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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 access two important (but different) challenges communication over wireless link handling mobile user who changes point of attachment to network第1页/共62页6-2Chapter 6 outline6.1 Introduction Wireless 6.2 Wireless links, characteristics CDMA 6.3 IEEE 802.11 wireless LANs (“wi-fi”) 6.4 Cellular Internet Access architecture standards (e.g., GSM)Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Handling mobility in cellular networks 6.8 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 mobility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 infrastructure 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 Mbps802.15802.11b802.11a,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 base 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 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 well multipath propagation: radio signal reflects 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 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 Multiple Access (CDMA) used in several wireless broadcast channels (cellular, satellite, etc) standards unique “code” assigned to each user; i.e., code set partitioning all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode dataencoded signal = (original data) X (chipping sequence)decoding: inner-product of encoded signal and chipping sequence allows 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 0channeloutputslot 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 Introduction Wireless 6.2 Wireless links, characteristics CDMA 6.3 IEEE 802.11 wireless LANs (“wi-fi”) 6.4 Cellular Internet Access architecture standards (e.g., GSM)Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Handling mobility in cellular networks 6.8 Mobility and higher-layer protocols6.9 Summary第14页/共62页6-15IEEE 802.11 Wireless LAN 802.11b 2.4-5 GHz unlicensed radio spectrum up to 11 Mbps direct sequence spread spectrum (DSSS) in physical layer all hosts use same chipping code widely deployed, using base stations 802.11a 5-6 GHz range up to 54 Mbps 802.11g 2.4-5 GHz range up to 54 Mbps All use CSMA/CA for multiple access All 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): base stationmad hoc mode: hosts onlyBSS 1BSS 2Internethub, switchor routerAPAP第16页/共62页6-17802.11: Channels, association 802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies AP admin chooses frequency for AP interference possible: channel can be same as that chosen by neighboring AP! host: must associate with an AP scans channels, listening for beacon frames containing APs name (SSID) and MAC address selects AP to associate with may perform authentication Chapter 8 will typically run DHCP to get IP address in APs subnet第17页/共62页6-18IEEE 802.11: multiple access avoid collisions: 2+ nodes transmitting at same time 802.11: CSMA - sense before transmitting dont collide with ongoing transmission by other node 802.11: 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, fading goal: 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 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 random access of data frames: avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyre short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes sender transmits data frame other stations 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6-22framecontroldurationaddress1address2address4address3payloadCRC22666260 - 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6-23InternetrouterAPH1R1AP 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 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 subnetrouter H1 remains in same IP subnet: IP address can remain same switch: 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 network less than 10 m diameter replacement for cables (mouse, keyboard, headphones) ad hoc: no infrastructure master/slaves: slaves request permission to send (to master) master grants requests 802.15: evolved from Bluetooth specification 2.4-2.5 GHz radio band up to 721 kbps第26页/共62页6-27Chapter 6 outline6.1 Introduction Wireless 6.2 Wireless links, characteristics CDMA 6.3 IEEE 802.11 wireless LANs (“wi-fi”) 6.4 Cellular Internet Access architecture standards (e.g., GSM)Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Handling mobility in cellular networks 6.8 Mobility and higher-layer protocols6.9 Summary第27页/共62页6-28Mobile 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 mobile 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 spectrum combined FDMA/TDMA: divide spectrum in frequency channels, divide each channel into time slots CDMA: code division multiple accessfrequencybandstime slots第29页/共62页6-30Cellular standards: brief survey2G systems: voice channels IS-136 TDMA: combined FDMA/TDMA (north america) GSM (global system for mobile communications): combined FDMA/TDMA most widely deployed IS-95 CDMA: code division multiple 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 channels for those who cant wait for 3G service: 2G extensions general packet radio service (GPRS) evolved from GSM 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 384K CDMA-2000 (phase 1) data rates up to 144K evolved from IS-95第31页/共62页6-32Cellular standards: brief survey3G systems: voice/data Universal Mobile Telecommunications Service (UMTS) GSM next step, but using CDMA CDMA-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 Wireless 6.2 Wireless links, characteristics CDMA 6.3 IEEE 802.11 wireless LANs (“wi-fi”) 6.4 Cellular Internet Access architecture standards (e.g., GSM)Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Handling mobility in cellular networks 6.8 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 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 mobilePermanent 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 networkvisited 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? 