计算机网络 自顶向下方法 英文.pptx

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1、6-1Chapter 6:Wireless and Mobile NetworksBackground:#wireless(mobile)phone subscribers now exceeds#wired phone subscribers!computer nets:laptops,palmtops,PDAs,Internet-enabled phone promise anytime untethered Internet accesstwo important(but different)challengescommunication over wireless linkhandli

2、ng mobile user who changes point of attachment to network第1页/共62页6-2Chapter 6 outline6.1 Introduction Wireless6.2 Wireless links,characteristicsCDMA6.3 IEEE 802.11 wireless LANs(“wi-fi”)6.4 Cellular Internet Accessarchitecturestandards(e.g.,GSM)Mobility6.5 Principles:addressing and routing to mobile

3、 users6.6 Mobile IP6.7 Handling mobility in cellular networks6.8 Mobility and higher-layer protocols6.9 Summary第2页/共62页6-3Elements of a wireless networknetwork infrastructurewireless hostsrlaptop,PDA,IP phonerrun applicationsrmay be stationary(non-mobile)or mobilemwireless does not always mean mobil

4、ity第3页/共62页6-4Elements of a wireless networknetwork infrastructure base stationrtypically connected to wired networkrrelay-responsible for sending packets between wired network and wireless host(s)in its“area”me.g.,cell towers 802.11 access points 第4页/共62页6-5Elements of a wireless networknetwork inf

5、rastructure wireless linkrtypically used to connect mobile(s)to base stationralso used as backbone link rmultiple access protocol coordinates link access rvarious data rates,transmission distance第5页/共62页6-6Characteristics of selected wireless link standards384 Kbps56 Kbps54 Mbps5-11 Mbps1 Mbps802.15

6、802.11b802.11a,gIS-95 CDMA,GSMUMTS/WCDMA,CDMA2000.11 p-to-p link2G3GIndoor10 30mOutdoor50 200mMid rangeoutdoor200m 4KmLong rangeoutdoor5Km 20Km第6页/共62页6-7Elements of a wireless networknetwork infrastructure infrastructure moderbase station connects mobiles into wired networkrhandoff:mobile changes b

7、ase station providing connection into wired network第7页/共62页6-8Elements of a wireless networkAd hoc moderno base stationsrnodes can only transmit to other nodes within link coveragernodes organize themselves into a network:route among themselves第8页/共62页6-9Wireless Link CharacteristicsDifferences from

8、 wired link.decreased signal strength:radio signal attenuates as it propagates through matter(path loss)interference from other sources:standardized wireless network frequencies(e.g.,2.4 GHz)shared by other devices(e.g.,phone);devices(motors)interfere as wellmultipath propagation:radio signal reflec

9、ts off objects ground,arriving ad destination at slightly different times.make communication across(even a point to point)wireless link much more“difficult”第9页/共62页6-10Wireless network characteristicsMultiple wireless senders and receivers create additional problems(beyond multiple access):ABCHidden

10、 terminal problemrB,A hear each otherrB,C hear each otherrA,C can not hear each othermeans A,C unaware of their interference at BABCAs signalstrengthspaceCs signalstrengthSignal fading:rB,A hear each otherrB,C hear each otherrA,C can not hear each other interferring at B第10页/共62页6-11Code Division Mu

11、ltiple Access(CDMA)used in several wireless broadcast channels(cellular,satellite,etc)standardsunique“code”assigned to each user;i.e.,code set partitioningall users share same frequency,but each user has own“chipping”sequence(i.e.,code)to encode dataencoded signal=(original data)X(chipping sequence)

12、decoding:inner-product of encoded signal and chipping sequenceallows multiple users to“coexist”and transmit simultaneously with minimal interference(if codes are“orthogonal”)第11页/共62页6-12CDMA Encode/Decodeslot 1slot 0d1=-11 1 111-1-1-1-Zi,m=di.cmd0=11 1 111-1-1-1-1 1 111-1-1-1-1 1 111-1-1-1-slot 0ch

