1、計算機網(wǎng)絡(luò)Computer Networks4-14-2Chapter 6 Wireless and Mobile NetworksAcknowledgementComputer Networking: A Top Down Approach ,4th edition. Jim Kurose, Keith RossAddison-Wesley, July 2007. EE284 Lecture NoteNick McKeownProfessor of Electrical Engineering and Computer Science, Stanford University6: Wirel

2、ess and Mobile Networks6-3Reading Assignment: Chapter 66: Wireless and Mobile Networks6-4Chapter 6: Wireless and Mobile NetworksBackground: # wireless (mobile) phone subscribers now exceeds # wired phone subscribers!computer nets: laptops, palmtops, PDAs, Internet-enabled phone promise anytime untet

3、hered Internet accesstwo important (but different) challengeswireless: communication over wireless linkmobility: handling the mobile user who changes point of attachment to network6: Wireless and Mobile Networks6-5Chapter 6 outline6.1 Introduction Wireless6.2 Wireless links, characteristicsCDMA6.3 I

4、EEE 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 Summary6: Wireless and Mobile Networks6-6Elem

5、ents of a wireless networknetwork infrastructurewireless hostslaptop, PDA, IP phonerun applicationsmay be stationary (non-mobile) or mobilewireless does not always mean mobility6: Wireless and Mobile Networks6-7Elements of a wireless networknetwork infrastructure base stationtypically connected to w

6、ired networkrelay - responsible for sending packets between wired network and wireless host(s) in its “area”e.g., cell towers, 802.11 access points 6: Wireless and Mobile Networks6-8Elements of a wireless networknetwork infrastructure wireless linktypically used to connect mobile(s) to base stationa

7、lso used as backbone link multiple access protocol coordinates link access various data rates, transmission distance6: Wireless and Mobile Networks6-9Characteristics of selected wireless link standardsIndoor10-30mOutdoor50-200mMid-rangeoutdoor200m 4 KmLong-rangeoutdoor5Km 20 Km.056.384145-1154IS-95,

8、 CDMA, GSM2GUMTS/WCDMA, CDMA20003G802.15802.11b802.11a,gUMTS/WCDMA-HSPDA, CDMA2000-1xEVDO3G cellularenhanced802.16 (WiMAX)802.11a,g point-to-point200802.11nData rate (Mbps)data6: Wireless and Mobile Networks6-10Elements of a wireless networknetwork infrastructure infrastructure modebase station conn

9、ects mobiles into wired networkhandoff: mobile changes base station providing connection into wired network6: Wireless and Mobile Networks6-11Elements of a wireless networkad hoc modeno base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network:

10、route among themselves6: Wireless and Mobile Networks6-12Wireless network taxonomysingle hopmultiple hopsinfrastructure(e.g., APs)noinfrastructurehost connects to base station (WiFi,WiMAX, cellular) which connects to larger Internetno base station, noconnection to larger Internet (Bluetooth, ad hoc

11、nets)host may have torelay through severalwireless nodes to connect to larger Internet: mesh netno base station, noconnection to larger Internet. May have torelay to reach other a given wireless nodeMANET, VANET6: Wireless and Mobile Networks6-13Wireless Link Characteristics (1)Differences from wire

12、d 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 signa

13、l reflects off objects ground, arriving ad destination at slightly different times. make communication across (even a point to point) wireless link much more “difficult” 6: Wireless and Mobile Networks6-14Wireless Link Characteristics (2)SNR: signal-to-noise ratiolarger SNR easier to extract signal

14、from noise (a “good thing”)SNR versus BER tradeoffsgiven physical layer: increase power - increase SNR-decrease BERgiven SNR: choose physical layer that meets BER requirement, giving highest thruputSNR may change with mobility: dynamically adapt physical layer (modulation technique, rate) 10203040QA

