网络WiFi-香港大学802.11n无线部署介绍_英文
IEEE 802.11p的英文文档
Runtime Optimization of IEEE802.11Wireless LANs PerformanceLuciano Bononi,Marco Conti,and Enrico GregoriAbstract—IEEE802.11is the standard for Wireless Local Area Networks(WLANs)promoted by the Institute of Electrical and Electronics Engineers.Wireless technologies in the LAN environment are becoming increasingly important and the IEEE802.11is the most mature technology to date.Previous works have pointed out that the standard protocol can be very inefficient and that an appropriate tuning of its congestion control mechanism(i.e.,the backoff algorithm)can drive the IEEE802.11protocol close to its optimal behavior.To perform this tuning,a station must have exact knowledge of the network contention level;unfortunately,in a real case,a station cannot have exact knowledge of the network contention level(i.e.,number of active stations and length of the message transmitted on the channel),but it,at most,can estimate it.This paper presents and evaluates a distributed mechanism for contention control in IEEE802.11Wireless LANs.Our mechanism,named Asymptotically Optimal Backoff(AOB),dynamically adapts the backoff window size to the current network contention level and guarantees that an IEEE802.11WLAN asymptotically achieves its optimal channel utilization.The AOB mechanism measures the network contention level by using two simple estimates:the slot utilization and the average size of transmitted frames.These estimates are simple and can be obtained by exploiting information that is already available in the standard protocol.AOB can be used to extend the standard802.11access mechanism without requiring any additional hardware.The performance of the IEEE802.11protocol,with and without the AOB mechanism,is investigated in the paper through simulation.Simulation results indicate that our mechanism is very effective,robust,and has traffic differentiation potentialities.Index Terms—Wireless LAN(WLAN),IEEE802.11,multiple access protocol(MAC),protocol capacity,performance analysis.æ1I NTRODUCTIONF OR decades,Ethernet has been the predominant networktechnology for supporting distributed computing.In recent years,the proliferation of portable and laptop computers has led to the development of the wireless LAN(WLAN)technology([28],[43]).The success of WLANs is connected to the development of networking products that can provide wireless network access at a competitive price.A major factor in achieving this goal is the availability of appropriate networking standards.IEEE Standard802.11defines a Medium Access Control(MAC) and Physical Layer(PHY)specification for a wireless local area network to provide wireless connectivity for fixed, portable,and moving stations within a local area[42].Two different approaches can be followed in the implementation of a WLAN:an infrastructure-based ap-proach or an ad hoc networking one([18],[25],[50]). Infrastructure-based802.11WLANs are currently widely used,while the use of IEEE802.11-based ad hoc networks is an open research issue([3],[21]).Since the wireless links will continue to have signifi-cantly lower capacity than wired links,the WLAN conges-tion is more problematic than in wired networks.In WLANs,the medium access control(MAC)protocol is the main element that manages congestion situations that may occur inside the network.For this reason,in this paper,we focus on the efficiency of the IEEE802.11MAC protocol and we propose a solution for increasing both protocol efficiency and protocol’s ability to react to congestion conditions.The IEEE802.11access scheme incorporates two access methods:Distributed Coordination Function(DCF)for asynchronous,contention-based,distributed access to the channel and Point Coordination Function(PCF)for centra-lized,contention-free access([42],[50]).PCF is intended to support real-time services(by using a centralized polling mechanism),but is not generally supported by current cards. Hereafter,we will concentrate our study on DCF only.The DCF is based on a Carrier Sensing Multiple Access protocol with Collision Avoidance,CSMA/CA,see,for example,([19],[38],[53]).The CSMA/CA protocol is typically adopted in a wireless environment due to its reliability, flexibility,and robustness.However,the performance of a WLAN based on the CSMA/CA protocol may be degraded by the presence of hidden terminals[54].A pair of stations is referred to as being hidden from each other if a station cannot hear the transmission from the other station.This event makes the carrier sensing unreliable as a station wrongly senses that the wireless medium has been idle while the other (hidden)station is transmitting.To avoid the hidden terminal problem,the CSMA/CA protocols are extended with a virtual carrier sensing mechanism,named Request To Send (RTS)/Clear To Send(CTS).This mechanism has been studied extensively;several variations and analyses of the RTS/CTS scheme can be found in the literature,see,for example,([4], [31],[29],[32]).IEEE802.11includes an optional RTS/CTS mechanism.In this work,we do not explicitly consider the RTS/CTS mechanism.The results presented hereafter always refer to the data transmission using the basic access only.A.L.Bononi is with the Department of Computer Science,University ofBologna,Mura Anteo Zamboni,7,40127Bologna,Italy.E-mail:bononi@cs.unibo.it..M.Conti