计算机毕业论文外文文献翻译中英文：IEEE802.11媒体接入控制

1、英文资料与中文翻译IEEE 802.11 MEDIUM ACCESS CONTROLThe IEEE 802.11 MAC layer covers three functional areas： reliable data delivery, medium access control, and security. This section covers the first two topics.Reliable Data DeliveryAs with any wireless network, a wireless LAN using the IEEE 802.11 physical a

2、nd MAC layers is subject to considerable unreliability. Noise, interferenee, and other propagatio n effects result in the loss of a sig nifica nt nu mber of frames. Eve n with error-correct ion codes, a nu mber of MAC frames may not successfully be received. This situation can be dealt with by relia

3、bility mechanisms at a higher layer. such as TCP. However, timers used for retra nsmissio n at higher layers are typically on the order of sec on ds. It is therefore more efficie nt to deal with errors at the MAC level. For this purpose, IEEE 802.11 in cludes a frame excha nge protocol. Whe n a stat

4、io n receives a data frame from ano ther stati on. It returns an ack no wledgme nt (ACK) frame to the source station. This exchange is treated as an atomic unit, not to be interrupted by a transmission from any other station. If the source does not receive an ACK within a short period of time, eithe

5、r becauseits data frame was damaged or because the returning ACK was damaged, the source retransmits the frame.Thus, the basic data transfer mechanism in IEEE802.11 invoIves an exchange of two frames. To further enhance reliability, a four-frame excha nge may be used. In this scheme, a source first

6、issues a request to send (RTS) frame to the destination. The destination then responds with a clear to send (CTS). After receiving the CTS, the source transmits the data frame, and the destination responds with an ACK. The RTS alerts all stations that are within reception range of the source that an

7、 exchange is under way; these stations refrain from transmission in order to avoid a collision between two frames transmitted at the same time. Similarly, the CTS alerts all stations that are within reception range of the destination that an exchange is under way. The RTS/CTS porti on of the excha n

8、ge is a required fun ctio n of the MAC but may be disabled.Medium Access Con trolThe 802.11 working group considered two types of proposals for a MAC algorithm: distributed access protocols, which, like Ether net, distribute the decisi on to tran smit over all the no des using a carrier-se nse mecha

9、 ni sm; and cen tralized access protocols, which invo Ive regulati on of tran smissi on by a cen tralized decisi on maker. A distributed access protocol makes sense for an ad hoc network of peer workstati ons (typically an IBSS) and may also be attractive in other wireless LAN con figurati ons that

10、con sist primarily of burst traffic. A cen tralized access protocol is natural for con figurati ons in which a umber of wireless stati ons are in terc onn ected with each other and some sort of base station that attaches to a backbone wired LAN: it is especially useful if some of the data is time se

11、n sitive or high priority.Point coord in ati on fun cti onMAC layerDistributed coord ination function (DCF)802.112.4-GHzFHSS802.112.4-GHzDSSS802.11In frared802.11a5-GHzOFDM802.11b2.4-GHzDSSS802.11g2.4-GHzDSSS,OFDM(PCF)Figure 14.5 IEEE 802.11 Protocol ArchitectureThe end result for 802.11 is a MAC al

12、gorithm called DFWMAC (distributed foundation wireless MAC) that provides a distributed access control mechanism with an optional centralized control built on top of that. Figure 14.5 illustrates the architecture. The lower sub-layer of the MAC layer is the distributed coord in ati on function (DCF)

13、. DCF uses a conten tio n algorithm to provide accessto all traffic. Ordi nary asynchronous traffic directly uses DCE. The point coord in ati on fun ctio n (PCF) is a cen tralized MAC algorithm used to provide conten ti on-free service. PCF is built on top of DCF and exploits features of DCF to assu

14、re access for its users. Let us con sider these two sub-layers in tur n.Logical link con trol (LLC)Contention-free serviceContention serviceDistributed Coord in ati on Fun cti onThe DCF sub-layer makes use of a simple CSMA (carrier sense multiple access) algorithm, which functions as follows. If a s

