Performance Evaluation of the IEEE 802.15.4 MAC for Low-Rate

第10卷第2期信息工程大学学报Vol 110No 122009年6月Journal of I nfor mati on Engineering University Jun 12009 收稿日期:2008-09-22;修回日期:2008-12-15 作者简介:任孝锋(1980-),男,硕士生,主要研究方向为通信与信息系统。

I EEE802.11WLAN 中DCF 延迟性能分析任孝锋,李建新(信息工程大学信息工程学院,河南郑州450002)摘要:通过二维马尔可夫链模型分析了I EEE 802.11DCF 在饱和状态下的延迟特性,给出了分布式协调功能(DCF )中分组传输延迟的理论分析方法。

利用NS2仿真软件进行仿真,对基本接入方式和RTS/CTS 方式两种传输方式的延迟性能进行分析比较,仿真结果验证了理论分析方法的有效性,为研究在无线局域网中传输实时业务提供了理论依据。

关键词:I EEE 802.11;DCF;传输延迟中图分类号:T N925.93 文献标识码:A文章编号:1671-0673(2009)02-0181-03D el ay Ana lysis of DCF i n I EEE 802.11WLANREN Xiao 2feng,L I J ian 2xin(I nstitute of I nf or mati on Engineering,I nf or mati on Engineering University,Zhengzhou 450002,China )Abstract:I n this paper,the delay perf or mance f or I EEE 802.11DCF in saturati on situati on is ana 2lyzed by intr oducing a t w o 2di m ensi onalMarkov chain model .The evaluati on of access delay by theo 2retical analysis is als o p resented .Si m ulati on is done by using net w ork si m ulating t ool NS2,and the delay perfor mance is compared both basic access mechanis m and RTS/CTS access mechanis m.Si m 2ulati on results validated the effectiveness of theoretical analysis,which p r ovides a research basis for real 2ti m e trans m itting of packets in wireless l ocal area net w orks .Key words:I EEE 802.11;DCF;access delay 随着局域网应用领域的扩展和通信中对移动和便携性要求的提高,无线局域网(WLAN )得到了飞速发展,相应产生了多种局域网标准,如I EEE 802.11、Ho meRF 、B luet ooth 等,其中I EEE 802.11[1]是目前应用最广泛的WLAN 标准,它提供了WLAN 物理层(PHY )和媒体接入控制层(MAC )的详细规范。