and E.Gregori are with the National Research Council(CNR),IIT Institute,Via G.Moruzzi,1,56124Pisa,Italy.E-mail:{marco.conti,enrico.gregori}@r.it.Manuscript received14Mar.2001;revised5Aug.2002;accepted29May2003.For information on obtaining reprints of this article,please send e-mail to:tpds@,and reference IEEECS Log Number113793.1045-9219/04/$17.00ß2004IEEE Published by the IEEE Computer Societymethodology for analyzing the optimal tuning of the backoff algorithm when a portion of the traffic is transmitted using the RTS/CTS mechanism can be found,for example,in([6], [13]).In addition,recent simulation and experimental results indicate that phenomena occurring at the physical layer make the effectiveness of the RTS/CTS mechanism arguable since the hidden station phenomenon rarely occurs([56],[11],[23]).The relevance of the IEEE802.11standard has generated extensive literature on its MAC protocol.A complete survey of the IEEE802.11literature is out of the scope of this paper.Below,we will show the main research areas together with some related references.Simulation studies of the IEEE802.11protocol performance are presented in([62] [2]).IEEE802.11analytical models are proposed and evaluated in([5],[6],[16],[17],[20],[59],[60]).The use of the PCF access method for supporting real-time applica-tions is investigated in([26],[57]).The optimization of the DCF mechanism from the power-saving standpoint is investigated in([7],[44]).Recently,considerable research activity has concentrated on supporting service differentia-tion on the IEEE802.11DCF access method(e.g.,[49],[58], [1],[47]),and on the use of IEEE802.11for constructing multihop ad hoc networks([63],[64]).In this paper,we propose and evaluate a mechanism, Asymptotically Optimal Backoff(AOB),for improving the efficiency of the IEEE802.11standard protocol.In the literature,it is extensively recognized that the backoff algorithm plays a crucial role in achieving a high aggregated throughput and a fair allocation of the channel to the stations,see[4].To meet this target,the backoff value should reflect the actual level of contention for the media. The IEEE802.11adopts a binary exponential backoff protocol([42],[36],[38])which does not always adequately guarantee the best time-spreading of the users’access for the current congestion level.Each station,to transmit a frame,accesses the channel within a random self-defined amount of time whose average length depends on the number of collisions previously experienced by the station for that frame.When the network is congested,for each transmitted frame,a station must experience several collisions to increase the backoff window size,thus achieving a time spreading of the transmission attempts that is adequate for the current congestion level.No experience from the previous transmitted frame is exploited.On the other hand,our AOB mechanism extends the binary exponential backoff algorithm of IEEE802.11to guarantee that the backoff interval always reflects the current congestion level of the system(in the standard backoff,any new transmission assumes a low congestion level in the system).Our mechanism forces the network stations to adopt a backoff window size that maximizes the channel utilization1for the current network condition. There are two main factors that reduce the channel utilization:collisions and idle periods(introduced by the spreading of accesses).As these two factors are conflicting (i.e.,reducing one causes an increase of the other),the optimal tuning of the backoff algorithm is approximately achieved by equating these two costs([15],[16],[30]).Since these costs change dynamically(depending on the network load),the backoff should adapt to congestion variations in the system.Unfortunately,in a real case,a station does not have an exact knowledge of the network and load configurations,but,at most,can estimate them.The most promising direction for improving backoff protocols is to obtain information of the network status through channel observation([34],[37],[45]).A great amount of work has been done on studying the information that can be obtained by observing the system’s parameters([33],[48],[55]).Our work follows the same direction of feedback-based proto-cols,but provides original contributions as it is based on an analytical characterization of the optimal channel utilization and uses a very simple feedback signal:slot utilization.Several authors have investigated the enhancement of the IEEE802.11backoff protocol to increase its performance. In[61],given the Binary Exponential Backoff scheme adopted by the Standard,heuristic solutions have been proposed for a better time spread of the transmission attempts.In([5],[6],[15],[16],[17]),feedback-based mechanisms have been proposed for adapting the station backoff to the network congestion and maximizing channel utilization.Recently,these mechanisms have been general-ized to achieve both optimal channel utilization and weighted fairness in an IEEE802.11network with traffic streams belonging to different classes[47].All the feedback-based mechanisms cited above are based on analytic models of an IEEE802.11network.These models provide the optimal setting of the backoff parameters for achieving the maximum channel utilization.Unfortunately,these methods require an estimation of the