數(shù)據(jù)與計算機通信課件17wirelesslans

1、William StallingsData and Computer Communications7th EditionChapter 17 Wireless LANs內(nèi)容 17.1 Overview 17.2 Wireless LAN Technology 17.3 IEEE 802.11 Architecture and Services 17.4 IEEE 802.11 Medium Access Control 17.5 IEEE 802.11 Physical Layer17.1 Overview A wireless LAN uses wireless transmission m

2、edium Used to have high prices, low data rates, occupational safety concerns, and licensing requirements Problems have been addressed Popularity of wireless LANs has grown rapidlyApplications - LAN Extension Saves installation of LAN cabling Eases relocation and other modifications to network struct

3、ure However, increasing reliance(信任) on twisted pair cabling for LANs Most older buildings already wired with Cat 3 cable Newer buildings are prewired with Cat 5 Wireless LAN to replace wired LANs has not happened In some environments, role for the wireless LAN Buildings with large open areas Manufa

4、cturing plants, stock exchange trading floors, warehouses Historical buildings Small offices where wired LANs not economical May also have wired LAN Servers and stationary workstationsSingle Cell Wireless LAN ConfigurationMulti-Cell Wireless LAN ConfigurationApplications Cross-Building Interconnect

5、Connect LANs in nearby buildings Point-to-point wireless link Connect bridges or routers Not a LAN per se(本質上)Usual to include this application under heading of wireless LAN Applications Nomadic(漫游) Access Link between LAN hub and mobile data terminal Laptop（膝上型電腦） or notepad computerEnable employee

6、 returning from trip to transfer data from portable computer to server Also useful in extended environment such as campus or cluster of buildingsUsers move around with portable computersMay wish access to servers on wired LANInfrastructure Wireless LANApplications Ad Hoc Networking Peer-to-peer netw

7、ork Set up temporarily to meet some immediate need E.g. group of employees, each with laptop or palmtop, in business or classroom meeting Network for duration of meeting -Ad Hoc的意思是“for this”引申為“for this purpose only”，即“為某種目的設置的，特別的”意思，即Ad hoc網(wǎng)絡是一種有特殊用途的網(wǎng)絡。 Ad Hoc LANAd Hoc LAN Ad hoc網(wǎng)絡的前身是分組無線網(wǎng)(Pac

8、ket Radio Network) 1972年，美國DARPA就啟動了分組無線網(wǎng)項目PRNET，研究在戰(zhàn)場環(huán)境下利用分組無線網(wǎng)進行數(shù)據(jù)通信。 DARPA于1983年啟動了高殘存性自適應網(wǎng)絡項目SURAN（Survivable Adaptive Network），研究如何將PRNET的研究成果加以擴展，以支持更大規(guī)模的網(wǎng)絡。 1994年，DARPA又啟動了全球移動信息系統(tǒng)GloMo(Globle Mobile Information Systems)項目，旨在對能夠滿足軍事應用需要的、可快速展開、高抗毀性的移動信息系統(tǒng)進行全面深入的研究。 1991年5月IEEE成立802.11標準委員會采用了

9、“Ad hoc網(wǎng)絡”一詞來描述這種特殊的自組織對等式多跳移動通信網(wǎng)絡，Ad hoc網(wǎng)絡就此誕生。 IETF也將Ad hoc網(wǎng)絡稱為MANET(移動Ad hoc網(wǎng)絡) Wireless LAN RequirementsSame as any LAN High capacity, short distances, full connectivity, broadcast capabilityThroughput: efficient use wireless mediumNumber of nodes:Hundreds of nodes across multiple cellsConnectio

10、n to backbone LAN: Use control modules to connect to both types of LANsService area: 100 to 300 mLow power consumption:Need long battery life on mobile stations Mustnt require nodes to monitor access points or frequent handshakesTransmission robustness and security:Interference prone and easily eave