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 exchange. routing tables indicate where each mobile located no 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 mobile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 located no 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 mobilenot scalable to millions of mobiles第39页/共62页6-40Mobility: registrationEnd result: Foreign agent knows about mobile Home 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”第40页/共62页6-41Mobility via Indirect Routingwide area networkhomenetworkvisitednetwork3241correspondent addresses packets using home 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: comments Mobile uses two addresses: permanent address: used by correspondent (hence mobile location is transparent to correspondent) care-of-address: used by home agent to forward datagrams to mobile foreign agent functions may be done by mobile itself triangle routing: correspondent-home-network-mobile inefficient when correspondent, mobile are in same network第42页/共62页6-43Indirect Routing: moving between networks suppose mobile user moves to another network registers with new foreign agent new foreign agent registers with home agent home agent update care-of-address for mobile packets continue to be forwarded to mobile (but with new care-of-address) mobility, changing foreign networks transparent: on going connections 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62页6-45Mobility via Direct Routing: comments overcome triangle routing problem non-transparent to correspondent: correspondent must get care-of-address from home agent what if mobile changes visited network?第45页/共62页6-46wide area network1foreign net visited at session startanchorforeignagent24new foreignagent35correspondentagentcorrespondentnew foreignnetworkAccommodating mobility with direct routing anchor foreign agent: FA in first visited network data always routed first to anchor FA when mobile moves: new FA arranges to have data forwarded from old FA (chaining)第46页/共62页6-47Chapter 6 outline6.1 Introduction Wireless 6.2 Wireless links, characteristics CDMA 6.3 IEEE 802.11 wireless LANs (“wi-fi”) 6.4 Cellular Internet Access architecture standards (e.g., GSM)Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Handling mobility in cellular networks 6.8 Mobility and higher-layer protocols6.9 Summary第47页/共62页6-48Mobile IP RFC 3220 has many features weve seen: home agents, foreign agents, foreign-agent registration, care-of-addresses, encapsulation (packet-within-a-packet) three components to standard: indirect routing of datagrams agent discovery registration with home agent第48页/共62页6-49Mobile IP: indirect routingdest: 128.119.40.186packet sent by correspondentdest: 79.129.13.2dest: 128.119.40.186packet sent by home agent to foreign agent: a packet within a packetdest: 128.119.40.186foreign-agent-to-mobile packet第49页/共62页6-50Mobile IP: agent discovery agent advertisement: foreign/home agents advertise service by broadcasting ICMP messages (typefield = 9) RBHFMGV bits reserved type = 16 type = 9 code = 0 = 9 checksum = 9 router address standard ICMP fields mobility agent advertisement extension length sequence # registration lifetime 0 or more care-of-addresses 0 8 16 24 R bit: registration requiredH,F bits: home and/or foreign agent第50页/共62页6-51Mobile IP: registration example visited network: 79.129.13/24 home agent HA: 128.119.40.7 foreign agent COA: 79.129.13.2 COA: 79.129.13.2 . ICMP agent adv. Mobile agent MA: 128.119.40.186 registration req. COA: 79.129.13.2 HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 9999 identification:714 . registration req. COA: 79.129.13.2 HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 9999 identification: 714 encapsulation format . registration reply HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 4999 Identification: 714 encapsulation format . registration reply HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 4999 Identification: 714 . time 第51页/共62页6-52Components of cellular network architecturecorrespondentMSCMSCMSCMSCMSCwired public telephonenetworkdifferent cellular networks,operated by different providersrecall:第52页/共62页6-53Handling mobility in cellular networkshome network: network of cellular provider you subscribe to (e.g., Sprint PCS, Verizon)home location register (HLR): database in home network containing permanent cell phone #, profile information (services, preferences, billing), information about current location (could be in another network)visited network: network in which mobile currently residesvisitor location register (VLR): database with entry for each user currently in network could be home network第53页/共62页6-54Public switched telephonenetwork mobileuserhomeMobile Switching CenterHLRhome networkvisitednetworkcorrespondentMobile Switching CenterVLRGSM: indirect routing to mobile1call routed to home network2home MSC consults HLR,gets roaming number ofmobile in visited network3home MSC sets up 2nd leg of callto MSC in visited network4MSC in visited network completescall through base station to mobile第54页/共62页6-55Mobile Switching CenterVLRold BSSnew BSSold routingnewroutingGSM: handoff with common MSC Handoff goal: route call via new base station (without interruption) reasons for handoff: stronger signal to/from new BSS (continuing connectivity, less battery drain) load balance: free up channel in current BSS GSM doesnt mandate why to perform handoff (policy), only how (mechanism) handoff initiated by old BSS第55页/共62页6-56Mobile Switching CenterVLRold BSS13245678GSM: handoff with common MSCnew BSS1. old BSS informs MSC of impending handoff, provides list of 1+ new BSSs 2. MSC sets up path (allocates resources) to new BSS3. new BSS allocates radio channel for use by mobile4. new BSS signals MSC, old BSS: ready 5. old BSS tells mobile: perform handoff to new BSS6. mobile, new BSS signal to activate new channel7. mobile signals via new BSS to MSC: handoff complete. MSC reroutes call8 MSC-old-BSS resources released第56页/共62页6-57home networkHome MSCPSTNcorrespondentMSCanchor MSCMSCMSC(a) before handoffGSM: handoff between MSCsanchor MSC: first MSC visited during cal call remains routed through anchor MSC new MSCs add on to end of MSC chain as mobile moves to new MSC IS-41 allows optional path minimization step to shorten multi-MSC chain第57页/共62页6-58home networkHome MSCPSTNcorrespondentMSCanchor MSCMSCMSC(b) after handoffGSM: handoff between MSCsranchor MSC: first MSC visited during calmcall remains routed through anchor MSCrnew MSCs add on to end of MSC chain as mobile moves to new MSCrI
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