number of users in the system that could prove expensive,difficult to obtain, and subject to significant error,especially in high contention situations[17].The AOB mechanism proposed in this paper goes a step further:1.By exploiting the analytical characterization of theoptimal IEEE802.11channel utilization presented in[16],we show that the optimal value is almostindependent of the network configuration(numberof active stations)and,hence,the maximum channelutilization can be obtained without any knowledgeof the number of active stations.2.The AOB mechanism tunes the backoff parameters tothe network contention level by using two simple andlow-cost load estimates(obtained by the informationprovided by the carrier sensing mechanism):slotutilization and average size of transmitted frames.3.AOB extends the standard802.11access mechanismwithout requiring any additional hardware. Specifically,AOB schedules the frames’transmission accord-ing to the IEEE802.11backoff algorithm,but adds an additional level of control before a transmission is enabled.A transmission already enabled by the standard backoff algorithm is postponed by AOB in a probabilistic way.The probability of postponing a transmission depends on the network congestion level and is equal to one if the channel utilization tends to exceed the optimal value.The postponed transmission is rescheduled as in the case of a collision,i.e., the transmission is delayed by a further backoff interval.In this paper,via simulation,we have extensively evaluated the performance of the IEEE802.11access scheme,with and without the AOB mechanism.The IEEE 802.11performance has been investigated both in steady-state and under transient conditions.Furthermore,we also1.In the literature,the maximum channel utilization is called protocol capacity;see[22].For this reason,hereafter,maximum channel utilization and protocol capacity are used interchangeably.investigate the mechanism robustness to errors and its potential for traffic differentiation.The work is organized as follows:In Section2,we present a brief explanation of the IEEE802.11standard,and we sketch the critical aspects connected to the contention level of the system.In Section3,we present a simple mechanism to extend the IEEE802.11standard and,in Section4,we discuss its tuning.In Sections5,6,and7,the AOB performance is deeply investigated through simula-tion.Section8discusses an AOB potential for traffic differentiation.Conclusions and future research are out-lined in Section9.2IEEE802.11In this section,we only sketch the portions of the IEEE802.11 standard that are relevant for this paper.A detailed description can be found in([42],[13],[27]).The IEEE802.11standard defines a MAC layer and a Physical Layer for WLANs.The basic access method in the IEEE802.11MAC protocol is the Distributed Coordination Function(DCF),which is a Carrier Sense Multiple Access with Collision Avoidance(CSMA/CA)MAC protocol.Besides the DCF,the IEEE802.11also incorporates an alternative access method known as the Point Coordination Function(PCF)—an access method that is similar to a polling system and uses a point coordinator to determine which station has the right to transmit.The DCF requires that every station,before transmitting, perform a carrier sensing activity to determine the state of the channel(idle or busy).If the medium is found to be idle for an interval exceeding the Distributed InterFrame Space(DIFS),the station continues with its transmission.If the medium is busy, the transmission is deferred until the ongoing transmission concludes.When the channel becomes idle,a Collision Avoidance mechanism is adopted.The IEEE802.11Collision Avoidance mechanism is a Binary Exponential Backoff scheme ([42],[36],[38],[39]).According to this mechanism,a station selects a random interval,called a backoff interval,that is used to initialize a backoff counter.When the channel is idle,the length of the time is measured in constant units(Slot_Time) indicated as slots in the following.The backoff interval is an integer number of slots and its value is uniformly chosen in the interval(0,CW_Size-1),where CW_Size,in each station is a local parameter defining the current station Contention Window size.Specifically,the backoff value is defined by the following expression[42]:Backo ff Counter¼INT RndðÞÁCW SizeðÞ; where Rnd()is a function that returns pseudorandom numbers uniformly distributed in[0,1).The backoff counter is decreased as long as the channel is sensed to be idle,stopped when a transmission is detected on the channel,and reactivated when the channel is sensed to be idle again for more than a DIFS.A station transmits when its backoff counter reaches zero.The Binary Exponential Backoff is characterized by the expression giving the dependency of the CW_Size parameter by the number of unsuccessful transmission attempts(N_A)already performed for a given frame.In[42],it is defined that the first transmission attempt for a given frame is performed adopting CW_Size equal to the minimum value CW_Size_min(assuming low contention). After each unsuccessful(re)transmission of the same frame,the station doubles CW_Size until it reaches the maximum value fixed by the standard,i.e.,CW_Size_MAX,as follows: CW SizeðN AÞ¼min CS Size MAX;CW Size minÁ2ðN AÀ1Þ: Positive acknowledgments are employed