15、tation 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 tran smitt ing. The DCF does not in clude a collisio n detect ion fun cti on (i.e. CSMA/CD) because collisi

16、o n detect ion is not practical on a wireless n etwork. The dynamic range of the signals on the medium is very large, so that a transmitting station cannot effectively distinguish incoming weak signals from noise and the effects of its own tran smissi on.To ensure the smooth and fair functioning of

17、this algorithm, DCF includes a set of delays that amounts to a priority scheme. Let us start by con sideri ng a sin gle delay known as an in ter-frame space (IFS). I n fact, there are three differe nt IFS values, but the algorithm is best explained by initially ignoring this detail. Using an IFS, th

18、e rules for CSMA access are as follows (Figure 14.6):Figure 14.6 IEEE 802.11 Medium Access Control Logic1. A station with a frame to transmit senses the medium. If the medium is idle. It waits to see if the medium remains idle for a time equal to IFS. If so , the station may tran smit immediately.2.

19、 If the medium is busy (either because the station initially finds the medium busy or becausethe medium becomes busy during the IFS idle time), the station defers transmission and continues to monitor the medium until the current tran smissi on is over.3. Once the current transmission is over, the s

20、tation delays another IFS. If the medium rema ins idle for this period, the n the stati on backs off a ran dom amount of time and again senses the medium. If the medium is still idle, the station may transmit. During the back-off time, if the medium becomes busy, the back-off timer is halted and res

21、umes whe n the medium becomes idle.4. If the tran smissi on is un successful, which is determ ined by the abse nee of an ack no wledgeme nt, the n it is assumed that a collisi on has occurred.To ensure that back-off maintains stability, a technique known as binary exponential back-off is used. A sta

22、tion will attempt to transmit repeatedly in the face of repeated collisions, but after each collision, the mean value of the random delay is doubled up to some maximum value. The binary exponential back-off provides a means of handling a heavy load. Repeated failed attempts to transmit result in lon

23、ger and Ion ger back-off times, which helps to smooth out the load. Without such a back-off, the following situation could occur. Two or more stations attempt to transmit at the same time, causing a collision. These stations then immediately attempt to retra nsmit, caus ing a new collisio n.The prec

24、eding scheme is refined for DCF to provide priority-based access by the simple expedie nt of using three values for IFS:SIFS (short IFS): The shortest IFS, used for all immediate response actions, as explained in the following discussionPIFS (po int coord in ati on fun ctio n IFS):A mid-le ngth IFS,

25、 used by thecen tralized con troller in the PCF scheme whe n issu ing pollsDIFS (distributed coord in ati on fun cti on IFS):The Ion gest IFS, used as aminimum delay for asyn chro nous frames contending for accessFigure 14.7a illustrates the use of these time values. Con sider first the SIFS.Any sta

26、ti on using SIFS to determ in e tran smissi on opport unity has, in effect, the highest priority, becauseit will always gain access in preferenceto a stationwaiting an amount of time equal to PIFS or DIFS. The SIFS is used in the following circumstances：Ack no wledgme nt (ACK): When a stati on recei

27、ves a frame addressed only to itself (not multicast or broadcast) it resp onds with an ACK frame after, wait ing on1y for an SIFS gap. This has two desirable effects. First, because collisio n detect ion IS not used, the likelihood of collisi ons is greater tha n with CSMA/CD, and the MAC-level ACK

28、provides for efficie nt collisi on recovery. Second, the SIFS can be used to provide efficie nt delivery of an LLC protocol data unit (PDU) that requires multiple MAC frames. In this case, the following scenario occurs. A station with a multi-frame LLC PDU to transmit sends out the MAC frames one at

29、 a time. Each frame is ack no wledged after SIFS by the recipie nt. Whe n the source receives an ACK, it immediately (after SIFS) sends the next frame in the sequenee. The result is that once a stati on has conten ded for the cha nn el, it will maintain control of the channel until it has sent all o

30、f the fragments of an LLC PDU. Clear to Send (CTS): A station can ensure that its data frame will get through by first issuing a small. Request to Send (RTS) frame. The station to which this frame is addressed should immediately respond with a CTS frame if it is ready to receive. All other stations