Performance Analysis of the IEEE802.11DistributedCoordination FunctionGiuseppe BianchiAbstract—Recently,the IEEE has standardized the802.11pro-tocol for Wireless Local Area Networks.The primary medium ac-cess control(MAC)technique of802.11is called distributed coor-dination function(DCF).DCF is a carrier sense multiple access with collision avoidance(CSMA/CA)scheme with binary slotted exponential backoff.This paper provides a simple,but nevertheless extremely accurate,analytical model to compute the802.11DCF throughput,in the assumption of finite number of terminals and ideal channel conditions.The proposed analysis applies to both the packet transmission schemes employed by DCF,namely,the basic access and the RTS/CTS access mechanisms.In addition,it also ap-plies to a combination of the two schemes,in which packets longer than a given threshold are transmitted according to the RTS/CTS mechanism.By means of the proposed model,in this paper we pro-vide an extensive throughput performance evaluation of both ac-cess mechanisms of the802.11protocol.Index Terms—802.11,collision avoidance,CSMA,performance evaluation.I.I NTRODUCTIONI N recent years,much interest has been involved in thedesign of wireless networks for local area communication [1],[2].Study group802.11was formed under IEEE Project 802to recommend an international standard for Wireless Local Area Networks(WLAN’s).The final version of the standard has recently appeared[3],and provides detailed medium access control(MAC)and physical layer(PHY)specification for WLAN’s.In the802.11protocol,the fundamental mechanism to access the medium is called distributed coordination function(DCF). This is a random access scheme,based on the carrier sense mul-tiple access with collision avoidance(CSMA/CA)protocol.Re-transmission of collided packets is managed according to bi-nary exponential backoff rules.The standard also defines an op-tional point coordination function(PCF),which is a centralized MAC protocol able to support collision free and time bounded services.In this paper we limit our investigation to the DCF scheme.DCF describes two techniques to employ for packet transmis-sion.The default scheme is a two-way handshaking technique called basic access mechanism.This mechanism is character-ized by the immediate transmission of a positive acknowledge-ment(ACK)by the destination station,upon successful recep-tion of a packet transmitted by the sender station.Explicit trans-Manuscript received November1998;revised July25,1999.this work was supported in part by CNR and MURST,Italy.The author is with the Universitádi Palermo,Dipartimento di Ingegneria Elet-trica,Viale delle Scienza,90128Palermo,Italy(e-mail:bianchi@elet.polimi.it). Publisher Item Identifier S0733-8716(00)01290-7.mission of an ACK is required since,in the wireless medium,a transmitter cannot determine if a packet is successfully received by listening to its own transmission.In addition to the basic access,an optional four way hand-shaking technique,known as request-to-send/clear-to-send (RTS/CTS)mechanism has been standardized.Before transmit-ting a packet,a station operating in RTS/CTS mode“reserves”the channel by sending a special Request-To-Send short frame. The destination station acknowledges the receipt of an RTS frame by sending back a Clear-To-Send frame,after which normal packet transmission and ACK response occurs.Since collision may occur only on the RTS frame,and it is detected by the lack of CTS response,the RTS/CTS mechanism allows to increase the system performance by reducing the duration of a collision when long messages are transmitted.As an important side effect,the RTS/CTS scheme designed in the 802.11protocol is suited to combat the so-called problem of Hidden Terminals[4],which occurs when pairs of mobile stations result to be unable to hear each other.This problem has been specifically considered in[5]and in[6],which,in addition,studies the phenomenon of packet capture.In this paper,we concentrate on the performance evaluation of the DCF scheme,in the assumption of ideal channel con-ditions and finite number of terminals.In the literature,perfor-mance evaluation of802.11has been carried out either by means of simulation[7],[8]or by means of analytical models with sim-plified backoff rule assumptions.In particular,constant or geo-metrically distributed backoff window has been used in[5],[9], [10]while[11]has considered an exponential backoff limited to two stages(maximum window size equal to twice the minimum size)by employing a two dimensional Markov chain analysis. In this paper,which revises and substantially extends[12],we succeed in providing an extremely simple model that accounts for all the exponential backoff protocol details,and allows to compute the saturation(asymptotic)throughput performance of DCF for both standardized access mechanisms(and also for any combination of the two methods).The key approximation that enables our model is the assumption of constant and indepen-dent collision probability of a packet transmitted by each station, regardless of the number of retransmissions already suffered.As proven by comparison with simulation,this assumption leads to extremely accurate(practically exact)results,especially when the number of stations in the wireless LAN is fairly large(say greater than ten).The paper is outlined as follows.In Section II we briefly re-view both basic access and RTS/CTS