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1、计算机网络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: r# wireless (mobile) phone subscribers now exceeds # wired phone subscribers!rcomputer nets: laptops, palmtops, PDAs, Internet-enabled phone promise anytime unt
3、ethered Internet accessrtwo important (but different) challengesmwireless: communication over wireless linkmmobility: handling the mobile user who changes point of attachment to network6: Wireless and Mobile Networks6-5Chapter 6 outline6.1 Introduction Wirelessr6.2 Wireless links, characteristicsmCD
4、MAr6.3 IEEE 802.11 wireless LANs (“wi-fi”)r6.4 Cellular Internet Accessmarchitecturemstandards (e.g., GSM)Mobilityr6.5 Principles: addressing and routing to mobile usersr6.6 Mobile IPr6.7 Handling mobility in cellular networksr6.8 Mobility and higher-layer protocols6.9 Summary6: Wireless and Mobile
5、Networks6-6Elements of a wireless networknetwork infrastructurewireless hostsrlaptop, PDA, IP phonerrun applicationsrmay be stationary (non-mobile) or mobilemwireless does not always mean mobility6: Wireless and Mobile Networks6-7Elements of a wireless networknetwork infrastructure base stationrtypi
6、cally 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 6: Wireless and Mobile Networks6-8Elements of a wireless networknetwork infrastructure wireless linkrtypically used to connect mob
7、ile(s) to base stationralso used as backbone link rmultiple access protocol coordinates link access rvarious data rates, transmission distance6: Wireless and Mobile Networks6-9Characteristics of selected wireless link standardsIndoor10-30mOutdoor50-200mMid-rangeoutdoor200m 4 KmLong-rangeoutdoor5Km 2
8、0 Km.056.384145-1154IS-95, 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 infrastruc
9、ture moderbase station connects mobiles into wired networkrhandoff: mobile changes base station providing connection into wired network6: Wireless and Mobile Networks6-11Elements of a wireless networkad hoc moderno base stationsrnodes can only transmit to other nodes within link coveragernodes organ
10、ize themselves into a network: 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 larg
11、er Internet (Bluetooth, ad hoc 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 Character
12、istics (1)Differences from wired link .mdecreased signal strength: radio signal attenuates as it propagates through matter (path loss)minterference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as wellm
13、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” 6: Wireless and Mobile Networks6-14Wireless Link Characteristics (2)rSNR: signal-to-noise ratiom
14、larger SNR easier to extract signal from noise (a “good thing”)rSNR versus BER tradeoffsmgiven physical layer: increase power - increase SNR-decrease BERmgiven SNR: choose physical layer that meets BER requirement, giving highest thruputSNR may change with mobility: dynamically adapt physical layer
15、(modulation technique, rate) 10203040QAM256 (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 probl
16、emrB, A hear each otherrB, C hear each otherrA, C can not hear each othermeans A, C unaware of their interference at BABCAs signalstrengthspaceCs signalstrengthSignal attenuation:rB, A hear each otherrB, C hear each otherrA, C can not hear each other interfering at B6: Wireless and Mobile Networks6-
17、16Code Division Multiple Access (CDMA)rused in several wireless broadcast channels (cellular, satellite, etc) standardsrunique “code” assigned to each user; i.e., code set partitioningrall users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode datarencoded signa
18、l = (original data) X (chipping sequence)rdecoding: inner-product of encoded signal and chipping sequencerallows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)6: Wireless and Mobile Networks6-17CDMA Encode/Decodeslot 1slot 0d1 = -11 1 11
19、1 -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 1channeloutputreceive
20、rcodereceivedinputDi = S Zi,m.cmm=1MM6: Wireless and Mobile Networks6-18CDMA: two-sender interference6: Wireless and Mobile Networks6-19Chapter 6 outline6.1 Introduction Wirelessr6.2 Wireless links, characteristicsmCDMAr6.3 IEEE 802.11 wireless LANs (“wi-fi”)r6.4 cellular Internet accessmarchitectur
21、emstandards (e.g., GSM)Mobilityr6.5 Principles: addressing and routing to mobile usersr6.6 Mobile IPr6.7 Handling mobility in cellular networksr6.8 Mobility and higher-layer protocols6.9 Summary6: Wireless and Mobile Networks6-20IEEE 802.11 Wireless LANr802.11bm2.4-5 GHz unlicensed spectrummup to 11