11、sdroppedCollocated network operation:Two or more wireless LANs in same areaLicense-free operationHandoff/roaming(漫游): Move from one cell to anotherDynamic configuration: Addition, deletion, and relocation of end systems without disruption to users17.2 Wireless Technology Infrared (IR) LANs: Individu

12、al cell of IR LAN limited to single roomIR light does not penetrate opaque walls Spread spectrum LANs: Mostly operate in ISM (industrial, scientific, and medical) bandsNo Federal Communications Commission (FCC) licensing is required in USA Narrowband microwave: Microwave frequencies but not use spre

13、ad spectrumSome require FCC licensingInfrared LANsStrengths and WeaknessesSpectrum virtually unlimited (光譜事實上沒有限制) Infrared spectrum is unregulated worldwide(紅外光譜全球均沒有收到限制) Extremely high data ratesInfrared shares some properties of visible light(紅外線共享了可見光的某些性質) Diffusely reflected(漫反射) by light-col

14、ored(淺色) objects Use ceiling reflection to cover entire room Does not penetrate(穿透) walls or other opaque (不透明) objects More easily secured against eavesdropping than microwave Separate installation in every room without interferenceInexpensive and simple(成本低且簡單) Uses intensity modulation(強度調制), so

15、receivers need to detect only amplitude(幅度)Background radiation(輻射) Sunlight, indoor lighting Noise, requiring higher power and limiting range Power limited by concerns of eye safety and power consumptionInfrared LANsTransmission Techniques Directed-beam IR(直接紅外線) Point-to-point links Range depends

16、on power and focusing Can be kilometers Used for building interconnect within line of sight Indoor use to set up token ring LAN IR transceivers positioned so that data circulate in ring Omnidirectional(全向) Single base station within line of sight of all other stations Typically, mounted on ceiling A

17、cts as a multiport repeater Other transceivers use directional beam aimed at ceiling unit Diffused(漫射) configuration Transmitters are focused and aimed at diffusely reflecting ceilingSpread Spectrum LANsHub Configuration Usually use multiple-cell arrangement Adjacent cells use different center frequ

18、encies Hub is typically mounted on ceiling (天花板) Connected to wired LAN Connect to stations attached to wired LAN and in other cells May also control access IEEE 802.11 point coordination function May also act as multiport repeater Stations transmit to hub and receive from hub Stations may broadcast

19、 using an omnidirectional antenna Logical bus configuration Hub may do automatic handoff Weakening signal, hand offSpread Spectrum LANsPeer-to-Peer Configuration No hub MAC algorithm such as CSMA used to control access Ad hoc LANs Spread Spectrum LANsTransmission Issues Licensing regulations differ

20、from one country to another USA FCC authorized two unlicensed applications within the ISM(Industrial Scientific Medical) band: Spread spectrum - up to 1 watt Very low power systems- up to 0.5 watts 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

21、also in Europe and Japan Higher frequency means higher potential bandwidth Interference Devices at around 900 MHz, including cordless telephones(無繩電話), wireless microphones, and amateur radio(業(yè)余無線電) Fewer devices at 2.4 GHz; microwave oven Little competition(競爭) at 5.8 GHz Higher frequency band, mor

22、e expensive equipmentNarrow Band Microwave LANs Just wide enough to accommodate(容納) signal Until recently, all products used licensed band At least one vendor has produced LAN product in ISM band ISM(Industrial Scientific Medical) Band，此頻段( 2.42.4835GHz)主要是開放給工業(yè)，科學、醫(yī)學，三個主要機構使用，該頻段是依據(jù)美國聯(lián)邦通訊委員會(FCC)所定

23、義出來，屬于Free License，并沒有所謂使用授權的限制。 Licensed Narrowband RF(需要許可證的窄帶無線電頻率)Microwave frequencies usable for voice, data, and video licensed within specific geographic areas to avoid interference Radium 28 km Can contain five licenses Each covering two frequencies Motorola holds 600 licenses (1200 frequen