to ascertain a successful transmission.This is accomplished by the receiver(immediately following the reception of the data frame),which initiates the transmission of an acknowl-edgment frame(ACK)after a time interval Short InterFrame Space(SIFS),which is less than DIFS.If the transmission generates a collision,2the CW_Size parameter is doubled for the new scheduling of the retransmission attempt,thus further reducing contention.The increase of the CW_Size parameter value after a collision is the reaction that the IEEE802.11standard DCF provides to make the access mechanism adaptive to channel conditions.In[8],by analyzing the behavior of the IEEE 802.11DCF mechanism,it was shown that the channel utilization is negatively affected by the increase of the contention level.This occurs because1)the increase in the CW_Size is obtained at the cost of a collision,and2)after a successful transmission,no memory of the actual contention level is maintained.3L OW-C OST D YNAMIC T UNING OF THEB ACKOFF W INDOW S IZEThe drawbacks of the IEEE802.11backoff algorithm, explained in the previous section,indicate a direction for improving the performance of a random access scheme by exploiting the information on the current network conges-tion level that is already available at the MAC level. Specifically,the utilization rate of the slots(Slot Utilization) observed on the channel by each station is used as a simple and effective estimate of the channel congestion level.The estimated Slot Utilization must be frequently updated.For this reason,in[9],it was proposed that an estimate be updated by each station in every Backoff interval,i.e.,the defer phase that precedes a transmission attempt.A simple and intuitive definition of the slot utilization ðS UÞis then given by:S U¼Num Busy SlotsNum Available Slots;where:.Num_Busy_Slots,hereafter referred to as busy slots,is the number of slots in the backoff interval in whichone or more stations start a transmission attempt.Atransmission attempt can be either a successfultransmission or a collision;and.Num_Available_Slots is the total number of slots available for transmission in the backoff interval,i.e.,the sum of idle and busy slots.In the IEEE802.11standard mechanism,every station performs a Carrier Sensing activity and,thus,the proposed S_U estimate is simple to obtain.The information required to estimate S_U is already available to an IEEE802.11 station and no additional hardware is required.The current S_U estimate can be used by each station (before trying a“blind”transmission)to evaluate the2.A collision is assumed whenever the ACK from the receiver is missing.opportunity to either perform or defer the scheduled transmission attempt.In other words,if a station knows that the probability of a successful transmission is low,it should defer its transmission attempt.This can be achieved in an IEEE 802.11network by exploiting the DCC mechan-ism proposed in [9].According to DCC,each IEEE 802.11station performs an additional control (beyond carrier sensing and backoff algorithm)before any transmission attempt.This control is based on a new parameter,named Probability of Transmission P_T(...),whose value depends on the current contention level of the channel,i.e.,S_U .The heuristic formula proposed in [9]for P_T (...)is:P T S U;N A ðÞ¼1ÀS U N A ;where,by definition,S U assumes values in the interval [0,1],and N_A is the number of attempts already performed by the station for the transmission of the current frame.3The N_A parameter is used to partition the set of active stations in such a way that each stations’subset is associated with a different level of privilege to access the channel.Stations that have performed several unsuccessful attempts have the highest transmission privilege [9].The P_T parameter allows filtering the transmission attempts.When,according to the standard protocol,a station is authorized to transmit (backoff counter is equal to zero and channel is idle)in the protocol extended with the Probability of Transmission,a station will perform a real transmission with probability P_T ;otherwise (i.e.,with probability 1-P_T )the transmission is rescheduled as a collision would have occurred,i.e.,a new backoff interval is sampled.To better understand the relationship between the P_T definition and the network congestion level,we can observe Fig.1.In Fig.1,we show the P_T curves (for users with different N_A )with respect to the estimated S_U values.Assuming S_U is close to zero,we can observe that each station,independently of its number of performed attempts,obtains a Probability of Transmission (P_T )close to 1.This means that the proposed mechanism has no effect on the system and each user performs its accesses as in the standard access scheme,without any additional contention control.This point is significant as it implies the absence ofoverhead introduced in low-load conditions.The differ-ences in the users’behavior as a function of their levels of privilege (related to the value of the N_A parameter)appear when the slot utilization grows.For example,assuming a slot utilization close to 1,say 0.8,we observe that the stations with the highest N_A value obtain a Probability of Transmission close to 1,while stations at the first transmis-sion attempt transmit with a probability equal to 0.2.It is worth