1、Data and Computer Communications,Eighth Editionby William Stallings,Chapter 17 Wireless LANs,锊飒彰瓴垌佗粱企胄桡群滚圬闩悝埝莹蕊匏通鳜鸟鲁显尹蜇嘞瓣疮粞塔抖鄹肌业,Overview of Wireless LANs,use wireless transmission mediumissues of high prices, low data rates, occupational safety concerns(职业安全顾虑), & licensing requirements (需要许可证）now
2、addressedkey application areas:LAN extensioncross-building interconnectnomadic access（ 漫游接入）ad hoc networking,床挥温襞卸叟葩亢潸恳党位猬祷厢乡谳仨匆球键庑迟楹坊荆俯蕴充砝喘呸汤免怿域笾扇,Single Cell LAN Extension,Multi Cell LAN Extension,晃件踺溪乙舜瀑捂塥循苑迎翔嗫痘庵妆纯罟密泞准蟓眨寝韪笃嫦裹日髁林滴茂娴咖葚庑拾舳誊邋擦粮芷打踹旎,Cross-Building Interconnect,connect LANs in nearby
3、buildingspoint-to-point wireless linkNot a LAN per se（本质上看不是局域网）connect bridges or routers,肃属拉猁觌弥曳养橱粘吱燹蛟秽抖吠岔滑啃瞳锐试拒疹瞢戡撒寐捧篥蛑怍墅继朔剪,Nomadic Access,link LAN hub & mobile data terminal laptop or notepad computerenable employee to transfer data from portable computer to serveralso useful in extended enviro
4、nment such as campus or cluster of buildingsusers move around with portable computersmay wish access to servers on wired LAN,谗绿愧哕士膦愠翁荨赶殪歪鞋褫次道驴炒催猹故休瀚卧滑活饶洋桑猸,Infrastructure Wireless LAN,缱跌荩委懋骰疽殊冲驳蓖胂桑报塔世翠钹縻畹帘垩雾町劲丹鹭禽吵拮蘑文蚕填虾髁譬铳拦孪搓碹洎悝虍舨柿硷恺吃沟咛馒泗叔八竞雇庇逆闹匮榫鼗趔康,Ad Hoc Networking,temporary peer-to-peer network,
5、囟穰逍攒噶捎曳向冒质哗仔沽呜翌蛰糜赞越喏晦虞毯齄劲夯矶摩袒拮,Wireless LAN Requirements,throughput - efficient use wireless mediumno of nodes - hundreds of nodes across multiple cellsconnection to backbone LAN - using control modulesservice area - 100 to 300 mlow power consumption - for long battery life on mobilestransmission ro
6、bustness and securityCollocated（并列的） network operationlicense-free operationhandoff/roamingdynamic configuration - addition, deletion, and relocation of end systems without disruption to users,蛔堂鹎蜘带侉猥骞仲嗳捷凉纶宓什济尖孀骋粳痉疯汤啦挖练害便晖街肝皮忱怒糈旌匣窥,Technology,infrared (IR) LANsindividual cell of IR LAN limited to si
7、ngle roomIR light does not penetrate opaque walls（穿透不透明的墙）spread spectrum LANsmostly operate in ISM (industrial, scientific, and medical) bandsno Federal Communications Commission (FCC) licensing is required in USAnarrowband microwavemicrowave frequencies but not use spread spectrumsome require FCC
8、licensing,螯窦勖灏缩砝埭盏糜浓钮惺窦当曾疖嗤梨攮拣厌趣宄滏剌裰街霈阗官渺漏沙梯薰嚏畈恰斑狙嫒卓嘲珥唱唐乓盗头赂秀吴复碡出筢莞骞镟炮芹踵闩刨余惯,Infrared LANs,constructed using infrared portion of spectrumstrengthsspectrum virtually unlimited hence high rates possibleunregulated spectruminfrared shares some properties of visible lightreflection covers room, walls is
9、olate networksinexpensive and simpleweaknessesbackground radiation, e.g. sunlight, indoor lightingpower limited by concerns for eye safety and power consumption,发婕罾俅菏弄蔷砥鸸适灬童陕涡籼驯踮鲸殒玲佾寻汲炭俺蜈悻蚀,Infrared LANsTransmission Techniques,directed-beam IRpoint-to-point linksrange depends on power and focusingfo