mechanisms of the DCF. In Section III we define the concept of Saturation Throughput, and in Section IV we provide an analytical technique to com-0733–8716/00$10.00©2000IEEEpute this performance figure.Section V validates the accuracy of the model by comparing the analytical results with that ob-tained by means of simulation.Additional considerations on the maximum throughput theoretically achievable are carried out in Section VI.Finally,the performance evaluation of both DCF ac-cess schemes is carried out in Section VII.Concluding remarks are given in Section VIII.II.802.11D ISTRIBUTED C OORDINATION F UNCTION This section briefly summarizes the DCF as standardized by the 802.11protocol.For a more complete and detailed presen-tation,refer to the 802.11standard [3].A station with a new packet to transmit monitors the channel activity.If the channel is idle for a period of time equal to a dis-tributed interframe space (DIFS),the station transmits.Other-wise,if the channel is sensed busy (either immediately or during the DIFS),the station persists to monitor the channel until it is measured idle for a DIFS.At this point,the station generates a random backoff interval before transmitting (this is the Colli-sion Avoidance feature of the protocol),to minimize the prob-ability of collision with packets being transmitted by other sta-tions.In addition,to avoid channel capture,a station must wait a random backoff time between two consecutive new packet transmissions,even if the medium is sensed idle in the DIFS time.1For efficiency reasons,DCF employs a discrete-time backoff scale.The time immediately following an idle DIFS is slotted,and a station is allowed to transmit only at the beginning of each slot time.The slot timesize,is called contention window,and de-pends on the number of transmissions failed for the packet.At the first transmissionattempt,is doubled,up to a maximumvaluereported in thefinal version of the standard [3]are PHY-specific and are sum-marized in Table I.The backoff time counter is decremented as long as the channel is sensed idle,“frozen”when a transmission is detected on the channel,and reactivated when the channel is sensed idle again for more than a DIFS.The station transmits when the backoff time reaches zero.Fig.1illustrates this operation.Two stations A and B share the same wireless channel.At the end of the packet transmis-1Asan exception to this rule,the protocol provides a fragmentation mecha-nism,which allows the MAC to split an MSDU (the packet delivered to the MAC by the higher layers)into more MPDUs (packets delivered by the MAC to the PHY layer),if the MSDU size exceeds the maximum MPDU payload size.The different fragments are then transmitted in sequence,with only a SIFS between them,so that only the first fragment must contend for the channel access.TABLE IS LOT T IME ,M INIMUM ,AND M AXIMUMC ONTENTION W INDOW V ALUES FOR THE THREE PHY S PECIFIED BY THE 802.11S TANDARD :F REQUENCY H OPPING S PREAD S PECTRUM (FHSS),D IRECTS EQUENCE S PREAD S PECTRUM (DSSS),AND I NFRARED(IR)sion,station B waits for a DIFS and then chooses a backoff time equal to 8,before transmitting the next packet.We assume that the first packet of station A arrives at the time indicated with an arrow in the figure.After a DIFS,the packet is transmitted.Note that the transmission of packet A occurs in the middle of the Slot Time corresponding to a backoff value,for station B,equal to 5.As a consequence of the channel sensed busy,the backoff time is frozen to its value 5,and the backoff counter decrements again only when the channel is sensed idle for a DIFS.Since the CSMA/CA does not rely on the capability of the sta-tions to detect a collision by hearing their own transmission,an ACK is transmitted by the destination station to signal the suc-cessful packet reception.The ACK is immediately transmitted at the end of the packet,after a period of time called short inter-frame space (SIFS).As the SIFS (plus the propagation delay)is shorter than a DIFS,no other station is able to detect the channel idle for a DIFS until the end of the ACK.If the transmitting sta-tion does not receive the ACK within a specified ACK_Timeout,or it detects the transmission of a different packet on the channel,it reschedules the packet transmission according to the given backoff rules.The above described two-way handshaking technique for the packet transmission is called basic access mechanism.DCF de-fines an additional four-way handshaking technique to be op-tionally used for a packet transmission.This mechanism,known with the name RTS/CTS,is shown in Fig.2.A station that wants to transmit a packet,waits until the channel is sensed idle for a DIFS,follows the backoff rules explained above,and then,in-stead of the packet,preliminarily transmits a special short frame called request to send (RTS).When the receiving station detects an RTS frame,it responds,after a SIFS,with a clear to send (CTS)frame.The transmitting station is allowed to transmit its packet only if the CTS frame is correctly received.The frames RTS and CTS carry the information of the length of the packet to be transmitted.This information can be read by any listening station,which is then able to update a network allocation vector (NA V)containing the information of the period of time in which the channel will remain busy.Therefore,when a station is hidden from either the transmitting or the receiving