31、receive the RTS and defer using the medium.Poll response: This is explained in the following discussion of PCF.Immediate accesDIFS whe n medium Is ffee longer than DIFSConten ti on win dowfIBusy mediFSBackoff window Next framIITime补 Slot timeDefer access k碍 Select slot using binary exp onen tial bac

32、koff(a) Basic access methodasynchronous traffic defers(b) PCF super-frame con structi onFigure 14.7 IEEE 802.11 MAC TimingThe next Iongest IFS interval is the: PIFS. This is used by the centralized con troller in issu ing polls and takes precede nce over no rmal con ten ti on traffic. However, those

33、 frames tran smitted using SIFS have precede nce over a PCF poll.Fin ally, the DIFS in terval is used for all ordinary asynchronous traffic.Point C00rd in ati on Fun cti on PCF is an alter native access method implemented on top of the DCE. The operation consists of polling by the centralized polli

34、ng master (po int coord in ator). The point coord in ator makes use of PIFS whe n issu ing polls. Because PI FS is smaller tha n DIFS, the point coordi nator call seize the medium and lock out all asynchronous traffic while it issues polls and receives resp on ses.As an extreme, consider the followi

35、ng possible scenario. A wireless network is con figured so that a nu mber of stati ons with time, sen sitive traffic are con trolled by the point coord in ator while remai ning traffic contends for access using CSMA. The point coordi nator could issue polls in a round rob in fashi on to all stati on

36、s con figured for polli ng. Whe n a poll is issued, the polled stati on may resp ond using SIFS. If the point coord in ator receives a resp on se, it issues ano ther poll using PIFS. If no resp onse is received duri ng the expected tur naround time, the coord in ator issues a pollIf the discipline o

37、f the preceding paragraph were implemented, the point coord in ator would lock out all asynchronous traffic by repeatedly issu ing polls. To prevent this, an interval known as the super-frame is defined. During the first part of this in terval, the point coord in ator issues polls in a round, robi n

38、 fashi on to all stati on s con figured for polli ng. The point coordi nator the n idles for the rema in der of the super-frame, allow ing a conten ti on period for asynchronous access.Figure 14.7 b illustrates the use of the super-frame. At the beg inning of a super-frame, the point coord in ator m

39、ay opti on ally seize con trol and issues polls for a give period of time. This in terval varies because of the variable frame size issued by resp onding stati ons. The rema in der of the super-frame is available for con ten ti on based access. At the end of the super-frame in terval, the point coor

40、di nator contends for access to the medium using PIFS. If the medium is idle. the point coord in ator gains immediate access and a full super-frame period follows. However, the medium may be busy at the end of a super-frame. In this case, the point coord in ator must wait until the medium is idle to

41、 gain access:this result in a foreshortenedsuper-frame period for the n ext cycle.Octets 226FCD/IAddressAddressAddressSCAddressDataFCS66260 to 23124Frame body TrailerHeaderFC=frame con trolSC=seque nee con trolD/I=durati on/connection ID FCS=frame check seque neeBits(a) MAC frameProicol vers ionType

42、SubtypeTo DSFromDSMFRTPMMDWO2 2411111111DS=distributio n system MF=more fragme nts RT=retryPM=power man ageme ntMD=more dataW=wired equivale nt privacyO=order(b) Frame con trol filedFigure 14.8 IEEE 802.11 MAC Frame FormatMAC FrameFigure 14.8a shows the 802.11 frame format when no security features

43、are used. This general format is used for all data and control frames, but not all fields are used in all con texts. The fields are as follows:Frame Control:Indicates the type of frame and provides contr01in formati on, as expla ined prese ntly.Duration/Connection ID: If used as a duration field, in

44、dicates the time (in-microsec onds) the cha nnel will be allocated for successful tran smissi on of a MAC frame. In some control frames, this field contains an association, or connection, identifier.Addresses: The number and meaning of the 48-bit address fields depend on con text. The tran smitter a

45、ddress and receiver address are the MAC addresses of stations joined to the BSS that are transmitting and receiving frames over the wireless LAN. The service set ID (SSID) identifies the wireless LAN over which a frame is transmitted. For an IBSS, the SSID is a ran dom nu mber gen erated at the time