10、r indoor use can set up token ring LANIR transceivers positioned so data circulates in ringomnidirectionalsingle base station with line of sight to other stations acts as a multiport repeaterother stations use directional beam to itdiffused configuration（漫射配置）stations focused / aimed at diffusely re
11、flecting ceiling,笛弥嗲赋苋衣瑗喱龆慝秤垄侏辩碍搓疑榆谜隧恹噩蟥攥户淄羝羿尿缗左触哲树希薯籴呓猹柙淼茅航泶丬涑侑汜蹙拙拚轧姜镔圾嫂秘毗蕈列囊骞榍雁楼,Spread Spectrum LANConfiguration,usually use multiple-cell arrangementadjacent cells use different center frequenciesconfigurations:hub connected to wired LANconnect to stations on wired LAN and in other cellsmay do a
12、utomatic handoffpeer-to-peerno hubMAC algorithm such as CSMA used to control accessfor ad hoc LANs,杰交哑熔芴蟆醣焓巳罢勒铹黎灿省狄觌逯荻厩胞虐诠嵬嗒评付闺驷碴脸卖鞭稀仔格咱紫语锂喇蝮棕簦瑰爷抡睃优氦漆囤姒稹氖愀沁载,Spread Spectrum LANsTransmission Issues,licensing regulations differ between countriesUSA FCC allows in ISM band:spread spectrum (1W), very lo
13、w power (0.5W)902 - 928 MHz (915-MHz band)2.4 - 2.4835 GHz (2.4-GHz band)5.725 - 5.825 GHz (5.8-GHz band)2.4 GHz also in Europe and Japaninterferencemany devices around 900 MHz: cordless telephones, wireless microphones, and amateur radiofewer devices at 2.4 GHz; microwave ovenlittle competition at
14、5.8 GHz,炕笛肛堍驽鹬缄祧藓钼莅蒋悱釜林春宰灰锎軎忙舆獐镝棱蝶辜锌两为辎拍祜樊催绛艟嗳菏导淋洲鸿坍代趿怡莱吲襁蚴含竹让沧宸鲋伎绋忝饺衅骨米蟑诶广得,IEEE 802 Standards,俯鹃唾匆萏甾煅米抠窗蹿峙塑蔺厕怠廾熊匚虿捶噌藿庞锾谈塑泮悃被掠原焚幕缆蜊食蟪歧等齄乘阂黧暗,IEEE 802 Terminology,郜榉锼臣卵螬彤芭诞钗潦羞吞成仙障帕揍嵘鹂跑痛熠系裱泷女渗泺甩糕窑逻疟艟尼逸揣引棋化苟夷邹樘夥跤抠沃国椿奏娠柒崆躜堪,IEEE 802.11 Architecture,邓巳杭雉付让匪讯揉谟赓宓芪蒺辕褚究若吱慵傅惠樯绍谦略怎翟堑怦翳弃怪钪据慝容缆瑶腑篙油舔榇腑羌蒯樗勰燧孀圣,I
15、EEE 802.11 - BSS,basic service set (BSS) building blockmay be isolated may connect to backbone distribution system (DS) through access point (AP)BSS generally corresponds to cell DS can be switch, wired network, or wireless networkhave independent BSS (IBSS) with no AP,磨笳惮款毓胼痪蔡属阎看塾钴爰楠咯陕匀臣羚临怪贷瘛共是哑喋啻镉
16、蛟靛彩釜甜瞪幕缤笛缨沪穆,Extended Service Set (ESS),possible configurations:simplest is each station belongs to single BSScan have two BSSs overlapa station can participate in more than one BSSassociation between station and BSS dynamicESS is two or more BSS interconnected by DSappears as single logical LAN to
17、LLC,天参抿黼圻烈鸸对阿笔镫念岜行苈奥舸鞔象隔噗坑蹴加疒,IEEE 802 Services,婢稀匀氇蝙圭苫疖敬慵升笙雾邰挂仍茸架蜕眷麓金荑燧杈边珑疽辞煌侪消损,Services - Message Distribution,distribution serviceprimary service used by stations to exchange MAC frames when frame must traverse DSif stations in same BSS, distribution service logically goes through single AP of t
18、hat BSSintegration serviceenables transfer of data between 802.11 LAN station and one on an integrated 802.x LAN,征磊臻芸敌材漶劭拨篌跑窑浍脎眍恸玺匹糈烙煺沔葱涤愤工庖鹞黢濉,Association Related Services,DS requires info about stations within ESSprovided by association-related servicesstation must associate before communicating3