noting a property of the DCC mechanism:The slot utilization of the channel never reaches the value 1.Assuming S_U is close to or equal to 1,the DCC mechanism reduces the Probabilities of Transmission for all stations close to zero,thus reducing the network contention level.This effect is due to the P_T definition and,in particular,to the explicit presence of the upper bound 1for the slot utilization estimate.The DCC choice to use 1as the asymptotic limit for the S_U is heuristic and does not guarantee the maximum channel utilization.To achieve the maximum channel utilization,we need to know the optimal congestion level,i.e.,the optimal upper bound for the S_U value (opt_S_U).It is worth noting that,if opt_S_U is known,the P_T mechanism can be easily tuned to guarantee that maximum channel utilization is achieved.Intuitively,if the slot-utilization boundary value (i.e.,the value one for DCC)is replaced by the opt_S_U value,we reduce all the probabilities of transmission to zero in correspondence with slot utilization values greater than or equal to the opt_S_U .This can be achieved by generalizing the definition for the Probability of Transmission:P T opt S U;S U;N A ðÞ¼1Àmin 1;S U opt S UNA:ð1ÞSpecifically,by applying this definition of the transmission probability,we obtain the P_T curves shown in Fig.2.These curves were obtained by applying the generalized P_T definition with opt_S_U =0.80.As expected,the curves indicate the effectiveness of the generalized P_T definition to limit S_U to the opt_S_U value.The generalized Probability of Transmission provides an effective tool for controlling the congestion inside an IEEE 802.11WLAN in an optimal way,provided that the opt_S_U value is known.In the following,we will present a simple mechanism to set the opt_S_U value.Our mechanism is named Asymptoti-cally Optimal Backoff as it guarantees that the optimal utilization is asymptotically achieved,i.e.,for large M values.3.Atthe first transmission attempt,N Ais equal to 1.Fig.1.DCC probability of transmission.Fig.2.Generalized probability of transmission.4A SYMPTOTICALLY O PTIMAL B ACKOFF(AOB) M ECHANISMThe aim of the AOB mechanism is to dynamically tune the backoff window size to achieve the theoretical capacity limit of the IEEE802.11protocol.The AOB mechanism is simpler, more robust,and has lower costs and overhead introduced than the contention mechanisms proposed in[16],[17]. Specifically,the AOB mechanism requires no estimate of the number M of active stations.An accurate M estimate may be very difficult to obtain because M may be highly variable in WLANs.In this section,we exploit the results obtained from the analysis of the theoretical capacity limits of the IEEE802.11 protocol to develop the AOB mechanism.For this reason, below,we briefly summarize the results derived in[16].In [16],to study the protocol capacity,a p-persistent IEEE 802.11protocol was defined.This protocol differs from the standard protocol only in the selection of the backoff interval.Instead of the binary exponential backoff used in the standard,the backoff interval of the p-persistent IEEE 802.11protocol is sampled from a geometric distribution with parameter p.Specifically,at the beginning of an empty slot,a station transmits(in that slot)with a probability p, while it defers the transmission with a probability1-p and then repeats the procedure at the next empty slot.4Hence, in this protocol,the average backoff time is completely identified by the p value.By setting p¼1=ðE½B þ1Þ(where E½B is the average backoff time of the standard protocol5), the p-persistent IEEE802.11model provides an accurate approximation(at least from a capacity analysis standpoint) of the IEEE802.11protocol behavior[16].The IEEE802.11p-persistent model is a useful and simple tool for analytically estimating the protocol capacity in a network with a finite number,M,of stations operating in asymptotic conditions.Furthermore,to simplify the discussion,hereafter we assume that stations transmit messages whose lengths are a geometrically distributed (with parameter q)number of slots.By denoting with t slot the length of a slot,the average message length, m,is: m¼t slot=ð1ÀqÞ.By exploiting the p-persistent model,in[16],a closed analytical formula for the channel utilization, ,is derived¼ m=fðM;p;qÞ:ð2ÞBy noting that fðÞis a function of the protocol and traffic parameters,it results that,for a fixed network and traffic configuration(i.e.,constant M and q),the maximum channel utilization corresponds to the p value,p min,that minimizes fðÞ.Due to the correspondence(from the capacity stand-point)between the standard protocol and the p-persistent one,the IEEE802.11maximum channel utilization is closely approximated by adopting,in the standard protocol,a contention window whose average size is identified by the optimal p value,i.e.,E½CW ¼2=p minÀ1.The previous analysis shows that the IEEE802.11 theoretical capacity is identified by p min.Hereafter,we will show the relationship between p min and the opt_S_U value of the AOB mechanism.To this end,we will further elaborate the capacity analysis presented in[16].4.1Theoretical Capacity Limits:An Invariant Figure Results presented in this section(see Table1)point out that the increase in the number of active stations has an almost negligible impact on the theoretical