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1、4 7 无线局域网求应前4.7 无线局域网 需求及应用前景 优势优势 工作模式 Ad Hoc Infrastructure 要解决的问题 技术经济政治环保健康安全 技术、经济、政治、环保、健康、安全有线通信有线通信优势 优势 Very reliabley Very high transfer rates Long distance Long distance 限制 Expensive to set up infrastructure Infrastructure is fixed once set upInfrastructure is fixed once set up No mobilit
2、y无线通信无线通信优势 优势 Allows mobility Much cheaper and easier to deploy, change, and upgrade! 限制 Exposed (unshielded) mediumExposed (unshielded) mediumSusceptible to physical phenomena (interference) Variable BER - Error correction may not suffice in all cases Slower data rates for wider distances OSI laye
3、red stack designed for wired medium Security: anyone in range hears transmission无线通信标准无线通信标准 Cellular 2G: GSM , CDMA 3G: CDMA2000，WCDMA， TD-SCDMA IEEE 802.11 5 0Gh bd 54Mb(25 Mbtit ) a: 5.0Ghz band, 54Mbps (25 Mbps operating rate) b: 2.4Ghz band, 11Mbps (4.5 Mbps operating rate) g: 2.4Ghz, 54Mbps (1
4、9 Mbps operating rate) Other versions to come （802 11n）Other versions to come （802.11n） IEEE 802.15 lower power wireless 802.15.1: 2.4Ghz, 2.1 Mbps (Bluetooth) 802.15.4: 2.4Ghz, 250 Kbps (Sensor Networks)Wireless Link CharacteristicsWireless Link CharacteristicsOther Wireless Link Characteristics Pa
5、th loss Silifif di Signal attenuation as a function of distance SNR decreases, make signal unrecoverable Multipath Propagation Signal reflects off surfaces effectively causing self Signal reflects off surfaces, effectively causing self interference Iffh Interference from other sources Internal Inter
6、ference (Hosts within range of each (g other collide with one anothers transmission ) External Interference ( Microwave is turned onExternal Interference ( Microwave is turned on and blocks your signal)A Wireless Link?A Wireless Link?A Wireless Link!A Wireless Link!Wireless Bit ErrorsWireless Bit Er
7、rors The lower the SNR (Signal/Noise) the higherThe lower the SNR (Signal/Noise) the higher the Bit Error Rate (BER) How can we deal with this? Make the signal strongergg Why is this not always a good idea? Id il tthi Increased signal strength requires more power Increases the interference range of
8、the sender, so you interfere with more nodes around you Error Correction schemes can correct someError Correction schemes can correct some problems802 11 Architecture802.11 Architecture802 11 f802.11 frames exchanges工作模式工作模式IEEE 802 11 IEEE 802.11Basic Service Access Point Setstationstationstationst
9、ationstationstationBSS: consisting of some number of stationsBSS: consisting of some number of stations executing the same MAC protocol and competing for access to the same shared mediumESSESSExtended Service SetDistributed SystemyBasic Service SetServer Basic Service Set802 11协议栈802.11协议栈上面各层逻辑链路控制
10、上面各层逻辑链路控制 数据 链路链路 层MAC 子层CSMA/CA802.11红 外线红 外线802.11 FHSS802.11 DSSS802.11a OFDM802.11b HR- DSSS802.11g OFDM物理层MAC层协议 Why not just use Ethernet algorithms? Ethernet: one shared “collision” domain 与以太网环境相比具有一定复杂性 与以太网环境相比，具有定复杂性 technically difficult to detect collisions Collisions are at receiver,
11、not sender even if we could, it wouldnt work Different transmitters have different coverage areas In addition, wireless links are much more prone to ,p loss than wired links Carrier Sense (CSMA) is OK; detection (CD) is Carrier Sense (CSMA) is OK; detection (CD) is notHidden TerminalsHidden Terminal
12、sABC A and C can both send to B but cant hear each othereach other A is a hidden terminal for C and vice versa CSMA/CD will be ineffective need to sense at receiversense at receiverExposed TerminalsExposed TerminalsABCD Exposed node: B sends a packet to A; CExposed node: B sends a packet to A; C hea
13、rs this and decides not to send a packet to D (despite the fact that thispacket to D (despite the fact that this will not cause interference)! )CSMA/CA (CSMA with Collision Avoidance) Since we cant detect collisions, we try to avoid them When medium busy, choose random interval (contention window)in
14、terval (contention window) When a collision is inferred, retransmit with binary exponential backoff (like Ethernet)Ethernet) Use ACK from receiver to infer “no collision” Uti l bk ff tdttti Use exponential backoff to adapt contention windowMACA (Multiple Acess with Collision Avoidance)基本思想 基本思想： 发送方
15、刺激一下接收方，让它输出一短帧，接收方 附近的站可以检测到该帧从而在接下去的数据附近的站可以检测到该帧，从而在接下去的数据 帧传输过程中就不再发送数据了。 方法 方法： 发送方发送RTS(Request To Send)帧 接收方发送CTS(ClTSd)帧 接收方发送CTS(Clear To Send)帧 其他站：虚拟信道机制，NAV信号 发送方未听到CTS执行退避算法 发送方未听到CTS，执行退避算法MACAW(MACA for Wireless)MACAW(MACA for Wireless)帧 引入ACK帧 引入载波检测引入载波检测 为每个单独的数据流运行后退算法 提供了各站间交换拥塞信
16、息的机制分片机制分片机制 问题： 无线网络有噪声，因而不可靠无线网络有噪声，因而不可靠 一帧成功传送的概率随帧长度的增加而减 少少 方法：分片传送方法：分片传送IEEE 802 11 protocol architectureIEEE 802.11 protocol architecture DCF uses a contention algorithm CttiContention-free to provide access to all traffic Point coordinationContentionserviceservice PCF is a centralized MAC algorithm used P
