信息工程外文翻譯----無線局域網dcf接入機制的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)設計（論文）外文文獻翻譯</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子層的體系結構，包括分布式協(xié)調功能（DCF），點協(xié)調功能（PCF），混合協(xié)調功能（HCF），并介紹了在9.1在IEEE 802.11局域網

2、共存。</p><p>　　9.1 MAC 體系結構</p><p>　　MAC架構可以描述成如圖9-1所示，提供了PCF和HCF通過DCF的服務。但是值得注意的是，在一個非QOS STA，HCF是不存在的。在QOS標準的實施中，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結構圖所示：</p><p><b>　　9.

3、1.1 DCF</b></p><p>　　在IEEE 802.11 MAC中的最基本的訪問方法是DCF，被稱為載波偵聽多路訪問沖突避免（CSMA/CA）。DCF應在所有STA中實現(xiàn)，用于在IBSS和基礎設施網絡配置中。</p><p>　　一個STA請求的發(fā)送，應識別媒介中是否有另一個STA在發(fā)送。如果確認介質是不忙碌的，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間間隔。 一個發(fā)射站應保證在所需的時間之前，傳輸介質是空閑的。 如果介質當前狀態(tài)是忙碌的，那么STA將推遲到當前的傳輸結束后執(zhí)行。 在延時之后，在成功的發(fā)送后，立馬優(yōu)先嘗試再發(fā)射， 當通道是空閑的，STA將選擇一個隨機退避間隔 并且消減退避間隔的計數(shù)器。一個傳輸成功時，ACK幀被接收，從STA尋址通過RA領域解決所傳送的幀，或當一個幀與一組地址在RA領域中被發(fā)送完全時。一個細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，進一步減少碰撞，在測定發(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，存在任何延時或補償。</p><p><b>　　9.2 DCF</b></p><p>　　基本的介質訪問協(xié)議DCF 允許自動介質之間在兼容的PHYs中共享，一個繁忙的信道狀態(tài)之后，通過使用CSMA/CA和一個隨機退避時間機制。除此之外，當發(fā)送方沒有收到ACK，所有單獨

6、的尋址方式使用即時確認（ACK幀）機制進行重發(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STA訪問之間的碰撞概率。在信道由空閑變成繁忙的時候，是當發(fā)生碰撞的概率最高的時候。這是因為多個STA從一直在等待中再次變?yōu)榭捎谩＿@種情況，就需要一個隨機退避過程去解決信道中的沖突問題。</p><p>　　CS的實行需要通過物理和虛擬的機制來進行。<

7、;/p><p>　　根據(jù)DCF發(fā)送數(shù)據(jù)幀時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數(shù)據(jù)的幀到LLC，但應當指明一個數(shù)據(jù)幀到LLC當替補類型是數(shù)據(jù)，即使幀的結構只包含零字節(jié)。</p><p>　　9.2.4 隨機退避機制</p><p>　　一個STA需要發(fā)起數(shù)據(jù)轉讓時，MPDU和MMPDIS

8、應調用CS機制來確定介質的狀態(tài)是處于忙碌還是空閑。如果介質是繁忙的，STA將推遲到媒介被確認為空閑的，沒有中斷一段時間，等價于當最后一幀被確認在通道中接收完成，或者在媒介被確認為空閑之后，沒有中斷的時間，等價于EIFS當最后一幀被確認在媒介中未被正確的接收。 在DIFS或者EIFS信道空閑的時間之后，STA將生成一個隨機的退避時間，在傳播之前有一個額外的延時，除非退避時間已經包含了一個非零的值，在不需要隨機數(shù)的情況下不執(zhí)行。這個過程可以

9、最大限度地減少 多個STA之間存在的延時競爭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為實際的退避時間，Random()為均勻分布在[0，CW]內的偽隨機數(shù)，CW為物理層中最小競爭窗口特征值與最大競爭窗口特征值之間的數(shù)，a Slot Time（時隙時間

10、）是物理層上的值，其大小均由物理層決定。</p><p>　　競爭窗口（CW）的參數(shù)應當取 aCWmin 的初始值。每一階段都應保持STA短重試計數(shù)（SSRC）以及一個STA長重試計數(shù)（SLRC），均應采取一個初始的零值。當任何短重試計數(shù)SRC聯(lián)合任何一個MPDU的數(shù)據(jù)類型被遞增，SSRC將會被遞增。 當任何長重試計數(shù)聯(lián)合任何一個MPDU的數(shù)據(jù)類型被遞增，SLRC也將遞增。 在連續(xù)的任何時間內有一個不成功，CW應