capacity bounds,while the average payload size(indicated as MFS in the following)greatly affects the optimal utilization level.Results presented in Table1are numerically derived by computing the optimal p value,i.e.,p min,according to formulas presented in[16].Specifically,in this table,we report,for various network and traffic configurations (defined by the(M,q)couples),the p min values derived analytically as explained before.In this table,we also report for each configuration the value MÁp min.It is worth noting that,while p min is highly affected by the M value,given a q-value,the product MÁp min is almost constant.Specifically,results indicate that,for a given message length,the product MÁp min has an asymptotic value with respect to M.Furthermore,when M!4,the MÁp min values are very close to the asymptotic value. This is the reason for calling MÁp min an invariant figure, i.e.,for a given MFS,it is almost constant.Hereafter,we will analytically investigate the rationale behind the MÁp min quasi-constant value(for a given MFS). To perform this analysis,instead of the exact p min derivation presented in[16](it is too complex for our purpose),we approximate p min with the p value that satisfies the following relationship:E½Coll ¼E½Idle p Át slot;ð3ÞOptimal pValues4.On the other hand,in the standard protocol,a station transmits in theempty slot selected uniformly inside the current contention window.5.Note that E½B ¼ðE½CW À1Þ=2,where E½CW is the average contention window.。
802.11n简述
802.11k
一个无线基站可以要求另一个基站测量和报告如下情况： • 在那个基站的覆盖范围内，一个信道上或者所有信道上的基站数量。 • 每一个基站的信标信号强度。 • 在一个具体时间间隔期间接收的帧的数量，负载情况。 • 每一个信道上的无线局域网活动水平。 • 便携式电话和微波炉等设备对每个信道产生的非无线局域网无线电 活动。
中国规定2.4GHz 频段范围是2.4~2.4835GHz, 1-13信道 (1信道为2.412GHz，每两个相邻信道间隔为5MHz); 5GHz频段范围是5.725~5.825GHz, 149/153/157/161/165信 道(149信道为5.745GHz)
Frame Aggregation
MIMO增强- diversity（分集）
•天线数量多于空间流数量，则把多个天线组合起来或选 择一个天线组合的最佳子集 • 更好利用多余天线
• 可获得更大的覆盖范围：使用两根天线的输出组合来接
收一个空间流，获得更远的传输距离 • 发送分集/接收分集
MIMO增强 –Beam-forming（聚束）
802.11n
600Mbps >100Mbps
20MHz /& 40MHz
DataRate和吞吐量
DateRate
DataRate指物理层传输速率，是传输信号的速度，不管这是 数据帧还是其他的控制帧。11N提供最高达600M的物理层传 输速率
吞吐量
吞吐量指的是真正的数据载荷部分传输的速率。一般测试结果 大致为总传输速率的一半左右它是不计算诸如：TCP负载，MAC 头负载，和PHY负载，以及控制帧管理帧，和空闲时间，冲突 造成的负载等
MAC层增进 QoS，增加了包突发机制 ------
802.11无线局域网缩写词及中文含义（一）解析
802.11无线局域网缩写词及中文含义（一）解析802.11无线局域网缩写词及中文含义（一）ACK (acknowledgment)应答AID (association identifier)关联识别码AP (accss point)访问点ATIM (announceent traffic indication message)广播传输指示消息BSA (basic service area)基本服务区BSS (basic service set)基本服务集BSSID (basic service set identification)基本服务集识别码CCA (clera channel assessment)干净信道评价CCK (complemenetary code keying)补码键控CF (contention free)无竞争CFP (contention-free period)无竞争期CID (connection identifier)连接标识符CP (contention period)竞争期CRC (cyclic redundancy code)循环冗余码CS (carrier serse)载波侦听CTS (clear to send)允许发送CW (contention window)竞争窗口DA (destination address)目的地址DBPSK (differential binary phase shift keying)差分二进制相移键控DCE (data communication equipment)数据通信设备DCF (distributed coordination function)分布式协调功能DCLA (direct current level adjustment)直接电平调整DIFS (distributed (coordination function)interframe space)分布式（协调功能）帧间间隔DLL (data link layer)数据链路层DP (desensitization)减敏现象DQPSK (differential quadrature phase shift keying)差分正交相移键控DS (ditribution system)分发系统DSAP (destination service access point)目的服务访问点DSM (distribution system medium)分发系统媒介DSS (distribution system service)分发系统服务DSSS (direct sequence spread spectrum)直接序列扩频DTIM (delivery traffic indication message)交付传输指示信息ED (energy detection) 能量检测EIFS (extended interframe space)扩展帧间间隔EIRP (equivalent isotropically radiated power)等效全向辐射功率ERS (extended rate set)扩展速率集ESA (extended service area) 扩展服务域ESS (extended service set) 扩展服务集FC (frame control) 帧控制FCS (frame check sequence) 帧校验序列FER (frame error ratio) 帧差错率FH (frequency hopping) 跳帧FHSS (frequency-hopping spread spectrum) 跳帧扩频FIFO (first in first out)先进先出GFSK (Gaussian frequency shift keying)高斯频移键控HEC (Header Error Check)头部差错校验HR/DSSS (High Rate direct sequence spreadd spectrum using the Long Preamble and header)使用长前导和长头部的高速直接序列扩频HR/DSSS/short (High Rate direct sequence spreadd spectrum using the optional Short Preamble and header mode)使用可选的短前导和短头部的高速直接序列扩频HR/DSSS/PBCC (High Rate direct sequence spreaddspectrum using the optional packet binary convolutional coding mode and the Long Preamble and header) 使用可选分组二进制卷积编码方式和长前导和长头部的高速直接序列扩频HR/DSSS/PBCC/short (High Rate direct sequence spreadd spectrum using the optional packet binary convolutional coding mode and the optional Short Preamble and header)使用可选分组二进制卷积编码方式和可选短前导和短头部的高速直接序列扩频IBSS (independent basic service set)独立基本服务集ICV (integrity check calue)完整性检验值IDU(inteface data unit) 接口数据单元IFS (interframe space)帧间间隔IMP (intermodulation)互调保护IR (infrared)红外线（的）ISM (industrial,scientific.and medical)工业科学医疗IV (initialization vector)初始化矢量LAN (local area network)局域网LLC (logical link control)逻辑链路控制LME (layer management entity)层管理实体LRC (long retry count)长重发计数器LSB (least significant bit)最低位比特MAC (medium access control)媒介访问控制MDF (management-defined field)管理定义域MIB (management information base)管理信息库协议层管理实体MMPDU (MAC management protocol data unit)媒介访问控制管理协议数据单元MPDU (MAC protocol data unit)媒介访问控制协议数据单元MSB(most significant bit)最高位比特MSDU(MAC service data unit)媒介访问控制服务数据单元N/A(not applicable)不可用NAV(network allocation vector)网络分配矢量PC(point coordinator)集中协调器PCF(point coordination function)集中协调功能PDU(protocol data unit)协调数据单元PHY(physical [layer])物理层PHY-SAP(physical layer service access point)物理层服务访问点PIFS(point [coordination function] interframe space)集中协调功能帧间间隔PLCP(physical layer convergence protocol)物理层收敛协议PLME(physical layer management entity)物理层管理实体PMD(physical edium dependent)物理媒介依赖PMD-SAP(physical medium dependent service access point)物理媒介依赖服务访问点PN(pseudo-noise [code sequence])随机噪声（码序列）PPDU(PLCP protocol data unit)物理层收敛协议协议数据单元ppm(paresper million)百万分率，百万分之。