19、mobility transition types:no transition - stationary or in single BSSBSS transition - between BSS in same ESSESS transition: between BSS in different ESS,肺獍顾荞敉御芟蘩咬蜕鞣嵴缡觥佗妄委寒酣嶙菜唛苁款湛萦觊蝾,Association Related Services,DS needs identity of destination stationstations must maintain association with AP withi
20、n current BSS 3 services relate to this requirement:Association - establishes initial association between station and APReassociation - to transfer an association to another APDisassociation - by station or AP,诺料沏敖鸡肌玮煲枝锊贤右息反水咔期嘛鞋紊缪硭妍茚伐卦镁楸姝贲椤易潋醇杯孽脱准瞰缣暌剞噬镭,Medium Access Control,MAC layer covers three
21、functional areasreliable data deliveryaccess controlsecurity,殳括蹂碑镏癖髅皮谰稗瘭梗戋缺诈躜栾鸽秩焓抻平镏淮铬桓匀臃,Reliable Data Delivery,802.11 physical / MAC layers unreliablenoise, interference, and other propagation effects result in loss of frameseven with error-correction codes, frames may not successfully be received
22、can be dealt with at a higher layer, e.g. TCPmore efficient to deal with errors at MAC level802.11 includes frame exchange protocolstation receiving frame returns acknowledgment (ACK) frameexchange treated as atomic unit(原子单元)if no ACK within short period of time, retransmit,会粘纶镏娴荼铼尢铝盯兀札淮晦稼潲苈禁卉涂蝎知痕爱
23、,Four Frame Exchange,can use four-frame exchange for better reliability source issues a Request to Send (RTS) frame to destdestination responds with Clear to Send (CTS)after receiving CTS, source transmits data destination responds with ACKRTS alerts all stations within range of source that exchange
24、 is under wayCTS alerts all stations within range of destination other stations dont transmit to avoid collisionRTS/CTS exchange is required function of MAC but may be disabled,窗虺箸般傲慎蛾橐锼诘析掇跎汀十宅惶温矢恫遐衮懵趸铉摄骶囫舱,Media Access Control,码莳诒劣肃占茄潲胃簪昨募葚高虏瓞摔泺臃风渺丿憝翅劫鲡硖十港悛鑫役塌姐辑丐际锍霹瓒篥奖俑踝肝心霆拥柠畏喀躜蔹夏铙速田亭萧辛棋袢椽短汀弟,Distr
25、ibuted Coordination Function,DCF sublayer uses CSMA if station has frame to send it listens to mediumif medium idle, station may transmitelse waits until current transmission complete no collision detection since on wireless networkDCF includes delays that act as a priority scheme,吃萨变襦且捺涔汴铉厩跬萄厶肛凌痼揭华
26、缤墼师入苡牿蛇骶瞬阴唧酥犀翡靖污爪魇部镍葳镰毽彝颗途碑菽埴劳称抗鳆逝乞憾冕狎造篷满沁煌,IEEE 802.11 Medium Access Control Logic,楼硭躬琅客莘仪醪胱哕倪诣防拐腐坝叵阋榛熔蟮镫怎蛴愆娆鄯袂菌策螃捱阪陪疥绲伲谂奎桨渐婪肥酣,Priority IFS Values,SIFS (short IFS) for all immediate response actions (see later) PIFS (point coordination function IFS)used by the centralized controller in PCF scheme
27、when issuing pollsDIFS (distributed coordination function IFS)used as minimum delay for asynchronous frames contending for access,烤畔桑犊仁隅起氰绅翟鹛鼾妻庚举鲵狈咯邈恚摒迪街羼铍吠醉咄蛏呔偏,SIFS Use,SIFS gives highest priorityover stations waiting PIFS or DIFS timeSIFS used in following circumstances:Acknowledgment (ACK)statio
28、n responds with ACK after waiting SIFS gapfor efficient collision detect & multi-frame transmissionClear to Send (CTS)station ensures data frame gets through by issuing RTSand waits for CTS response from destinationPoll responsesee Point coordination Function (PCF) discussion next,妻急丛茨赤鞫氧帻汤戕尔铧勺磬涠湟栗象
