数据与计算机通信课件17wirelesslans

1、William StallingsData and Computer Communications7th EditionChapter 17 Wireless LANs内容 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(本质上)Usual to include this application under heading of wireless LAN Applications Nomadic(漫游) Access Link between LAN hub and mobile data terminal Laptop（膝上型电脑） 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”引申为“for this purpose only”，即“为某种目的设置的，特别的”意思，即Ad hoc网络是一种有特殊用途的网络。 Ad Hoc LANAd Hoc LAN Ad hoc网络的前身是分组无线网(Pac

8、ket Radio Network) 1972年，美国DARPA就启动了分组无线网项目PRNET，研究在战场环境下利用分组无线网进行数据通信。 DARPA于1983年启动了高残存性自适应网络项目SURAN（Survivable Adaptive Network），研究如何将PRNET的研究成果加以扩展，以支持更大规模的网络。 1994年，DARPA又启动了全球移动信息系统GloMo(Globle Mobile Information Systems)项目，旨在对能够满足军事应用需要的、可快速展开、高抗毁性的移动信息系统进行全面深入的研究。 1991年5月IEEE成立802.11标准委员会采用了

9、“Ad hoc网络”一词来描述这种特殊的自组织对等式多跳移动通信网络，Ad hoc网络就此诞生。 IETF也将Ad hoc网络称为MANET(移动Ad hoc网络) 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 (光谱事实上没有限制) Infrared spectrum is unregulated worldwide(红外光谱全球均没有收到限制) Extremely high data ratesInfrared shares some properties of visible light(红外线共享了可见光的某些性质) Diffusely reflected(漫反射) by light-col

14、ored(浅色) 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(成本低且简单) Uses intensity modulation(强度调制), so

15、receivers need to detect only amplitude(幅度)Background radiation(辐射) 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(直接红外线) 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(无绳电话), wireless microphones, and amateur radio(业余无线电) Fewer devices at 2.4 GHz; microwave oven Little competition(竞争) at 5.8 GHz Higher frequency band, mor

22、e expensive equipmentNarrow Band Microwave LANs Just wide enough to accommodate(容纳) signal Until recently, all products used licensed band At least one vendor has produced LAN product in ISM band ISM(Industrial Scientific Medical) Band，此频段( 2.42.4835GHz)主要是开放给工业，科学、医学，三个主要机构使用，该频段是依据美国联邦通讯委员会(FCC)所定

23、义出来，属于Free License，并没有所谓使用授权的限制。 Licensed Narrowband RF(需要许可证的窄带无线电频率)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(非交叠) 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 (不需要许可证的窄带无线电频率) 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(推选) one node as dynamic master Based on location, interference, and signal strength Master can change automatically as conditions change Includes dynamic relay(中继) 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(转换) 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(认证,鉴权): 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 局域网的 MAC 层协议1. CSMA/CA 协议 无线局域网却不能简单地搬用 CSMA/CD 协议。这里主要有两个原因。 CSMA/CD 协议要求一个站点在发送本站数据的同时，还必须不间断地检测信道，但在无线局域网的设备中要实现这种功能就花费过大。 即使我们能够实现碰撞检测的功能，并且当我们在发送数据时检测到信道是空闲的，在接收端仍然有可能发生碰撞。 无线局域网的特殊问题 当 A 和

50、C 检测不到无线信号时，都以为 B 是空闲的，因而都向 B 发送数据，结果发生碰撞。这种未能检测出媒体上已存在的信号的问题叫做隐蔽站问题(hidden station problem) A 的作用范围C 的作用范围ABCD无线局域网的特殊问题 B 向 A 发送数据，而 C 又想和 D 通信。C 检测到媒体上有信号，于是就不敢向 D 发送数据。 其实 B 向 A 发送数据并不影响 C 向 D 发送数据这就是暴露站问题(exposed station problem) ADCB？B 的作用范围C 的作用范围CSMA/CA 协议 无线局域网不能使用 CSMA/CD，而只能使用改进的 CSMA 协议。

51、 改进的办法是把 CSMA 增加一个碰撞避免(Collision Avoidance)功能。 802.11 就使用 CSMA/CA 协议。而在使用 CSMA/CA 的同时，还增加使用停止等待协议。 下面先介绍 802.11 的 MAC 层。 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(等价于优先级策略的时延) Interframe space(帧间间隔)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帧间间隔 IFS 所有的站在完成

58、发送后，必须再等待一段很短的时间（继续监听）才能发送下一帧。这段时间的通称是帧间间隔 IFS (InterFrame Space)。 帧间间隔长度取决于该站欲发送的帧的类型。高优先级帧需要等待的时间较短，因此可优先获得发送权。 若低优先级帧还没来得及发送而其他站的高优先级帧已发送到媒体，则媒体变为忙态因而低优先级帧就只能再推迟发送了。这样就减少了发生碰撞的机会。 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