13、anneloutputslot 1channeloutputchannel output Zi,msendercodedatabitsslot 1slot 0d1=-1d0=11 1 111-1-1-1-1 1 111-1-1-1-1 1 111-1-1-1-1 1 111-1-1-1-slot 0channeloutputslot 1channeloutputreceivercodereceivedinputDi=S Zi,m.cmm=1MM第12页/共62页6-13CDMA:two-sender interference第13页/共62页6-14Chapter 6 outline6.1 I

14、ntroduction Wireless6.2 Wireless links,characteristicsCDMA6.3 IEEE 802.11 wireless LANs(“wi-fi”)6.4 Cellular Internet Accessarchitecturestandards(e.g.,GSM)Mobility6.5 Principles:addressing and routing to mobile users6.6 Mobile IP6.7 Handling mobility in cellular networks6.8 Mobility and higher-layer

15、 protocols6.9 Summary第14页/共62页6-15IEEE 802.11 Wireless LAN802.11b2.4-5 GHz unlicensed radio spectrumup to 11 Mbpsdirect sequence spread spectrum(DSSS)in physical layerall hosts use same chipping codewidely deployed,using base stations802.11a 5-6 GHz rangeup to 54 Mbps802.11g 2.4-5 GHz rangeup to 54

16、MbpsAll use CSMA/CA for multiple accessAll have base-station and ad-hoc network versions第15页/共62页6-16802.11 LAN architecturerwireless host communicates with base stationmbase station=access point(AP)rBasic Service Set(BSS)(aka“cell”)in infrastructure mode contains:mwireless hostsmaccess point(AP):ba

17、se stationmad hoc mode:hosts onlyBSS 1BSS 2Internethub,switchor routerAPAP第16页/共62页6-17802.11:Channels,association802.11b:2.4GHz-2.485GHz spectrum divided into 11 channels at different frequenciesAP admin chooses frequency for APinterference possible:channel can be same as that chosen by neighboring

18、 AP!host:must associate with an APscans channels,listening for beacon frames containing APs name(SSID)and MAC addressselects AP to associate withmay perform authentication Chapter 8will typically run DHCP to get IP address in APs subnet第17页/共62页6-18IEEE 802.11:multiple accessavoid collisions:2+nodes

19、 transmitting at same time802.11:CSMA-sense before transmittingdont collide with ongoing transmission by other node802.11:no collision detection!difficult to receive(sense collisions)when transmitting due to weak received signals(fading)cant sense all collisions in any case:hidden terminal,fadinggoa

20、l:avoid collisions:CSMA/C(ollision)A(voidance)ABCABCAs signalstrengthspaceCs signalstrength第18页/共62页6-19IEEE 802.11 MAC Protocol:CSMA/CA802.11 sender1 if sense channel idle for DIFS then transmit entire frame(no CD)2 if sense channel busy then start random backoff timetimer counts down while channel

21、 idletransmit when timer expiresif no ACK,increase random backoff interval,repeat 2802.11 receiver-if frame received OK return ACK after SIFS(ACK needed due to hidden terminal problem)senderreceiverDIFSdataSIFSACK第19页/共62页6-20Avoiding collisions(more)idea:allow sender to“reserve”channel rather than

22、random access of data frames:avoid collisions of long data framessender first transmits small request-to-send(RTS)packets to BS using CSMARTSs may still collide with each other(but theyre short)BS broadcasts clear-to-send CTS in response to RTSRTS heard by all nodessender transmits data frameother s

23、tations defer transmissions Avoid data frame collisions completely using small reservation packets!第20页/共62页6-21Collision Avoidance:RTS-CTS exchangeAPABtimeRTS(A)RTS(B)RTS(A)CTS(A)CTS(A)DATA(A)ACK(A)ACK(A)reservation collisiondefer第21页/共62页6-22framecontroldurationaddress1address2address4address3payl

24、oadCRC22666260-23124seqcontrol802.11 frame:addressingAddress 2:MAC addressof wireless host or AP transmitting this frameAddress 1:MAC addressof wireless host or AP to receive this frameAddress 3:MAC addressof router interface to which AP is attachedAddress 4:used only in ad hoc mode第22页/共62页6-23Inte