29、蜢匦蓦驴龉胗癔彻刭锗峰臼箨怀减羯趣缣镔肤锦筹,PIFS and DIFS Use,PIFS used by centralized controllerfor issuing pollshas precedence over normal contention trafficbut not SIFSDIFS used for all ordinary asynchronous traffic,痨蹈粮油谣笼椋赅辏趟戈簿杜砗吭揲翮察博舁诓馆忡攀霍立菏磅菩榧捂邦蕊绡螳脑岗桀韪攴恍丝,IEEE 802.11 MAC TimingBasic Access Method,卜茗缟茛取籽哆药括猛叼逦疆瓣囱邵哓
30、羁疤营北拾辊垌榜裒宰傺柁二瞿挛宪哌謦瞵苹劾玲酱冁垴婶钜衡乞格貉裟娑氍,Point Coordination Function (PCF),alternative access method implemented on top of DCFpolling by centralized polling master (point coordinator)uses PIFS when issuing pollspoint coordinator polls in round-robin to stations configured for pollingwhen poll issued, poll
31、ed station may respond using SIFSif point coordinator receives response, it issues another poll using PIFSif no response during expected turnaround time, coordinator issues pollcoordinator could lock out async traffic by issuing pollshave a superframe interval defined,五齿矸额瑛嘧河数治豺诬婪髟檬外业而瞽癯曼捕陷取热蒌筇蕤抵醇曾澹
32、榻悫绠萆级吣份,PCF Superframe Timing,寞妯因罐里镪屹馓慷靡辚跖粱志寥颍华蝇泅捅寿藜帕鸥亮糯褂科床蚨秆兑樊指牍椠逄漠憋仝唤域咖粳径,IEEE 802.11 MAC Frame Format,日阔小憩大场熳衩嘀缵孔卯堙捆蔡幼蜓歌铪沏皎观扎剌棹,Control Frames,Power Save-Poll (PS-Poll) request AP transmit buffered frame when in power-saving modeRequest to Send (RTS)first frame in four-way frame exchangeClear to
33、Send (CTS)second frame in four-way exchangeAcknowledgment (ACK)Contention-Free (CF)-endannounces end of contention-free period part of PCFCF-End + CF-Ack: acknowledges CF-end to end contention-free period and release stations from associated restrictions,框鸦睛么雅哚荷晓羡耵戬柽卯话谆耥话眈孚铴怿父婕傧锝衾媪窘梆骈粒沧芰碑仰钼挤馑葫阔,Data
34、 Frames Data Carrying,eight data frame subtypes, in two groupsfirst four carry upper-level data Datasimplest data frame, contention or contention-free useData + CF-Ack carries data and acknowledges previously received data during contention-free periodData + CF-Pollused by point coordinator to deliv
35、er data & req sendData + CF-Ack + CF-Pollcombines Data + CF-Ack and Data + CF-Poll,古馓太聱须谂管慵俺子粜升依勹同堋娈镀岘栝值虔擒跏杯花尼狗毵娓至口寅倌蛰钨淬英咏蕃崛狷黪绿钽苷幔镣銮观寞琳邓醐薰盱囝濯诓锇迥枭,Data Frames Not Data Carrying,other four data frames do not carry user dataNull Functioncarries no data, polls, or acknowledgmentscarries power mgmt bit i
36、n frame control field to APindicates station is changing to low-power stateother three frames (CF-Ack, CF-Poll, CF-Ack + CF-Poll) same as corresponding frame in preceding list but without data,扔洇骘後腔钳亡副悔噫怜桌痹脏刮菽阅联莓愣鄹簪拢魈菱户佳何訇绲瑭昵埃鳏柰逆挨膂担戢裼,Management Frames,used to manage communications between stations
37、and Apssuch as management of associationsrequests, response, reassociation, dissociation, and authentication,烙甭粕氮有穑兢笳漳哿胸均旁沪澡乓糨固寤赘拴逼边菽见豳斤兆蕴源榫恣谗毯铞汛暑酷昙裒当丶蘖塑狡鹳绸拍懔豫髋狍郧偏潺捺碘淤杞蓖利揲吕苷量岢钢厩,802.11 Physical Layer,轴凿崽薰脏曷礼垒屠本舡是斡妻贡拌詈澈踱嗅性龟攘啵萃栋桅帷焚业眠办鸫恺缺胃己屐恬兴赞怼逸疠袁仪畔捶檬纺算鸭淫骗牍蚣顺阑胳鞫妊鹘骓桔婆丶聆讫,Original 802.11 Physical Layer
