules vary slightly between the DCF and the PCF.</p><p>　　9.2.5.1 Basic access</p><p>　　Basic access refers to the core mechanism a STA u

55、ses to determine whether it may transmit.In general, a STA may transmit a pending MPDU when it is operating under the DCF access method, either in the absence of a PC, or in the CP of the PCF access method, when the STA

56、determines that the medium is idle for greater than or equal to a DIFS period, or an EIFS period if the immediately preceding medium-busy event was caused by detection of a frame that was not received at this STA with a

57、correct MAC</p><p>　　In a STA having an FH PHY, control of the channel is lost at the dwell time boundary and the STA shall have to contend for the channel after that dwell boundary. It is required that STAs

58、 having an FH PHY complete transmission of the entire MPDU and associated acknowledgment (if required) before the dwell time boundary. If, when transmitting or retransmitting an MPDU, there is not enough time remaining i

59、n the dwell to allow transmission of the MPDU plus the acknowledgment (if required), the STA s</p><p>　　The basic access mechanism is illustrated in Figure 9-5.</p><p>　　9.2.5.2 Backoff procedur

60、e for DCF</p><p>　　This subclause describes backoff procedure that is to be invoked when DCF is used. For the backoff procedure when EDCA is used, see 9.9.1.5.</p><p>　　The backoff procedure

61、shall be invoked for a STA to transfer a frame when finding the medium busy as indicated by either the physical or virtual CS mechanism (see Figure 9-6). The backoff procedure shall also be invoked when a

62、transmitting STA infers a failed transmission as defined in 9.2.5.7 or 9.2.8.</p><p>　　To begin the backoff procedure, the STA shall set its Backoff Timer to a random backoff time using the

63、 equation in 9.2.4. All backoff slots occur following a DIFS period during which the medium is determined to be idle for the duration of the DIFS period, or following an EIFS period during which the medi

64、um is determined to be idle for the duration of the EIFS period, as appropriate (see 9.2.3).</p><p>　　A STA performing the backoff procedure shall use the CS mechanism (see 9.2.1) to determine whether there

65、 is activity during each backoff slot. If no medium activity is indicated for the duration of a particular backoff slot,then the backoff procedure shall decrement its backoff time by aSlotTime.</p><p>　　If

66、the medium is determined to be busy at any time during a backoff slot, then the backoff procedure is suspended; that is, the backoff timer shall not decrement for that slot. The medium shall be determined to be id

67、le for the duration of a DIFS period or EIFS, as appropriate (see 9.2.3), before the backoff procedure is allowed to resume. Transmission shall commence when the Backoff Timer reaches zero.</p><p>　　A ba

68、ckoff procedure shall be performed immediately after the end of every transmission with the More Fragments bit set to 0 of an MPDU of type Data, Management, or Control with subtype PS-Poll, even if no addit

69、ional transmissions are currently queued. In the case of successful acknowledged transmissions, this backoff procedure shall begin at the end of the received ACK frame. In the case of unsuccessful transmissio

70、ns requiring acknowledgment, this backoff procedure</p><p>　　The effect of this procedure is that when multiple STAs are deferring and go into random backoff, then the STA selecting the smallest backoff

71、time using the random function will win the contention (assuming all of the contending STAs detect the same instances of WM activity at their respective receivers).</p><p>　　In an IBSS, the backoff time for

72、a pending nonbeacon or non-ATIM transmission shall not decrement in the period from the TBTT until the expiration of the ATIM window, and the backoff time for a pending ATIM</p><p>　　9.2.5.4 Setting and rese

73、tting the NAV</p><p>　　STAs receiving a valid frame shall update their NAV with the information received in the Duration field for all frames where the new NAV value is greater than the current NAV value, ex

74、cept the NAV shall not be updated where the RA is equal to the receiving STA’s MAC address. Upon receipt of a PS-Poll frame, a STA shall update its NAV settings as appropriate under the data rate selection rules using a

75、duration value equal to the time, in microseconds, required to transmit one ACK frame plus one S</p><p>　　Figure 9-7 indicates the NAV for STAs that may receive the RTS frame, while other STAs may only recei

76、ve the CTS frame, resulting in the lower NAV bar as shown (with the exception of the STA to which the RTS was addressed).</p><p>　　A STA that used information from an RTS frame as the most recent basis to u

77、pdate its NAV setting is permitted to reset its NAV if no PHY-RXSTART.indication is detected from the PHY during a period with a duration of (2 × aSIFSTime) + (CTS_Time) + aPHY-RX-START-Delay + (2 

78、5; aSlotTime) starting at the PHY-RXEND.indication corresponding to the detection of the RTS frame. The “CTS_Time” shall be calculated using the length of the CTS frame and the data rate at which the RTS frame</p

79、><p>　　9.2.5.6 RTS/CTS usage with fragmentation</p><p>　　The following is a description of using RTS/CTS for a fragmented MSDU or MMPDU. The RTS/CTS frames define the duration of the following fram

80、e and acknowledgment. The Duration/ID field in the data and ACK frames specifies the total duration of the next fragment and acknowledgment. This is illustrated in Figure 9-9.</p><p>　　Each frame contains in

81、formation that defines the duration of the next transmission. The duration information from RTS frames shall be used to update the NAV to indicate busy until the end of ACK 0. The duration information

82、 from the CTS frame shall also be used to update the NAV to indicate busy until the end of ACK 0.Both Fragment 0 and ACK 0 shall contain duration information to update the NAV to indicate busy until the end of ACK 1.

83、 This shall be done by using the</p><p>　　In the case where an acknowledgment is sent but not received by the source STA, STAs that heard the fragment, or ACK, will mark the cha

84、nnel busy for the next frame exchange due to the NAV having been updated from these frames. This is the worst-case situation, and it is shown in Figure 9-10. If an acknowledgment is not sent b