22、 Mbpsmdirect sequence spread spectrum (DSSS) in physical layerall hosts use same chipping coder802.11a m5-6 GHz rangemup to 54 Mbpsr802.11g m2.4-5 GHz rangemup to 54 Mbpsr802.11n: multiple antennaem2.4-5 GHz rangemup to 200 Mbpsrall use CSMA/CA for multiple accessrall have base-station and ad-hoc ne
23、twork versions6: Wireless and Mobile Networks6-21802.11 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
24、2Internethub, switchor routerAPAP6: Wireless and Mobile Networks6-22802.11: Channels, associationr802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequenciesmAP admin chooses frequency for APminterference possible: channel can be same as that chosen by neighboring AP!rhost: m
25、ust associate with an APmscans channels, listening for beacon frames containing APs name (SSID) and MAC addressmselects AP to associate withmmay perform authentication Chapter 8mwill typically run DHCP to get IP address in APs subnet6: Wireless and Mobile Networks6-23802.11: passive/active scanningA
26、P 2AP 1H1BBS 2BBS 112234Active Scanning: (1)Probe Request frame broadcast from H1(2)Probes response frame sent from APs(3)Association Request frame sent: H1 to selected AP (4)Association Response frame sent: H1 to selected APAP 2AP 1H1BBS 2BBS 11231Passive Scanning: (1)beacon frames sent from APs(2)
27、association Request frame sent: H1 to selected AP (3)association Response frame sent: H1 to selected AP6: Wireless and Mobile Networks6-24IEEE 802.11: multiple accessravoid collisions: 2+ nodes transmitting at same timer802.11: CSMA - sense before transmittingmdont collide with ongoing transmission
28、by other noder802.11: no collision detection!mdifficult to receive (sense collisions) when transmitting due to weak received signals (fading)mcant sense all collisions in any case: hidden terminal, fadingmgoal: avoid collisions: CSMA/C(ollision)A(voidance)ABCABCAs signalstrengthspaceCs signalstrengt
29、h6: 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 channel busy then start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK, increase random backoff
30、 interval, repeat 2802.11 receiver- if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) senderreceiverDIFSdataSIFSACK6: Wireless and Mobile Networks6-26Avoiding collisions (more)idea: allow sender to “reserve” channel rather than random access of data frames: avoid
31、 collisions of long data framesrsender first transmits small request-to-send (RTS) packets to BS using CSMAmRTSs may still collide with each other (but theyre short)rBS broadcasts clear-to-send CTS in response to RTSrCTS heard by all nodesmsender transmits data framemother stations defer transmissio
32、ns avoid data frame collisions completely using small reservation packets!6: Wireless and Mobile Networks6-27Collision Avoidance: RTS-CTS exchangeAPABtimeRTS(A)RTS(B)RTS(A)CTS(A)CTS(A)DATA (A)ACK(A)ACK(A)reservation collisiondefer6: Wireless and Mobile Networks6-28framecontroldurationaddress1address
33、2address4address3payloadCRC22666260 - 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
34、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. address source address 802.3 frame802.11 frame: addressing6: Wireless and Mobile Networks6-30framecontroldurationaddress1address2a
35、ddress4address3payloadCRC22666260 - 23124seqcontrolTypeFromAPSubtypeToAPMore fragWEPMoredataPowermgtRetryRsvdProtocolversion22411111111802.11 frame: moreduration of reserved transmission time (RTS/CTS)frame seq #(for RDT)frame type(RTS, CTS, ACK, data)6: Wireless and Mobile Networks6-31hub or switch
36、AP 2AP 1H1BBS 2BBS 1802.11: mobility within same subnetrouterrH1 remains in same IP subnet: IP address can remain samerswitch: which AP is associated with H1?mself-learning (Ch. 5): switch will see frame from H1 and “remember” which switch port can be used to reach H16: Wireless and Mobile Networks6
37、-32802.11: advanced capabilitiesRate Adaptationrbase station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)10203040SNR(dB)BER10-110-210-310-510-610-710-4operating point1. SNR decreases, BER inc
38、rease 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 capabilitiesPower Managementrnode-to-AP: “I am going to sleep until next beacon frame”mAP knows not to transmit frames to th
39、is nodemnode wakes up before next beacon framerbeacon frame: contains list of mobiles with AP-to-mobile frames waiting to be sentmnode will stay awake if AP-to-mobile frames to be sent; otherwise sleep again until next beacon frame6: Wireless and Mobile Networks6-34Mradius ofcoverageSSSPPPPMSMaster
40、deviceSlave deviceParked device (inactive)P802.15: personal area networkrless than 10 m diameterrreplacement for cables (mouse, keyboard, headphones)rad hoc: no infrastructurermaster/slaves:mslaves request permission to send (to master)mmaster grants requestsr802.15: evolved from Bluetooth specifica