11、當取接下來的值，并嘗試去傳輸一個MPDU原因，STA也將再次相反的去遞增，直到連續(xù)波到達aCWmax的值。一個重新定義為整個幀序列發(fā)送的間隔，被SIFS間隔時間分開，在試圖提供一個MPDU。一旦達到aCWmax，CW應保持在值aCWmax直到連續(xù)復位。這提高了在高負載下的訪問協(xié)議的穩(wěn)定性。如圖9-2：</p><p><b>　　圖9-2</b></p><p>　　

12、CW應復位到aCWmin，在每成功發(fā)送MPDU或MMPDU后，在SLRC到達dot11LongRetryLimit階段，或者當SSRC到達dot11LongRetryLimit階段。 SSRC應當被重置為0當CTS幀來響應一個RTS幀接收，當ACK幀響應接收一個MPDU或者MMPDU的傳輸，或者當一個幀和一組地址在地址1場內被傳輸。SLRC應復位到0當ACK幀響應一個MPDU發(fā)送或接收MMPDU長度大于dot11rtsthreshold

13、，或當一幀中的地址1組地址字段被傳輸。</p><p>　　連續(xù)值的設定應依次上升2，整數(shù)次冪減1，開始與物理層明確的aCWmin的值，繼續(xù)直至并包括物理層具體的aCWmax價值。</p><p>　　9.2.5 DCF訪問過程</p><p>　　CSMA/CA接入方法基礎是DCF。執(zhí)行方式規(guī)則在DCF和PCF之間有細微的不同。</p><p&

14、gt;　　9.2.5.1 基礎訪問</p><p>　　基本的訪問是指 STA的使用，確定它是否可以成為傳遞的核心機制。一般來說，一個站可以發(fā)送一個掛起的MPDU 當它是DCF接入方式下的訪問方式，無論是否缺少PC，或者在PCF的CP接入方式，當STA確定介質是空閑的，大于或等于DIFS時間，或EIFS期間如果立刻在先前的介質繁忙的事件是人為偵查的幀沒有被接收，在這個階段有一個正確的MAC FCS值。如果，在這些

15、條件下，媒介是由CS機制確定的繁忙的，當一個STA請求開始最初的幀的一個幀交換，專用的CF階段，隨機退避程序應遵循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，隨機退避程序應當遵循甚至嘗試去發(fā)起一個幀去交換序列。</p><p>　　在STA中具有跳頻PHY，對信道的控制實在丟失的保留時間邊界和STA將競爭信道，在居住邊界之后。它要求有一個跳頻PHY完成STA整個層傳輸和相關的

16、確認（如果需要）前停留時間的邊界。如果當發(fā)送或者程序存在一個MPDU，沒有足夠的剩余時間去允許傳送MPDU的確認信息（是否所需的），STA將會通過選擇一個隨機退避時間，推遲傳送，利用現(xiàn)有的CW（沒有推進到系列中的一個值）。 這個短和長的重試計數(shù)器的MSDU計數(shù)不受影響。</p><p>　　基本訪問機制如圖9-3所示。</p><p>　　圖9-3 基本訪問機制圖</p>&

17、lt;p>　　9.5.2.2 DCF退避過程</p><p>　　本小節(jié)介紹了退避過程，使用時要調用的DCF。</p><p>　　退避機制為STA傳送一個幀時，當發(fā)現(xiàn)信道繁忙時，表明無論是物理的或虛擬的CS的機制，退避程序也應當調用一個發(fā)射站推斷傳輸失敗。如圖9-6所示：</p><p>　　開始退避機制時，STA將設置他的退避時間器區(qū)一個隨機退避時間使用

18、的方程式。所有的退避時隙發(fā)生之后，一個DIFS 時間在信道被確認閑置的DIFS期間或之后期間，或者跟在一個EIFS期間之后當信道被確認為空閑的。</p><p>　　STA執(zhí)行退避程序應使用CS機制以確定是否有每個退避期間的活動。如果沒有信道活動去表明一個特定的退避時隙的時間，然后退避程序將會通過a Slot Time.消耗退避時間。</p><p>　　如果信道在所有退避機制中被確認是繁