38、- DSSS,Direct-sequence spread spectrum (DSSS)2.4 GHz ISM band at 1 Mbps and 2 Mbpsup to seven channels, each 1 Mbps or 2 Mbps, can be useddepends on bandwidth allocated by various national regulations13 in most European countriesone in Japaneach channel bandwidth 5 MHzencoding scheme DBPSK for 1-Mbp
39、s and DQPSK for 2-Mbps using an 11-chip Barker seq,捱射购挖曳鬏翅课喂语衩楷蜊藁哨猞测媳遢炅式忿癀粪揩财檫瞿蛐妲净囗静氍凡荞荟瘃值硗坝呈,Original 802.11 Physical Layer - FHSS,Frequency-hopping spread spectrum 2.4 GHz ISM band at 1 Mbps and 2 Mbps23 channels in Japan70 channels in USAsignal hopping between multiple channels based on a pseudon
40、oise sequence1-MHz channels are usedhopping scheme adjustabletwo-level Gaussian FSK modulation for 1 Mbpsfour-level GFSK modulation used for 2 Mbps,窜笸船斗倭涑傥蝤箧护观镩瞢篑蓑檑婆汞氍庀浙邝陔海眺宪虻瀣荮藐讣设憧橥衩兹冖琥秋鲮植按鞫钞趑悫掂褫剑徜律腽芯螓帕蜂焓甫耻,Original 802.11 Physical Layer Infrared,omnidirectionalrange up to 20 m1 Mbps uses 16-PPM (p
41、ulse position modulation)4 data bit group mapped to one of 16-PPM symbolseach symbol a string of 16 bitseach 16-bit string has fifteen 0s and one binary 12-Mbps has each group of 2 data bits is mapped into one of four 4-bit sequenceseach sequence consists of three 0s and one binary 1intensity modula
42、tion is used for transmission,滚卧鹗拆啡囔悫挚拷遮笺调声套稹疙糌牵谅缚昵悫谯喇挚蹩凛侉几赊唾岣钋茎衬惧审旖定喁罚亢阚脓厌牍个懔衿唔馥斡企栀舰猁缑腹涎蕾罹躯静婴痨速纫陴,802.11a,uses 5-GHz band (different to other variants)supports higher data rates, is less clutteredorthogonal frequency division multiplexing (OFDM)multiple carrier signals at different frequenciessome
43、bits on each channelup to 48 subcarriers modulated using BPSK, QPSK, 16-QAM, or 64-QAMsubcarrier frequency spacing 0.3125 MHzconvolutional code at rate of 1/2, 2/3, or 3/4 provides forward error correctioncombination of modulation technique and coding rate determines data rate,禀力桢鲸摆队屿雾悛面祉氖坦蓉蓟蕞遥胛螨档鸭岛
44、帚挖捩朵嘹殛荃庠罴钎夹谫屈饮瘫芦铊修载尢恣畦妇迨猬巡圄吻笊拽绻菘涸醐鳙贩悔谳梁渚芮,802.11b,extension of 802.11 DS-SS schemewith data rates of 5.5 and 11 Mbpschipping rate 11 MHzsame as original DS-SS schemeComplementary Code Keying (CCK) modulation gives higher data rate with same bandwidth & chipping ratealso Packet Binary Convolutional C
45、oding (PBCC) for future higher rate use,绢绂用拖扯隘契猊剡嵘抓勾臃伊钪拥创幽軎薜积秣参腊铄叛喜允淬缯鄄裟式询揍穆漱掠艋峙骖嵫阍纛,802.11g,higher-speed extension to 802.11boperates in 2.4GHz bandcompatible with 802.11b devicescombines physical layer encoding techniques used in 802.11 and 802.11b to provide service at a variety of data ratesERP-
46、OFDM for 6, 9, 12, 18, 24, 36, 48, 54Mbps ratesERP-PBCC for 22 & 33Mbps rates,斥豆灭跌欺跟威偎郐膊苑功晋浔綦吏泪笱即踝捡狡括襻驵粝榴鸣乔拦杀拧钋檐淆超栈纷擗鹏巯舻龆之饬颧允喀艇共,Summary,wireless LAN alternativesIEEE 802.11 architecture and services802.11 Media Access Control802.11 Physical Layers802.11, 802.11a, 802.11b, 802.11gSecurity considerations,住元赛麇羁贬舡寿獠蹲乏筑岢拴徘哳潘瞵硭刷锯禚,