24、cies) in the 18-GHz range Cover all metropolitan areas with populations of 30,000 or more in USAUse of cell configurationAdjacent cells use nonoverlapping(非交疊) frequency bands Motorola controls frequency band Can assure nearby independent LANs do not interfereAll transmissions are encryptedLicensed

25、narrowband LAN guarantees interference-free communicationLicense holder has legal right tointerference-free data channelUnlicensed Narrowband RF (不需要許可證的窄帶無線電頻率) 1995, RadioLAN introduced narrowband wireless LAN using unlicensed ISM spectrum Used for narrowband transmission at low power 0.5 watts or

26、 less Operates at 10 Mbps 5.8-GHz band 50 m in semiopen office and 100 m in open office Peer-to-peer configuration Elects(推選) one node as dynamic master Based on location, interference, and signal strength Master can change automatically as conditions change Includes dynamic relay(中繼) function Stati

27、ons can act as repeater to move data between stations that are out of range of each other17.3 IEEE 802.11 - BSS MAC protocol and physical medium specification for wireless LANs Smallest building block is basic service set (BSS) Number of stations Same MAC protocol Competing for access to same shared

28、 wireless medium May be isolated or connect to backbone distribution system (DS) through access point (AP) AP functions as bridge MAC protocol may be distributed or controlled by central coordination function in AP BSS generally corresponds to cell DS can be switch, wired network, or wireless networ

29、kIEEE 802.11 ArchitectureBSS Configuration Simplest: each station belongs to single BSSWithin range only of other stations within BSS Can have two BSSs overlapStation could participate in more than one BSS Association between station and BSS dynamicStations may turn off, come within range, and go ou

30、t of rangeExtended Service Set (ESS) Two or more BSS interconnected by DSTypically, DS is wired backbone but can be any network Appears as single logical LAN to LLCAccess Point (AP) Logic within station that provides access to DSProvides DS services in addition to acting as station To integrate IEEE

31、 802.11 architecture with wired LAN, portal used Portal logic implemented in device that is part of wired LAN and attached to DSE.g. Bridge or routerServicesServiceProviderCategoryAssociation Distribution systemMSDU(MAC Service Data Units) delivery Authentication StationLAN access and security Deaut

32、hentication StationLAN access and security Dissassociation Distribution systemMSDU delivery Distribution Distribution systemMSDU delivery Integration Distribution systemMSDU delivery MSDU deliveryStationMSDU delivery PrivacyStationLAN access and security Reassocation Distribution systemMSDU delivery

33、 Categorizing Services Station services implemented in every 802.11 station Including AP stations Distribution services provided between BSSs May be implemented in AP or special-purpose device Three services used to control access and confidentiality Six services used to support delivery of MAC serv

34、ice data units (MSDUs) between stations Block of data passed down from MAC user to MAC layer Typically LLC PDU If MSDU too large for MAC frame, fragment and transmit in series of frames (see later)Distribution of Messages Within a DS Distribution is primary service used by stations to exchange MAC f

35、rames when frame must traverse DS From station in one BSS to station in another BSS Transport of message through DS is beyond scope of 802.11 If stations within same BSS, distribution service logically goes through single AP of that BSS Integration service enables transfer of data between station on

36、 802.11 LAN and one on an integrated 802.x LAN Integrated refers to wired LAN physically connected to DS Stations may be logically connected to 802.11 LAN via integration service Integration service takes care of address translation and media conversionAssociation Related Services Purpose of MAC lay

37、er transfer MSDUs between MAC entities Fulfilled by distribution service (DS) DS requires information about stations within ESS Provided by association-related services Station must be associated before communicating Three transition(轉換) types of based on mobility No transition: Stationary or moves