15、M256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)SNR(dB)BER10-110-210-310-510-610-710-46: Wireless and Mobile Networks6-15Wireless network characteristicsMultiple wireless senders and receivers create additional problems (beyond multiple access):ABCHidden terminal problemB, A hear each otherB, C hear each ot

16、herA, C can not hear each othermeans A, C unaware of their interference at BABCAs signalstrengthspaceCs signalstrengthSignal attenuation:B, A hear each otherB, C hear each otherA, C can not hear each other interfering at B6: Wireless and Mobile Networks6-16Code Division Multiple Access (CDMA)used in

17、 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)decoding

18、: inner-product of encoded signal and chipping sequenceallows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)6: Wireless and Mobile Networks6-17CDMA Encode/Decodeslot 1slot 0d1 = -111111-1-1-1-Zi,m= di.cmd0 = 111111-1-1-1-11111-1-1-1-1111

19、1-1-1-1-slot 0channeloutputslot 1channeloutputchannel output Zi,msendercodedatabitsslot 1slot 0d1 = -1d0 = 111111-1-1-1-11111-1-1-1-11111-1-1-1-11111-1-1-1-slot 0channeloutputslot 1channeloutputreceivercodereceivedinputDi = S Zi,m.cmm=1MM6: Wireless and Mobile Networks6-18CDMA: two-sender interferen

20、ce6: Wireless and Mobile Networks6-19Chapter 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 users6.6 Mobile IP6.7 Han

21、dling mobility in cellular networks6.8 Mobility and higher-layer protocols6.9 Summary6: Wireless and Mobile Networks6-20IEEE 802.11 Wireless LAN802.11b2.4-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layerall hosts use same chipping code802.11a 5-6 GHz ran

22、geup to 54 Mbps802.11g 2.4-5 GHz rangeup to 54 Mbps802.11n: multiple antennae2.4-5 GHz rangeup to 200 Mbpsall use CSMA/CA for multiple accessall have base-station and ad-hoc network versions6: Wireless and Mobile Networks6-21802.11 LAN architecturewireless host communicates with base stationbase sta

23、tion = access point (AP)Basic Service Set (BSS) (aka “cell”) in infrastructure mode contains:wireless hostsaccess point (AP): base stationad hoc mode: hosts onlyBSS 1BSS 2Internethub, switchor routerAPAP6: Wireless and Mobile Networks6-22802.11: Channels, association802.11b: 2.4GHz-2.485GHz spectrum

24、 divided into 11 channels at different frequenciesAP admin chooses frequency for APinterference possible: channel can be same as that chosen by neighboring AP!host: must associate with an APscans channels, listening for beacon frames containing APs name (SSID) and MAC addressselects AP to associate

25、withmay perform authentication Chapter 8will typically run DHCP to get IP address in APs subnet6: Wireless and Mobile Networks6-23802.11: passive/active scanningAP 2AP 1H1BBS 2BBS 112234Active Scanning: Probe Request frame broadcast from H1Probes response frame sent from APsAssociation Request frame

26、 sent: H1 to selected AP Association Response frame sent: H1 to selected APAP 2AP 1H1BBS 2BBS 11231Passive Scanning: beacon frames sent from APsassociation Request frame sent: H1 to selected AP association Response frame sent: H1 to selected AP6: Wireless and Mobile Networks6-24IEEE 802.11: multiple

27、 accessavoid collisions: 2+ nodes 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 collisio

28、ns in any case: hidden terminal, fadinggoal: avoid collisions: CSMA/C(ollision)A(voidance)ABCABCAs signalstrengthspaceCs signalstrength6: Wireless and Mobile Networks6-25IEEE 802.11 MAC Protocol: CSMA/CA802.11 sender1 if sense channel idle for DIFS then transmit entire frame (no CD)2 if sense channe

29、l busy then start random backoff timetimer counts down while channel 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) senderreceiverDIFSdataSIFSACK6: Wireless an

30、d Mobile Networks6-26Avoiding collisions (more)idea: allow sender to “reserve” channel rather than 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