station,by detecting just one frame among the RTS and CTS frames,it can suitably delay further transmission,and thus avoid collision.The RTS/CTS mechanism is very effective in terms of system performance,especially when large packets are considered,as it reduces the length of the frames involved in the contention process.In fact,in the assumption of perfect channel sensing by every station,collision may occur only when two (or more)packets are transmitted within the same slot time.If both trans-BIANCHI:PERFORMANCE ANALYSIS OF THE IEEE 802.11DCF537Fig.1.Example of basic access mechanism.mitting stations employ the RTS/CTS mechanism,collision oc-curs only on the RTS frames,and it is early detected by the transmitting stations by the lack of CTS responses.A quanti-tative analysis will be carried out in Section VII.III.M AXIMUM AND S ATURATION T HROUGHPUT P ERFORMANCE In this paper we concentrate on the “Saturation Throughput”.This is a fundamental performance figure defined as the limit reached by the system throughput as the offered load increases,and represents the maximum load that the system can carry in stable conditions .It is well known that several random access schemes exhibit an unstable behavior.In particular,as the offered load increases,the throughput grows up to a maximum value,referred to as “maximum throughput.”However,further increases of the offered load lead to an eventually significant decrease in the system throughput.This results in the practical impossibility to operate the random access scheme at its maximum throughput for a “long”period of time,and thus in the practical mean-ingless of the maximum throughput as performance figure for the access scheme.The mathematical formulation and interpretation of this instability problem is the object of a wide and general discussion in [13].Indeed,the 802.11protocol is known to exhibits some form of instability (see,e.g.,[5],and [11]).To visualize the unstable behaviour of 802.11,in Fig.3we have run simulations in which the offered load linearly increases with the simulation time.The general simulation model and parameters employed are summa-rized in Section V .The results reported in the figure are obtained with 20stations.The straight line represents the ideal offered load,normalized with respect of the channel capacity.The sim-ulated offered load has been generated according to a Poisson arrival process of fixed size packets (payload equal to 8184bits),where the arrival rate has been varied throughout the simulation to match the ideal offered load.The figure reports both offered load and system throughput measured over 20s time intervals,and normalized with respect to the channel rate.From the figure,we see that the measured throughput follows closely the measured offered load for the first 260s of sim-ulation,while it asymptotically drops to the value 0.68in the second part of the simulation run.This asymptotic throughput value is referred to,in this paper,as saturation throughput,and represents the system throughput in overload conditions.Note than,during the simulation run,the instantaneous throughput temporarily increases over the saturation value (up to 0.74inFig.2.RTS/CTS AccessMechanism.Fig.3.Measured Throughput with slowly increasing offered load.the example considered),but ultimately it decreases and stabi-lizes to the saturation value.Queue build-up is observed in such a condition.IV .T HROUGHPUT A NALYSISThe core contribution of this paper is the analytical evalu-ation of the saturation throughput,in the assumption of ideal channel conditions (i.e.,no hidden terminals and capture [6]).In the analysis,we assume a fixed number of stations,each al-ways having a packet available for transmission.In other words,we operate in saturation conditions,i.e.,the transmission queue of each station is assumed to be always nonempty.The analysis is divided into two distinct parts.First,we study the behavior of a single station with a Markov model,and we obtain the stationaryprobability.A.Packet Transmission Probability Consider a fixednumber538IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS,VOL.18,NO.3,MARCH2000Fig.4.Markov Chain model for the backoff window size.for transmission,after the completion of each successful trans-mission.Moreover,being all packets “consecutive,”each packet needs to wait for a random backoff time before transmitting.Letand,as it may include a packettransmission.In what follows,unless ambiguity occurs,with the term slot time we will refer to either the (constant)valueis non-Markovian.However,define forconvenience .Let,and let us adopt thenotation ,wherebe the sto-chastic process representing the backoffstage.The key approximation in our model is that,at each transmis-sion attempt,and regardless of the number of retransmissions suffered,each packet collides with constant and independentprobabilityand will be referred to asconditional collision probability ,meaning that this is the prob-ability of a collision seen by a packet being transmitted on the channel.Once independence is assumed,and;k ;k ;b (t +1)=k ;b (t )=kBIANCHI:PERFORMANCE ANALYSIS OF THE IEEE802.11DCF539 stage0,and thus the backoff is initially uniformly chosen in therange,the backoff stage increases,and the newinitial backoff value is uniformly chosen in therange,it is not increased in subsequentpacket transmissions.Let,(3)rewritesas(4)Thus,by relations(2)and(4),all thevaluesand of the