38、within range of single BSS BSS transition: From one BSS to another within same ESS Requires addressing capability be able to recognize new location ESS transition: From BSS in one ESS to BSS in another ESS Only supported in sense that the station can move Maintenance of upper-layer connections not g

39、uaranteed Disruption of service likelyStation Location DS needs to know where destination station is Identity of AP to which message should be delivered Station must maintain association with AP within current BSS Three services relate to this requirement: Association: Establishes initial associatio

40、n between station and AP To make identity and address known Station must establish association with AP within particular BSS AP then communicates information to other APs within ESS Reassociation: Transfer established association to another AP Allows station to move from one BSS to another Disassoci

41、ation: From either station or AP that association is terminated Given before station leaves ESS or shuts MAC management facility protects itself against stations that disappear without notificationAccess and Privacy Services - AuthenticationOn wireless LAN, any station within radio range other devic

42、es can transmitAny station within radio range can receiveAuthentication(認證,鑒權): Used to establish identity of stations to each other Wired LANs assume access to physical connection conveys authority to connect to LAN Not valid assumption for wireless LANs Connectivity achieved by having properly tun

43、ed antenna Authentication service used to establish station identity 802.11 supports several authentication schemes Allows expansion of these schemes Does not mandate() any particular scheme Range from relatively insecure handshaking to public-key encryption schemes 802.11 requires mutually acceptab

44、le, successful authentication before associationAccess and Privacy Services - Deauthentication and Privacy Deauthentication: Invoked whenever an existing authentication is to be terminated Privacy: Used to prevent messages being read by others 802.11 provides for optional use of encryption17.4 Mediu

45、m Access Control MAC layer covers three functional areas Reliable data delivery Access control SecurityBeyond our scopeReliable Data Delivery 802.11 physical and MAC layers subject to unreliability Noise, interference, and other propagation effects result in loss of frames Even with error-correction

46、 codes, frames may not successfully be received Can be dealt with at a higher layer, such as TCP However, retransmission timers at higher layers typically order of seconds More efficient to deal with errors at the MAC level 802.11 includes frame exchange protocol Station receiving frame returns ackn

47、owledgment (ACK) frame Exchange treated as atomic unit Not interrupted by any other station If noACK within short period of time, retransmitFour Frame Exchange Basic data transfer involves exchange of two frames To further enhance reliability, four-frame exchange may be used Source issues a Request

48、to Send (RTS) frame to destination Destination responds with Clear to Send (CTS) After receiving CTS, source transmits data Destination responds with ACK RTS alerts all stations within range of source that exchange is under way CTS alerts all stations within range of destination Stations refrain fro

49、m transmission to avoid collision RTS/CTS exchange is required function of MAC but may be disabled802.11 局域網(wǎng)的 MAC 層協(xié)議1. CSMA/CA 協(xié)議 無線局域網(wǎng)卻不能簡單地搬用 CSMA/CD 協(xié)議。這里主要有兩個原因。 CSMA/CD 協(xié)議要求一個站點在發(fā)送本站數(shù)據(jù)的同時，還必須不間斷地檢測信道，但在無線局域網(wǎng)的設備中要實現(xiàn)這種功能就花費過大。 即使我們能夠實現(xiàn)碰撞檢測的功能，并且當我們在發(fā)送數(shù)據(jù)時檢測到信道是空閑的，在接收端仍然有可能發(fā)生碰撞。 無線局域網(wǎng)的特殊問題 當 A 和

50、C 檢測不到無線信號時，都以為 B 是空閑的，因而都向 B 發(fā)送數(shù)據(jù)，結果發(fā)生碰撞。這種未能檢測出媒體上已存在的信號的問題叫做隱蔽站問題(hidden station problem) A 的作用范圍C 的作用范圍ABCD無線局域網(wǎng)的特殊問題 B 向 A 發(fā)送數(shù)據(jù)，而 C 又想和 D 通信。C 檢測到媒體上有信號，于是就不敢向 D 發(fā)送數(shù)據(jù)。 其實 B 向 A 發(fā)送數(shù)據(jù)并不影響 C 向 D 發(fā)送數(shù)據(jù)這就是暴露站問題(exposed station problem) ADCB？B 的作用范圍C 的作用范圍CSMA/CA 協(xié)議 無線局域網(wǎng)不能使用 CSMA/CD，而只能使用改進的 CSMA 協(xié)議。