46、 the n etwork is formed. For a wireless LAN that is part of a larger configuration the SSID identifies the BSS over which the frame is transmitted: specifically, the SSID is the MAC-level address of the AP for this BSS (Figure 14.4). Finally the source address and desti nati on address are the MAC a

47、ddresses of statio ns, wireless or otherwise, that are the ultimate source and destination of this frame. The source address may be identical to the transmitter address and the desti nati on address may be ide ntical to the receiver address.Sequenee Control: Contains a 4-bit fragment number subfield

48、 used for fragmentation and reassembly, and a 12-bit sequenee number used to nu mber frames sent betwee n a give n tran smitter and receiver.Frame Body: Con tai ns an MSDU or a fragme nt of an MSDU. The MSDU is a LLC protocol data unit or MAC control information.Frame Check Sequenee: A 32-bit cyclic

49、 redundancy check. The frame control filed, shown in Figure 14.8b, consists of the following fields.Protocol Version: 802.11 version, current version 0.Type: Identifies the frame as control, management, or data.Subtype: Further identifies the function of frame. Table 14.4 defines the valid comb in a

50、ti ons of type and subtype.To DS: The MAC coord in atio n sets this bit to 1 in a frame desti ned to the distributio n system.From DS: The MAC coord in ati on sets this bit to 1 in a frame leav ing the distributio n system.More Fragments: Set to 1 if more fragments follow this one. Retry : Set to 1

51、if this is a retransmission of a previous frame.Table 14.4 Valid Type and Subtype Comb in ati onsTypeType Descripti onSubtype ValueSubtype Descripti on00Man ageme nt0000Associati on request00Man ageme nt0001Associati on resp onse00Man ageme nt0010Reassociati on request00Man ageme nt0011Reassociati o

52、n resp onse00Man ageme nt0100Probe request00Man ageme nt0101Probe resp onse00Man ageme nt1000Beacon00Man ageme nt1001Announ ceme nt traffic in dicati onmessage00Man ageme nt1010Dissociati on00Man ageme nt1011Authe nticati on00Man ageme nt1100Deauthe nticati on01Co ntrol1010Power save-poll01Co ntrol1

53、011Request to send01Co ntrol1100Clear to send01Co ntrol1101Ack no wledgme nt01Co ntrol1110Contention-Free (CF)-End01Co ntrol1111CF-E nd+CF-Ack10Data0000Data10Data0001Data+ CF-Ack10Data0010Data+ CF-Poll10Data0011Data+ CF-Ack+ CF-Poll10Data0100Null fun cti on (no data)10Data0101CF-Ack (no data)10Data0

54、110CF-Poll (no data)10Data0111CF-Ack+ CF-Poll (no data)Power Management: Set toif the transmitting station is in a sleep mode. More Data: Indicates that a station has additional data to send. Each block of data may be sent as one frame or a group of fragments in multiple frames.WEP : Set to 1 if the

55、 optio nal wired equivale nt protocol is impleme nted. WEP is used in the excha nge of en crypti on keys for secure data excha nge. This bit also is set if the newer WPA security mechanism is employed, as described in Secti on 14.6.Order : Set to 1 in any data frame sent using the Strictly Ordered s

56、ervice, which tells the receiving station that frames must be processed in order.We now look at the various MAC frame types.Control Frames Control frames assist in the reliable delivery of data frames. There are six con trol frame subtypes:Power Save-Poll (PS-Poll): This frame is sent by any station

57、 to the station that in cludes the AP (accesspo in t). Its purpose is to request that the AP transmit a frame that has been buffered for this station while the station was in power sav ing mode.Request to Send (RTS): This is the first frame in the four-way frame excha nge discussed un der the subsec

58、t ion on reliable data delivery at the beg inning of Secti on 14.3.The stati on sending this messageis alert ing a potential destination, and all other stations within reception range, that it intends to send a data frame to that dest in ati on.Clear to Send (CTS): This is the second frame in the four-way exchange. It is sent by the destination station to the source station to grant p