conditional collisionprobability is finally determined by imposing the nor-malization condition,that simplifies asfollows:that a station trans-mits in a randomly chosen slot time.As any transmission occurswhen the backoff time counter is equal to zero,regardless of thebackoff stage,itis,i.e.,no exponential backoff is considered,theprobability,and(7)becomes the much simplerone independently found in[9]for the constant backoff windowproblem(8)However,ingeneral,,which is still unknown.To find the valueofthat a transmittedpacket encounters a collision,is the probability that,in a timeslot,at least one ofthe remaining stations transmit.Thefundamental independence assumption given above implies thateach transmission“sees”the system in the same state,i.e.,insteady state.At steady state,each remaining station transmits apacket withprobability(9)Equations(7)and(9)represent a nonlinear system in the twounknowns,which can be solved using numerical tech-niques.It is easy to prove that this system has a unique solution.In fact,inverting(9),weobtaindefined by(7)is also continuous intherangecan bealternatively writtenasis trivially shown to be a monotone decreasing function thatstartsfrom and reduces uptoandbe the normalized system throughput,defined as thefraction of time the channel is used to successfully transmit pay-load bits.Tocomputebe the probability that thereis at least one transmission in the considered slot time.Since(10)Theprobability(11)We are now able toexpress,540IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS,VOL.18,NO.3,MARCH2000Fig.5.T for basic access and RTS/CTS mechanisms.the slot time is empty;withprobabilityit con-tains a collision.Hence,(12)becomesis the duration of an empty slot time.Ofcourse,thevalues must be expressed withthe same unit.Note that the throughput expression (13)has been obtained without the need to specify the access mechanism employed.To specifically compute the throughput for a given DCF ac-cess mechanism it is now necessary only to specify the corre-spondingvaluesbe the propagation delay.As shownin Fig.5,in the basic access case weobtainis the the average length of the longest packetpayload involved in a collision.In the case all packets have the same fixedsize,for the packet's payload size.Letof collidingpackets,writes asfollows:(15)When the probability of three or more packets simultaneously colliding is neglected,(15)simplifiesto(16)BIANCHI:PERFORMANCE ANALYSIS OF THE IEEE 802.11DCF 541only if they exceed a given predetermined threshold on the packet's payload size.More specifically,being,again,theprobability distribution function of the packetsize,(19)Tocompute;2)collision between two packets transmitted viabasic access,withprobabilitythe respective average collisiondurations,weobtain(20)The average collision durations adopted in (20)detail as fol-lows.Letbe the extra length of the packet header with respect of the RTS frame,andlethas been already computed in thecase(22)Finally,noting that in the case of collision between two basic ac-cess packets,the probability distribution function of the length of the longest packet payload involved in a collision is the square of the conditional probability distribution function of the packet sizedistribution(23)By substituting (21),(22),and (23)in (20),we finallyobtain(24)For simplicity,in the rest of this paper we restrict our numer-ical investigation to the case of fixed packet size,and therefore we will evaluate the performance of systems in which all sta-tions operate either according to the basic access mechanism or according to the RTS/CTS mechanism (i.e.,never operating in the hybrid mode.)3V .M ODEL V ALIDATIONTo validate the model,we have compared its results with that obtained with the 802.11DCF simulator used in [9].Ours is an event-driven custom simulation program,written in the C++programming language,that closely follows all the 802.11pro-tocol details for each independently transmitting station.In par-ticular,the simulation program attempts to emulate as closely as possible the real operation of each station,including propa-gation times,turnaround times,etc.The values of the parameters used to obtain numerical results,for both the analytical model and the simulation runs,are sum-marized in Table II.The system values are those specified for the frequency hopping spread spectrum (FHSS)PHY layer [3].The channel bit rate has been assumed equal to 1Mbit/s.The frame sizes are those defined by the 802.11MAC specifications,and the PHY header is that defined for the FHSS PHY .The values of the ACK_Timeout and CTS_Timeout reported in Table II,and used in the simulation runs only (our analysis neglects the effect of these timeouts)are not specified in the standard,and they have been set equal to 300µs.This numerical value has been chosen as it is sufficiently long to contain a SIFS,the ACK transmission and a round trip delay.Unless otherwise specified,we have used in the simulation runs a constant packet payload size of 8184bits,which is about one fourth of the maximum MPDU size specified for the FHSS PHY ,while it is the maximum MPDU size for the DSSS PHY .Fig.6shows that the analytical model is extremely accurate:analytical results (lines)practically coincide with the simulation3Adetailed performance analysis of the hybrid mode requires to assume one or more suitable probability distribution functions for the packet's payload size,and also to determine