英文版04 IEEE 802.11无线网
3
infrastructure vs. ad-hoc networks
infrastructure network
AP
AP wired network AP: Access Point
AP
ad-hoc network
4
802.11 - Architecture of an infrastructure network
clear channel assessment signal (carrier sense) modulation, coding channel selection, MIB
MAC Management
PMD Physical Medium Dependent
PHY Management
Direct communication within a limited range
802.11 LAN
STA1
IBSS1
STA3
Station (STA): terminal with access mechanisms to the wireless medium Independent Basic Service Set (IBSS): group of stations using the same radio frequency
STA2
IBSS2 STA5
STA4
802.11 LAN
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IEEE standard 802.11
fixed terminal mobile terminal
infrastructure network access point
application
网络WiFi-20100811-HKU Introduction
Introducing University of Hong Kong
Peter Li Associate IT Director
August 2010
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The University of Hong Kong Information Technology Services
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The University of Hong Kong Information Technology Services
Experience in HPC and research computing :
Advancing HPC services
Deploy high performance and large scale computing facilities, and the latest software technology
Select & implement a single centrally supported LMS
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The University of Hong Kong Information Technology Services
ITIL Roadmap
Assessment and project starts Post
Architecture, Arts, Business and Economics, Dentistry, Education, Engineering, Law, Science, Social Sciences, Medicine Graduate School School of Professional and Continuing Education (HKU SPACE) 21600 students 6,500 staffs
802.11N协议基础及网络部署
802.11g
54Mbps (实际25Mbps）
带宽的大幅度提升使得每个接入点可以接入更多的客户端。
全面向下兼容a/b/g
2.4 GHz 5.8 GHz
Not support
b only
g only, b/g mix, b only
n only, b/g/n mix, g/n mix,b/g mix, g only, b only n only, a/n mix, a only
技术要求
在传输速率方面，802.11n可以将WLAN的传输速 率由目前802.11a及802.11g提供的54Mbps，提供 到300Mbps甚至高达600Mbps；覆盖范围可以扩 大到好几平方公里，使WLAN移动性极大提高； 兼容其他标准(包括3G) 。
802.11N技术简介
产生原因
Wi－Fi联盟在802.11a/b/g后面的一个无线传输标 准协议， 为了实现高带宽、高质量的WLAN服务， 使无线局域网达到以太网的性能水平，802.11任 务组N（TGn）应运而生。
目录
802.11n 概述 802.11n 关键技术 影响802.11n速率的关键因素 802.11n设备部署
802.11n部署——部署策略
2010年Q4-2011年Q4：
新建网络：以11n设备为主。 原有网络扩容：新增11n设备，与原有11g设备 混合部署。 利用2-3年时间逐步替换原有11g设备，实现11g 向11n的平滑升级。
IP报文长度（byte）
1*1实际速率 （无加密） Mb/s
1500
512
88
20MHz 60 40MHz 120
802.11n无线局域网部署指南
射频链路
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空间复用
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保护间隔
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20 MHz 与 40 MHz OFDM 信道
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天线分集
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传输波束成形
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数据链路层增强
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帧聚合ቤተ መጻሕፍቲ ባይዱ
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块确认
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速度与吞吐量
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调制编码方式
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合规、设计与集成
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Wi-Fi CERTIFIED™ n 认证项目
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PoE
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现有的基础架构
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2.4 GHz 与 5 GHz 频段
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20 MHz 与 40 MHz 模式
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向后兼容
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客户设备
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VoWiFi
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安全
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站点勘测
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迁移与部署策略
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结语
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802.11n 无线局域网部署指南
概述
1997 年，电子与电气工程师协会（IEEE）在 802.11 原始标准中对无线局域网（WLAN） 技术首次做出了定义。最初的 WLAN 设备采用称为直接序列扩频（DSSS）或跳频扩频（FHSS） 的射频技术，支持 1 Mbps 与 2 Mbps 的数据速率。从 1997 年开始，随着 IEEE 对 802.11 标 准进行修订以支持新的技术，WLAN 的数据速率逐渐增加。1999 年批准通过的 802.11a 修正 案采用正交频分复用（OFDM）技术，在 5 GHz 频段上支持 6～54 Mbps 的数据速率。同年， 802.11b 修正案定义的高速直接序列扩频（HR-DSSS）机制可在广泛使用的 2.4 GHz 频段上 支持 5.5 Mbps 与 11 Mbps 的数据速率。2003 年，IEEE 批准通过了 802.11g 修正案，后者同 样采用 OFDM 技术，在 2.4 GHz 频段上支持 6～54 Mbps 的数据速率。用户与企业一直期待 802.11-2009 修正案，它定义的高吞吐量（HT）技术可支持高达 600 Mbps 的数据速率。802.11n 技术增强并优化了物理层与 MAC 子层，从而支持如此高的数据速率。
WLAN--802.11N协议基础及网络部署
TD-SCDMA 2.8M TD-LTE 173M
802.11g
54Mbps (实际25Mbps）
带宽的大幅度提升使得每个接入点可以接入更多的客户端。
全面向下兼容a/b/g
2.4 GHz 5.8 GHz
Not support
b only
g only, b/g mix, b only
n only, b/g/n mix, g/n mix,b/g mix, g only, b only n only, a/n mix, a only
802.11n 关键技术 802.11n 的技术优势 影响802.11n速率的关键因素 802.11n设备部署
影响802.11n速率的关键因素——CSMA/CA机制
WLAN 采用CSMA/CA(Carrier Sense Multiple Access with Collision Avoidance) 机制来解决冲突问题。 终端在发送数据前首先监听信道，如果信道空闲，则随机退避一 个时间，如果这个时间内没有其他终端抢占信道，终端等待完后 可以立即占用信道并传输数据。 CSMA/CA决定了：终端越多，则冲突规避开销越大，信道可用 带宽越低。