31、broadcasts clear-to-send CTS in response to RTSCTS heard by all nodessender transmits data frameother stations defer transmissions avoid data frame collisions completely using small reservation packets!6: Wireless and Mobile Networks6-27Collision Avoidance: RTS-CTS exchangeAPABtimeRTS(A)RTS(B)RTS(A)

32、CTS(A)CTS(A)DATA (A)ACK(A)ACK(A)reservation collisiondefer6: Wireless and Mobile Networks6-28framecontroldurationaddress1address2address4address3payloadCRC22666260 - 23124seqcontrol802.11 frame: addressingAddress 2: MAC addressof wireless host or AP transmitting this frameAddress 1: MAC addressof wi

33、reless host or AP to receive this frameAddress 3: MAC addressof router interface to which AP is attachedAddress 4: used only in ad hoc mode6: Wireless and Mobile Networks6-29InternetrouterAPH1R1AP MAC addr H1 MAC addr R1 MAC addraddress 1address 2address 3802.11 frameR1 MAC addr H1 MAC addr dest. ad

34、dress source address 802.3 frame802.11 frame: addressing6: Wireless and Mobile Networks6-30framecontroldurationaddress1address2address4address3payloadCRC22666260 - 23124seqcontrolTypeFromAPSubtypeToAPMore fragWEPMoredataPowermgtRetryRsvdProtocolversion22411111111802.11 frame: moreduration of reserve

35、d transmission time (RTS/CTS)frame seq #(for RDT)frame type(RTS, CTS, ACK, data)6: Wireless and Mobile Networks6-31hub or switchAP 2AP 1H1BBS 2BBS 1802.11: mobility within same subnetrouterH1 remains in same IP subnet: IP address can remain sameswitch: which AP is associated with H1?self-learning (C

36、h. 5): switch will see frame from H1 and “remember” which switch port can be used to reach H16: Wireless and Mobile Networks6-32802.11: advanced capabilitiesRate Adaptationbase station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies QAM2

37、56 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)10203040SNR(dB)BER10-110-210-310-510-610-710-4operating point1. SNR decreases, BER increase as node moves away from base station2. When BER becomes too high, switch to lower transmission rate but with lower BER6: Wireless and Mobile Networks6-33802.11: advanced

38、capabilitiesPower Managementnode-to-AP: “I am going to sleep until next beacon frame”AP knows not to transmit frames to this nodenode wakes up before next beacon framebeacon frame: contains list of mobiles with AP-to-mobile frames waiting to be sentnode will stay awake if AP-to-mobile frames to be s

39、ent; otherwise sleep again until next beacon frame6: Wireless and Mobile Networks6-34Mradius ofcoverageSSSPPPPMSMaster deviceSlave deviceParked device (inactive)P802.15: personal area networkless than 10 m diameterreplacement for cables (mouse, keyboard, headphones)ad hoc: no infrastructuremaster/sl

40、aves:slaves request permission to send (to master)master grants requests802.15: evolved from Bluetooth specification2.4-2.5 GHz radio bandup to 721 kbps6: Wireless and Mobile Networks6-35802.16: WiMAXlike 802.11 & cellular: base station modeltransmissions to/from base station by hosts with omnidirec

41、tional antennabase station-to-base station backhaul with point-to-point antennaunlike 802.11:range 6 miles (“city rather than coffee shop”)14 Mbpspoint-to-multipointpoint-to-point6: Wireless and Mobile Networks6-36802.16: WiMAX: downlink, uplink schedulingtransmission framedown-link subframe: base s

42、tation to node uplink subframe: node to base stationpream.DL-MAPUL-MAPDLburst 1SS #1DLburst 2DLburst nInitialmaint.requestconn.downlink subframeSS #2SS #kuplink subframebase station tells nodes who will get to receive (DL map) and who will get to send (UL map), and whenWiMAX standard provide mechani

43、sm for scheduling, but not scheduling algorithm6: Wireless and Mobile Networks6-37Chapter 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 an