25、rnetrouterAPH1R1AP MAC addr H1 MAC addr R1 MAC addraddress 1address 2address 3802.11 frameR1 MAC addr AP MAC addr dest.address source address 802.3 frame802.11 frame:addressing第23页/共62页6-24framecontroldurationaddress1address2address4address3payloadCRC22666260-23124seqcontrolTypeFromAPSubtypeToAPMore

26、 fragWEPMoredataPowermgtRetryRsvdProtocolversion22411111111802.11 frame:moreduration of reserved transmission time(RTS/CTS)frame seq#(for reliable ARQ)frame type(RTS,CTS,ACK,data)第24页/共62页6-25hub or switchAP 2AP 1H1BBS 2BBS 1802.11:mobility within same subnetrouterH1 remains in same IP subnet:IP add

27、ress can remain sameswitch:which AP is associated with H1?self-learning(Ch.5):switch will see frame from H1 and“remember”which switch port can be used to reach H1第25页/共62页6-26Mradius ofcoverageSSSPPPPMSMaster deviceSlave deviceParked device(inactive)P802.15:personal area networkless than 10 m diamet

28、erreplacement for cables(mouse,keyboard,headphones)ad hoc:no infrastructuremaster/slaves:slaves request permission to send(to master)master grants requests802.15:evolved from Bluetooth specification2.4-2.5 GHz radio bandup to 721 kbps第26页/共62页6-27Chapter 6 outline6.1 Introduction Wireless6.2 Wireles

29、s links,characteristicsCDMA6.3 IEEE 802.11 wireless LANs(“wi-fi”)6.4 Cellular Internet Accessarchitecturestandards(e.g.,GSM)Mobility6.5 Principles:addressing and routing to mobile users6.6 Mobile IP6.7 Handling mobility in cellular networks6.8 Mobility and higher-layer protocols6.9 Summary第27页/共62页6

30、-28Mobile Switching CenterPublic telephonenetwork,andInternetMobile Switching CenterComponents of cellular network architectureq connects cells to wide area netq manages call setup(more later!)q handles mobility(more later!)MSCq covers geographical regionq base station(BS)analogous to 802.11 APq mob

31、ile users attach to network through BSq air-interface:physical and link layer protocol between mobile and BScellwired network第28页/共62页6-29Cellular networks:the first hopTwo techniques for sharing mobile-to-BS radio spectrumcombined FDMA/TDMA:divide spectrum in frequency channels,divide each channel

32、into time slotsCDMA:code division multiple accessfrequencybandstime slots第29页/共62页6-30Cellular standards:brief survey2G systems:voice channelsIS-136 TDMA:combined FDMA/TDMA(north america)GSM(global system for mobile communications):combined FDMA/TDMA most widely deployedIS-95 CDMA:code division mult

33、iple accessIS-136GSMIS-95GPRSEDGECDMA-2000UMTSTDMA/FDMADont drown in a bowlof alphabet soup:use thisoor reference only第30页/共62页6-31Cellular standards:brief survey2.5 G systems:voice and data channelsfor those who cant wait for 3G service:2G extensionsgeneral packet radio service(GPRS)evolved from GS

34、M data sent on multiple channels(if available)enhanced data rates for global evolution(EDGE)also evolved from GSM,using enhanced modulation Date rates up to 384KCDMA-2000(phase 1)data rates up to 144Kevolved from IS-95第31页/共62页6-32Cellular standards:brief survey3G systems:voice/dataUniversal Mobile

35、Telecommunications Service(UMTS)GSM next step,but using CDMACDMA-2000 TDS-CDMA .more(and more interesting)cellular topics due to mobility(stay tuned for details)第32页/共62页6-33Chapter 6 outline6.1 Introduction Wireless6.2 Wireless links,characteristicsCDMA6.3 IEEE 802.11 wireless LANs(“wi-fi”)6.4 Cell