51、 改進的辦法是把 CSMA 增加一個碰撞避免(Collision Avoidance)功能。 802.11 就使用 CSMA/CA 協(xié)議。而在使用 CSMA/CA 的同時，還增加使用停止等待協(xié)議。 下面先介紹 802.11 的 MAC 層。 Media Access Control Distributed wireless foundation MAC (DWFMAC)Distributed access control mechanismOptional centralized control on top Lower sublayer is distributed coordination

52、 function (DCF)Contention algorithm to provide access to all trafficAsynchronous traffic Point coordination function (PCF)Centralized MAC algorithmContention freeBuilt on top of DCFIEEE 802.11 Protocol ArchitectureDistributed Coordination Function DCF sublayer uses a simple CSMA If station has frame

53、 to transmit, it listens to medium If medium idle, station may transmit Otherwise must wait until current transmission complete No collision detection Because collision detection is practical Not practical on wireless network Dynamic range of signals very large Transmitting station cannot distinguis

54、h incoming weak signals from noise and effects of own transmission DCF includes delays Amounts to priority scheme(等價于優(yōu)先級策略的時延) Interframe space(幀間間隔)Interframe SpaceSingle delay known as interframe space (IFS)Using IFS, rules for CSMA:1. Station with frame senses mediumIf idle, wait to see if remain

55、s idle for one IFS. If so, may transmit immediately2. If busy (either initially or becomes busy during IFS) station defers transmissionContinue to monitor until current transmission is over3. Once current transmission over, delay another IFSIf remains idle, back off random time and again senseIf med

56、ium still idle, station may transmitDuring backoff time, if becomes busy, backoff timer is halted and resumes when medium becomes idleTo ensure stability, binary exponential backoff usedIEEE 802.11 Medium Access Control LogicPriority Use three values for IFS SIFS (short IFS): Shortest IFS For all im

57、mediate response actions (see later) PIFS (point coordination function IFS): Midlength IFS Used by the centralized controller in PCF scheme when issuing polls DIFS (distributed coordination function IFS): Longest IFS Used as minimum delay for asynchronous frames contending for access幀間間隔 IFS 所有的站在完成

58、發(fā)送后，必須再等待一段很短的時間（繼續(xù)監(jiān)聽）才能發(fā)送下一幀。這段時間的通稱是幀間間隔 IFS (InterFrame Space)。 幀間間隔長度取決于該站欲發(fā)送的幀的類型。高優(yōu)先級幀需要等待的時間較短，因此可優(yōu)先獲得發(fā)送權。 若低優(yōu)先級幀還沒來得及發(fā)送而其他站的高優(yōu)先級幀已發(fā)送到媒體，則媒體變?yōu)槊B(tài)因而低優(yōu)先級幀就只能再推遲發(fā)送了。這樣就減少了發(fā)生碰撞的機會。 SIFS Use - ACK Station using SIFS to determine transmission opportunity has highest priority In preference to station

59、 waiting PIFS or DIFS time SIFS used in following circumstances: Acknowledgment (ACK): Station responds with ACK after waiting SIFS gap No collision detection so likelihood of collisions greater than CSMA/CD MAC-level ACK gives efficient collision recovery SIFS provide efficient delivery of multiple

60、 frame LLC PDU Station with multiframe LLC PDU to transmit sends out MAC frames one at a time Each frame acknowledged after SIFS by recipient When source receives ACK, immediately (after SIFS) sends next frame in sequence Once station has contended for channel, it maintains control of all fragments