19、忙的，那么退避機制將會被暫停。那么，退避計時器將不會被消耗。當信道在持續(xù)的DIFS時期或者EIFS被確認為空閑時，作為適當?shù)?，在退避過程允許恢復之前。當退避計時器達到零時開始傳輸。</p><p>　　退避程序應當在每個傳輸?shù)慕Y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置為0，管理，或者控制PS-Poll的圖表類型，即使沒有額外的傳送在當前的隊列等待。在傳輸成功的情況下，退避過程應當在收到ACK幀結

20、束后開始。在不成功傳輸?shù)那闆r下</p><p>　　需要確認，這種退避程序應在確認的超時時間結束后開始。一個不成功的傳輸是一個ACK幀沒有接收從STA尋址的一個單獨的地址。如果發(fā)送成功，CW值恢復到之前aCWmin隨機退避間隔選擇，SSRC和SLRC被更新。這確保了發(fā)送幀從STA總是由至少一個退避間隔。</p><p>　　這個程序的作用是，當多個站點正在推遲進入隨機退避，然后STA選擇最

21、小的退避時間使用隨機函數(shù)將贏得競爭 （假設所有的STAs檢測WM活動相同的實例在各自的接收機）。</p><p>　　在IBSS中，時間退避機制。在IBSS，一個單獨的退避間隔應產生Beacon幀的傳輸。</p><p>　　9.2.5.4 設置和重置 NAV</p><p>　　STA接收到有效的幀應該更新他的NAV，在持續(xù)的收到的所有幀的信息，當新的NAV的值

22、大于當前NAV的值，排除NAV不被更新，當RA等于接收到的STA的MAC地址時。在收入一個PS-Poll幀，一個STA應被更新他的NAV，適當設置，在數(shù)據(jù)的選擇規(guī)則下使用一個持續(xù)的值相同的時間，在微秒內，需要發(fā)送一個ACK幀加一個SIFS間隔，但是只有當新的NAV的值大于當前NAV的值。如果計算時間包括小數(shù)微秒，值圍捕高一個整數(shù)。各種附加的條件可以被設置或重置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設置所示：</p><p>　　一個STA使用從一個RTS幀作為最新的基礎去更新他的NAV設置，并允許重置他的NAV如果沒有PHY-RXSTART。指示被發(fā)現(xiàn)從物理層的時期有一段持續(xù)的時間（(2× a SIFS Time)+(CTS Time)+a PHY-RX

24、-START-Delay+(2×a Slot Time)開始于PHY-RXEND。在RTS幀檢測相應的指示。“CTS Time” 應該被計算出用于CTS幀的長度，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>　　以下是一個描述使用RTS/CTS對小部分 MSDU 或者 MMPDU。RTS/CT

25、S幀定義下列持續(xù)的幀和確認的。在數(shù)據(jù)和ACK的時間/ID字段幀指定的持續(xù)的一個片段和確認的總時間，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)時間的信息。RTS幀的時間信息將被用于更新NAV去表明繁忙的直到ACK結束為0。兩片段0和ACK 0應包含時間信息去更新NAV去表示繁忙直到ACK為1結束。這將通過使用時間/ ID字段中的數(shù)據(jù)和ACK幀。這將繼續(xù)下去直到最后一個片段，這將持續(xù)一個

26、ACK時間加上一個SIFS時間，及其應答，這將它的持續(xù)時間/標識字段設置為0。每個片段和ACK作為一個虛擬的RTS和CTS；因此，沒有進一步的RTS/CTS幀需要在 RTS/CTS幀序列開始交換之后發(fā)生，即使隨后的碎片可能會比dot11rtsthreshold大。在STA的使用fhphy，當沒有足夠的時間在存在于分界之前去傳輸后來的片段，那么STA起始幀交換序列可能設置時間/ ID字段結構在數(shù)據(jù)末尾或者管理幀去發(fā)射，在邊界一個ACK的時

27、間前加上一個SIFS時間。</p><p>　　在這種情況下，確認發(fā)送但是不接收源STA。 STA取得片段，或者ACK，將標志信道忙碌，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。如果確認不是由目的站發(fā)送的STA，STA只能聽到目的STA，將不會更新他的NAV并且嘗試去使用通道，當他的NAV更新來自事先接收的幀的層級為0.所有的STA收聽到源將會最有的使用通道當他們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