36、ular Internet Accessarchitecturestandards(e.g.,GSM)Mobility6.5 Principles:addressing and routing to mobile users6.6 Mobile IP6.7 Handling mobility in cellular networks6.8 Mobility and higher-layer protocols6.9 Summary第33页/共62页6-34What is mobility?spectrum of mobility,from the network perspective:no

37、mobilityhigh mobilitymobile wireless user,using same access pointmobile user,passing through multiple access point while maintaining ongoing connections(like cell phone)mobile user,connecting/disconnecting from network using DHCP.第34页/共62页6-35Mobility:Vocabularyhome network:permanent“home”of mobileP

38、ermanent address:address in home network,can always be used to reach mobilehome agent:entity that will perform mobility functions on behalf of mobile,when mobile is remotewide area networkcorrespondent第35页/共62页6-36Mobility:more vocabularyCare-of-address:address in visited network.wide area networkvi

39、sited network:network in which mobile currently resides Permanent address:remains constant(foreign agent:entity in visited network that performs mobility functions on behalf of mobile.correspondent:wants to communicate with mobile第36页/共62页6-37How do you contact a mobile friend:search all phone books

40、?call her parents?expect her to let you know where he/she is?I wonder where Alice moved to?Consider friend frequently changing addresses,how do you find her?第37页/共62页6-38Mobility:approachesLet routing handle it:routers advertise permanent address of mobile-nodes-in-residence via usual routing table

41、exchange.routing tables indicate where each mobile locatedno changes to end-systemsLet end-systems handle it:indirect routing:communication from correspondent to mobile goes through home agent,then forwarded to remotedirect routing:correspondent gets foreign address of mobile,sends directly to mobil

42、e第38页/共62页6-39Mobility:approachesLet routing handle it:routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange.routing tables indicate where each mobile locatedno changes to end-systemslet end-systems handle it:indirect routing:communication from corresponde

43、nt to mobile goes through home agent,then forwarded to remotedirect routing:correspondent gets foreign address of mobile,sends directly to mobilenot scalable to millions of mobiles第39页/共62页6-40Mobility:registrationEnd result:Foreign agent knows about mobileHome agent knows location of mobilewide are

44、a networkhome networkvisited network1mobile contacts foreign agent on entering visited network2foreign agent contacts home agent home:“this mobile is resident in my network”第40页/共62页6-41Mobility via Indirect Routingwide area networkhomenetworkvisitednetwork3241correspondent addresses packets using h

45、ome address of mobilehome agent intercepts packets,forwards to foreign agentforeign agent receives packets,forwards to mobilemobile replies directly to correspondent第41页/共62页6-42Indirect Routing:commentsMobile uses two addresses:permanent address:used by correspondent(hence mobile location is transp

46、arent to correspondent)care-of-address:used by home agent to forward datagrams to mobileforeign agent functions may be done by mobile itselftriangle routing:correspondent-home-network-mobileinefficient when correspondent,mobile are in same network第42页/共62页6-43Indirect Routing:moving between networks

47、suppose mobile user moves to another networkregisters with new foreign agentnew foreign agent registers with home agenthome agent update care-of-address for mobilepackets continue to be forwarded to mobile(but with new care-of-address)mobility,changing foreign networks transparent:on going connectio

48、ns can be maintained!第43页/共62页6-44Mobility via Direct Routingwide area networkhomenetworkvisitednetwork4241correspondent requests,receives foreign address of mobilecorrespondent forwards to foreign agentforeign agent receives packets,forwards to mobilemobile replies directly to correspondent3第44页/共6

49、2页6-45Mobility via Direct Routing:commentsovercome triangle routing problemnon-transparent to correspondent:correspondent must get care-of-address from home agentwhat if mobile changes visited network?第45页/共62页6-46wide area network1foreign net visited at session startanchorforeignagent24new foreigna

50、gent35correspondentagentcorrespondentnew foreignnetworkAccommodating mobility with direct routinganchor foreign agent:FA in first visited networkdata always routed first to anchor FAwhen mobile moves:new FA arranges to have data forwarded from old FA(chaining)第46页/共62页6-47Chapter 6 outline6.1 Introd