MIMO：具有多个独立的接收和传输路径的无 线电射频系统，是11n的核心技术。 MIMO-OFDM：将高速码流拆分成多个低速码 流，每个低速码流在相同的频点上分别由不同 的天线同时发送。码流在接收端进行整合。
802.11n关键技术——信道捆绑技术
1Mbps 20MHz Channel 1Mbps 20MHz Channel 40MHz Channel 2Mbps
直接信号
反射信号
反射信号
802.11n关键技术-帧聚合技术
帧聚合技术原理：
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The University of Hong Kong Information Technology Services
Coverage of WiFi Network
Total no. of APs > 1500 Locations:
Classrooms & Lecture Theatres Libraries Residential Halls Departmental Offices and Premises Amenities Centres, PC laboratories and Open Areas (podia)
802.11b&g -> 802.11n
2008: 200 units of “IEEE 802.11n draft 2.0” Aruba access points as pilot test. Scope:
replaced 100 units of 802.11b access points installed over 5 years. added 100 new 802.11n access points to augment WiFi coverage.
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The University of Hong Kong Information Technology Services
Recent Major Upgrades
Robert Black College
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The University of Hong Kong Information Technology Services
WiFi Network Accessibility
PC Laboratories
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The University of Hong Kong Information Technology Services
Schematic Diagram of HKU WiFi Network
Staff Network
530 new APs for student residential halls 810 mainly for replacing existing 802.11b&g Aps
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The University of Hong Kong Information Technology Services
0
501
2004/05
537
2005/06
570
587
2006/07
2007/08
1,527 840
2008/09
2009/10
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The University of Hong Kong Information Technology Services
WiFi Users
<presenter> Computer Centre
Recent Major Upgrades
WiFi for Student Residential Halls
Internet
Router
Access control
WiFi Network
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The University of Hong Kong Information Technology Services
Simplified Thin-AP System Configuration
WiFi controller
Brief History
Year 2000-2003 2004-2005 2006-2007 2008 2009-2010
Standard 802.11b 802.11b/g
802.11b/g 802.11b/g/n (draft 2.0) 802.11b/g/n
AP type Fat AP Fat AP Thin AP Thin AP Thin AP
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The University of Hong Kong Information Technology Services
WiFi Network Accessibility
Lecture Theatres
Departmental Conference Rooms
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The University of Hong Kong Information Technology Services
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The University of Hong Kong Information Technology Services
Brief history of WiFi development in HKU
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The University of Hong Kong Information Technology Services
Components of WiFi network
DHCP Server
Radius Server
Core switch
WiFi Controllers
Tunnel
Tunnel
ITS Building
Router
Access switch
AP Distribution switch AP AP Access switch
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The University of Hong Kong Information Technology Services
Recent Major Upgrades
WiFi for Student Residential Halls
1998: wired network connection for all student residential halls 2010: 570 APs for 17 halls, 1 guest house; 4700 places
WiFi controller
Primary link
Resilient links
Primary link
Campus network
Thin access points
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The University of Hong Kong Information Technology Services
Schematic Diagram of HKU WiFi Network
Brand Orinoco/Proxim Orinoco/Proxim 3Com Aruba H3C
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The University of Hong Kong Information Technology Services
Coverage, topology and usage statistics
Notebook
Notebook Mobile Device
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The University of Hong Kong Information Technology Services
Schematic Diagram of WiFi Network – inside building
AP Access switch
2006/07
2007/08
2008/09
2009/10
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The University of Hong Kong Information Technology Services
Number of WiFi Access Points
1,600 1,400 1,200 1,000
800 600 400 200
<date>
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The University of Hong Kong Information Technology Services
WiFi Usage
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The University of Hong Kong Information Technology Services
Recent Major Upgrades:
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The University of Hong Kong Information Technology Services
Recent Major Upgrades
St. John’s College University Hall
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The University of Hong Kong Information Technology Services
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The University of Hong Kong Information Technology Services
Recent Major Upgrades
802.11b&g -> 802.11n
2009: wide deployment of 802.11n-ready access points in HKU 1340 H3C access points (H3C WA2620EAGN)
Recent Major Upgrades
802.11b&g -> 802.11n
6 H3C WX6103 controllers (3+3 redundancy) All 802.11n APs in HKU after this upgrade Upgrade expected to be completed before end of 2010.
Recent Major Upgrades
Ricci Hall
Simon K. Y. Lee Hall
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The University of Hong Kong Information Technology Services
Recent Major Upgrades
Graduate House
Swire Hall