44、d routing to mobile users6.6 Mobile IP6.7 Handling mobility in cellular networks6.8 Mobility and higher-layer protocols6.9 Summary6: Wireless and Mobile Networks6-38Mobile Switching CenterPublic telephonenetwork, andInternetMobile Switching CenterComponents of cellular network architecture connects

45、cells to wide area net manages call setup (more later!) handles mobility (more later!)MSC covers geographical region base station (BS) analogous to 802.11 AP mobile users attach to network through BS air-interface: physical and link layer protocol between mobile and BScellwired network6: Wireless an

46、d Mobile Networks6-39Cellular networks: the first hopTwo techniques for sharing mobile-to-BS radio spectrumcombined FDMA/TDMA: divide spectrum in frequency channels, divide each channel into time slotsCDMA: code division multiple accessfrequencybandstime slots6: Wireless and Mobile Networks6-40Cellu

47、lar 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 multiple accessIS-136GSMIS-95GPRSEDGECDMA-2000UMTSTDMA/FDMADont drown in a bowlof alphab

48、et soup: use thisfor reference only6: Wireless and Mobile Networks6-41Cellular 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 GSM data sent on multiple channels (if available)enhanced d

49、ata rates for global evolution (EDGE)also evolved from GSM, using enhanced modulation data rates up to 384KCDMA-2000 (phase 1)data rates up to 144Kevolved from IS-956: Wireless and Mobile Networks6-42Cellular standards: brief survey3G systems: voice/dataUniversal Mobile Telecommunications Service (U

50、MTS)data service: High Speed Uplink/Downlink packet Access (HSDPA/HSUPA): 3 MbpsCDMA-2000: CDMA in TDMA slotsdata service: 1xEvlution Data Optimized (1xEVDO) up to 14 Mbps . more (and more interesting) cellular topics due to mobility (stay tuned for details)6: Wireless and Mobile Networks6-43Chapter

51、 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 users6.6 Mobile IP6.7 Handling mobility in cellular networks6.8 Mobil

52、ity and higher-layer protocols6.9 Summary6: Wireless and Mobile Networks6-44What is mobility?spectrum of mobility, from the network perspective:no mobilityhigh mobilitymobile wireless user, using same access pointmobile user, passing through multiple access point while maintaining ongoing connection

53、s (like cell phone)mobile user, connecting/ disconnecting from network using DHCP. 6: Wireless and Mobile Networks6-45Mobility: Vocabularyhome network: permanent “home” of mobile(e.g., 128.119.40/24)Permanent address: address in home network, can always be used to reach mobilee.g., 128.119.40.186hom

54、e agent: entity that will perform mobility functions on behalf of mobile, when mobile is remotewide area networkcorrespondent6: Wireless and Mobile Networks6-46Mobility: more vocabularyCare-of-address: address in visited network.(e.g., 79,129.13.2) wide area networkvisited network: network in which

55、mobile currently resides (e.g., 79.129.13/24)Permanent address: remains constant (e.g., 128.119.40.186)foreign agent: entity in visited network that performs mobility functions on behalf of mobile. correspondent: wants to communicate with mobile6: Wireless and Mobile Networks6-47How do you contact a

56、 mobile friend:search all phone books?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?6: Wireless and Mobile Networks6-48Mobility: approachesLet routing handle it: routers advertise permanent

57、 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 correspondent to mobile goes through home agent, then forwarded to remotedirect routing: co

58、rrespondent gets foreign address of mobile, sends directly to mobile6: Wireless and Mobile Networks6-49Mobility: 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 chan

59、ges 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 mobilenot scalable to millions of mobiles6: Wireless and Mobile N

60、etworks6-50Mobility: registrationEnd result:Foreign agent knows about mobileHome agent knows location of mobilewide area networkhome networkvisited network1mobile contacts foreign agent on entering visited network2foreign agent contacts home agent home: “this mobile is resident in my network”6: Wire