the sensitivity of the throughput on the assumed distri-butions.Such a straightforward,but lengthy,study is out of the scopes of the present work.542IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS,VOL.18,NO.3,MARCH2000 results(symbols),in both basic access and RTS/CTS cases.Allsimulation results in the plot are obtained with a95%confidenceinterval lower than0.002.Negligible differences,well below1%,are noted only for a small number of stations(results forthe extreme case of as low as2and3stations are tabulated inTable III).VI.M AXIMUM S ATURATION T HROUGHPUTThe analytical model given above is very convenient to de-termine the maximum achievable saturation throughput.Let usrearrange(13)toobtain(25)As,are constants,thethroughput.Taking the derivative of(26)with respecttoholds,and yields the following approximatesolution:that each sta-tion should adopt in order to achieve maximum throughput per-formance within a considered network scenario(i.e.,number ofstations)maximum performance can be,in principle,achieved for everynetwork scenario,through a suitable sizing of the transmissionprobabilitydepends only on the networksize and on the systemparameters.Asa n d.T h i s p r o b l e m h a s b e e n s p e c i f i c a l l y c o n s i d e r e d i n[9]f o r t h ec a s e o f f i x ed b a c k o f f w i n d o w s i z e(i.e.,).I n s u c h a c a s e,T A B L E I IF H S S SY S T E MP A R A M E T E R S A N DA D D I T I O N A LP A R A M E T E R SU S E D T OO B T A I N N U M E R I C A LR E S U L TSF i g.6.S a t u r a t i o n T h r o u g h p u t:a n a l y s i s v e r s u s s i m uT A B L E I I IA N A L Y S I SV E R S U S S I M U L A T I O N:C OMPARISON FOR A V ERY L OW N UMBER OFS TATIONS—W=32;m=32BIANCHI:PERFORMANCE ANALYSIS OF THE IEEE802.11DCF543Fig.7.Throughput versus the transmission probability for the basic access method.Table II.The figure reports also the different throughput values obtained in the case of exact and approximate solutionforleads to similar throughput values.The accuracy of the throughput obtained by the approxi-mate solution is better testified by the numerical values reported in Table IV.Note that the agreement is greater in the basic ac-cess case,assufficientlylarge(29).Using the numerical valuesof Table V,weobtain.An advantage of the RTS/CTS scheme is that the throughputis less sensitive on the transmissionprobabilityaxis scale)thata small variation in the optimal valueofANDTn n n nu n n ne nn544IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS,VOL.18,NO.3,MARCH2000Fig.9.Saturation Throughput versus initial contention window size for the basic access mechanism.network size,the lower is the throughput.The only partial ex-ception is thecaseincreases.In fact,the throughput is practically constantfor,and even increases with the number of mobile sta-tionswhenwe have reported in Figs.9and 10the saturation throughput versus thevalue.The figures report four different networksizes,i.e.,number ofstations,and the optimal valueof(e.g.,1024)gives excellent throughput performancein the case of50contending stations,while it drastically penal-izes the throughput in the case of small number(e.g.,5)of con-tending stations.This behavior is seen also in Fig.10,where theRTS/CTS mechanism is rge valuesofthat a transmission occurs onthe channel is equal to theprobabilityequal to0,,and particularly in correspondence of thevaluesgets closer to64.Moreover,the throughputsignificantly decreases as the number of stations increases.Onthe contrary,Fig.10shows that the throughput obtained withthe RTS/CTS mechanism is almost independent of thevalue,com-puted according to(14)and(17),in the assumption of systemand channel parameters of Table II.The differencebetweenis the average number of slot times spent onthe channel in order to have a successful transmission.Of thoseslot times,afraction is empty,and each empty slot timelasts,is plotted in Fig.11versus thenetwork size,for three different values of the initial contentionwindowBIANCHI:PERFORMANCE ANALYSIS OF THE IEEE 802.11DCF 545of idle slot times per packet transmission is very low,particu-larly when compared with thevaluesgets greater (thecaseis theaverage number of collided transmissions per each successful transmission,which is multipliedbyand the networksize.It shows that,for the basic access mechanism,the amount of channel time wasted in collisions is extremely large for a smallvalueandof backoff stages is marginal.The figurereports the cases of both Basic and RTS/CTS access schemes,with(similar behaviour is observed for other values of theparameter,i.e.,in correspondenceof the standardized engineering parameters of the DSSS PHY (Table I).We see that the choiceofis greater than four or five.The only case in which the throughput still grows,foris the probability,seen by thestation,that its transmitted packet collides.Owing to the model's key assumption of independence at each retransmission,the av-erage number of transmissions that each station must perform in order to successfully complete a packet transmission is givenby.This value is reported in Fig.14,obtained with the same system parameters of Figs.9and 10.Fig.14shows that the number of transmissions per packet significantly increases as the initial backoffwindow.In particular,the lowerisis related to the fact,com-mented above by means of (33)and Fig.11,that the number of idle slot times per packet transmission is extremely small.A changeofand。