41、tionm2.4-2.5 GHz radio bandmup to 721 kbps6: Wireless and Mobile Networks6-35802.16: WiMAXrlike 802.11 & cellular: base station modelmtransmissions to/from base station by hosts with omnidirectional antennambase station-to-base station backhaul with point-to-point antennarunlike 802.11:mrange 6 mile
42、s (“city rather than coffee shop”)m14 Mbpspoint-to-multipointpoint-to-point6: Wireless and Mobile Networks6-36802.16: WiMAX: downlink, uplink schedulingrtransmission framemdown-link subframe: base station to node muplink subframe: node to base stationpream.DL-MAPUL-MAPDLburst 1SS #1DLburst 2DLburst
43、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 whenrWiMAX standard provide mechanism for scheduling, but not scheduling algorithm6: Wireless and Mobile Networks6-37Chapter 6 outline6
44、.1 Introduction Wirelessr6.2 Wireless links, characteristicsmCDMAr6.3 IEEE 802.11 wireless LANs (“wi-fi”)r6.4 Cellular Internet Accessmarchitecturemstandards (e.g., GSM)Mobilityr6.5 Principles: addressing and routing to mobile usersr6.6 Mobile IPr6.7 Handling mobility in cellular networksr6.8 Mobili
45、ty and higher-layer protocols6.9 Summary6: Wireless and Mobile Networks6-38Mobile 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!)M
46、SCq 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 network6: Wireless and Mobile Networks6-39Cellular networks: the first hopTwo techniques for sharing mob
47、ile-to-BS radio spectrumrcombined FDMA/TDMA: divide spectrum in frequency channels, divide each channel into time slotsrCDMA: code division multiple accessfrequencybandstime slots6: Wireless and Mobile Networks6-40Cellular standards: brief survey2G systems: voice channelsrIS-136 TDMA: combined FDMA/
48、TDMA (north america)rGSM (global system for mobile communications): combined FDMA/TDMA mmost widely deployedrIS-95 CDMA: code division multiple accessIS-136GSMIS-95GPRSEDGECDMA-2000UMTSTDMA/FDMADont drown in a bowlof alphabet soup: use thisfor reference only6: Wireless and Mobile Networks6-41Cellula
49、r standards: brief survey2.5 G systems: voice and data channelsrfor those who cant wait for 3G service: 2G extensionsrgeneral packet radio service (GPRS)mevolved from GSM mdata sent on multiple channels (if available)renhanced data rates for global evolution (EDGE)malso evolved from GSM, using enhan
50、ced modulation mdata rates up to 384KrCDMA-2000 (phase 1)mdata rates up to 144Kmevolved from IS-956: Wireless and Mobile Networks6-42Cellular standards: brief survey3G systems: voice/datarUniversal Mobile Telecommunications Service (UMTS)mdata service: High Speed Uplink/Downlink packet Access (HSDPA
51、/HSUPA): 3 MbpsrCDMA-2000: CDMA in TDMA slotsmdata 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 6 outline6.1 Introduction Wirelessr6.2 Wireless links, charact
52、eristicsmCDMAr6.3 IEEE 802.11 wireless LANs (“wi-fi”)r6.4 Cellular Internet Accessmarchitecturemstandards (e.g., GSM)Mobilityr6.5 Principles: addressing and routing to mobile usersr6.6 Mobile IPr6.7 Handling mobility in cellular networksr6.8 Mobility and higher-layer protocols6.9 Summary6: Wireless
53、and Mobile Networks6-44What is mobility?rspectrum 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/ disconne
54、cting 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., 128.119.40.186home agent: entity that will perform mobility functions
55、 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 mobile currently resides (e.g., 79.129.13/24)Permane
56、nt 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 mobile friend:rsearch all phone books?rcall her par
57、ents?rexpect 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: approachesrLet routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual
58、routing table exchange.mrouting tables indicate where each mobile locatedmno changes to end-systemsrLet end-systems handle it: mindirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remotemdirect routing: correspondent gets foreign address of mobile,
59、 sends directly to mobile6: Wireless and Mobile Networks6-49Mobility: approachesrLet routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange.mrouting tables indicate where each mobile locatedmno changes to end-systemsrlet end-systems handl
60、e it: mindirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remotemdirect routing: correspondent gets foreign address of mobile, sends directly to mobilenot scalable to millions of mobiles6: Wireless and Mobile Networks6-50Mobility: registrationEnd
61、result:rForeign agent knows about mobilerHome 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: Wireless and Mobile Networks6-51Mobilit