ieee 2标准IEEE 802标准，全称为IEEE 802 LAN/MAN Standards Committee，是由IEEE （Institute of Electrical and Electronics Engineers）制定的一系列局域网和城域网标准。

IEEE 802标准涵盖了各种网络技术，包括以太网、Wi-Fi、蓝牙等，为网络通信领域的发展做出了重要贡献。

IEEE 802标准的制定始于上世纪80年代，当时以太网技术正在迅速发展，各种不同的局域网技术层出不穷。

为了统一这些技术，IEEE成立了802工作组，旨在制定一系列通用的局域网标准。

至今，IEEE 802标准已经发展成为一个庞大而复杂的体系，涵盖了多个子标准和相关技术。

IEEE 802标准的重要性不言而喻。

它不仅为各种网络设备的互操作性提供了基础，同时也推动了全球范围内的网络通信技术的发展。

在当今数字化、信息化的时代，网络已经成为人们生活和工作中不可或缺的一部分，而IEEE 802标准则是支撑这一切的重要基石。

在IEEE 802标准体系中，最为人熟知的莫过于IEEE 802.11，即Wi-Fi标准。

Wi-Fi技术的普及和发展，使得人们能够随时随地连接互联网，享受高速无线网络带来的便利。

而IEEE 802.3则是以太网标准，是局域网中应用最为广泛的一种技术，几乎所有的有线网络都在使用这一标准。

除了这两个最为常见的标准之外，IEEE 802标准还包括了诸如IEEE 802.15.1（蓝牙）、IEEE 802.16（WiMAX）等多个子标准，涵盖了不同的网络场景和应用需求。

总的来说，IEEE 802标准的制定和发展，对于推动网络通信技术的发展起到了举足轻重的作用。

它不仅为不同厂商的设备提供了统一的标准，也为用户提供了更加便利和高效的网络体验。

随着物联网、5G等新兴技术的不断发展，IEEE 802标准也在不断演进和完善。

未来，我们有理由相信，IEEE 802标准将继续发挥重要作用，推动网络通信技术不断向前发展，为人们的生活和工作带来更多的便利和可能。

IEEE802系列标准
IEEE802系列标准
802.1--高层及其交互工作。提供高层标准的框架，包 括端到端协议、网络互连、网络管理、路由选择、桥接和 性能测量。 802.1X(基于端口的访问控制Port Based Network Access Control) ，协议起源于802.11协议，后者是标 准的无线局域网协议，802.1x协议的主要目的是为了解决 无线局域网用户的接入认证问题。 IEEE 802.1d (生成树协议Spanning Tree) IEEE 802.1w, RSTP算法 IEEE 802.1s, MSTP算法 IEEE 802.1P，讲述的是交换机与优先级相关的流量 处理的协议。 IEEE 802.1q，虚拟局域网Virtual LANs：VLan)虚 拟桥接局域网协议，定义了VLAN以及封装技术，包括 GARP协议及其源码、GVRP源码。 协议及其源码C (Logical Link Control，逻辑链 路控制)，802.2--连接链路控制LLC，提供OSI数据链路 层的高子层功能，提供LAN 、MAC子层与高层协议间的一 致接口。 IEEE 802.3 是一篇非常重要的业界规范文档。其中 最主要的就是规定了以太网的电气指标，从物理层的电路 结构到链路层的MAC操作都有介绍。802.3--以太网规范， ， 定义CSMA/CD标准的媒体访问控制（MAC）子层和物理 层规范。 802.3u (快速以太网Fast Ethernet) 802.3z (千兆以太网Gigabit Ethernet) IEEE802.3ab — 传输距离为100米的5类无屏蔽双绞 线介质千兆以太网标准规范 IEEE802.3ad，链路聚合 IEEE802.3X，全双工流控
IEEE802系列标准
4、802.11z 是一种专门为了加强无线局域网安全的标准。 802.12--按需优先（100VG-ANYLAN ）。定义使用按需 优先访问方法的100Mpbs 的以太网标准。 802.13 IEEE什么都没有定义 802.14 (有线电视 (CATV)) 802.15 (无线PAN (Personal Area Network))无线 个人局域网 802.16：宽带无线接入 Broadband Wireless Access 802.17 (弹性分组环 (Resilient Packet Ring)) 802.18：无线管制 Radio Regulatory TAG 802.19：共存 Coexistence TA 802.20：移动宽带无线接入 Mobile Broadband Wireless Access (MBWA) 802.21：媒质无关切换 Media Independent Handoff