62、y 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 correspondent6: Wireless and Mobile Net
63、works6-52Indirect Routing: commentsrMobile uses two addresses:mpermanent address: used by correspondent (hence mobile location is transparent to correspondent)mcare-of-address: used by home agent to forward datagrams to mobilerforeign agent functions may be done by mobile itselfrtriangle routing: co
64、rrespondent-home-network-mobileminefficient when correspondent, mobile are in same network6: Wireless and Mobile Networks6-53Indirect Routing: moving between networksrsuppose mobile user moves to another networkmregisters with new foreign agentmnew foreign agent registers with home agentmhome agent
65、update care-of-address for mobilempackets continue to be forwarded to mobile (but with new care-of-address)rmobility, changing foreign networks transparent: on going connections can be maintained!6: Wireless and Mobile Networks6-54Mobility via Direct Routingwide area networkhomenetworkvisitednetwork
66、4241correspondent requests, receives foreign address of mobilecorrespondent forwards to foreign agentforeign agent receives packets, forwards to mobilemobile replies directly to correspondent36: Wireless and Mobile Networks6-55Mobility via Direct Routing: commentsrovercome triangle routing problemrn
67、on-transparent to correspondent: correspondent must get care-of-address from home agentmwhat if mobile changes visited network?6: Wireless and Mobile Networks6-56wide area network1foreign net visited at session startanchorforeignagent24new foreignagent35correspondentagentcorrespondentnew foreignnetw
68、orkAccommodating mobility with direct routingranchor foreign agent: FA in first visited networkrdata always routed first to anchor FArwhen mobile moves: new FA arranges to have data forwarded from old FA (chaining)6: Wireless and Mobile Networks6-57Chapter 6 outline6.1 Introduction Wirelessr6.2 Wire
69、less links, characteristicsmCDMAr6.3 IEEE 802.11 wireless LANs (“wi-fi”)r6.4 Cellular Internet Accessmarchitecturemstandards (e.g., GSM)Mobilityr6.5 Principles: addressing and routing to mobile usersr6.6 Mobile IPr6.7 Handling mobility in cellular networksr6.8 Mobility and higher-layer protocols6.9
70、Summary6: Wireless and Mobile Networks6-58Mobile IPrRFC 3344rhas many features weve seen: mhome agents, foreign agents, foreign-agent registration, care-of-addresses, encapsulation (packet-within-a-packet)rthree components to standard:mindirect routing of datagramsmagent discoverymregistration with
71、home agent6: Wireless and Mobile Networks6-59Mobile IP: indirect routingPermanent address: 128.119.40.186Care-of address: 79.129.13.2dest: 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.
72、40.186foreign-agent-to-mobile packet6: Wireless and Mobile Networks6-60Mobile IP: agent discoveryragent advertisement: foreign/home agents advertise service by broadcasting ICMP messages (typefield = 9)R bit: registration requiredH,F bits: home and/or foreign agent6: Wireless and Mobile Networks6-61
73、Mobile IP: registration example6: Wireless and Mobile Networks6-62Components of cellular network architecturecorrespondentMSCMSCMSCMSCMSCwired public telephonenetworkdifferent cellular networks,operated by different providersrecall:6: Wireless and Mobile Networks6-63Handling mobility in cellular net
74、worksrhome network: network of cellular provider you subscribe to (e.g., Sprint PCS, Verizon)mhome location register (HLR): database in home network containing permanent cell phone #, profile information (services, preferences, billing), information about current location (could be in another networ
75、k)rvisited network: network in which mobile currently residesmvisitor location register (VLR): database with entry for each user currently in networkmcould be home network6: Wireless and Mobile Networks6-64Public switched telephonenetwork mobileuserhomeMobile Switching CenterHLRhome networkvisitedne
76、tworkcorrespondentMobile 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 mobile6
77、: Wireless and Mobile Networks6-65Mobile Switching CenterVLRold BSSnew BSSold routingnewroutingGSM: handoff with common MSCrHandoff goal: route call via new base station (without interruption)rreasons for handoff:mstronger signal to/from new BSS (continuing connectivity, less battery drain)mload bal
78、ance: free up channel in current BSSmGSM doesnt mandate why to perform handoff (policy), only how (mechanism)rhandoff initiated by old BSS6: Wireless and Mobile Networks6-66Mobile Switching CenterVLRold BSS13245678GSM: handoff with common MSCnew BSS1. old BSS informs MSC of impending handoff, provid
79、es 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 M
80、SC: handoff complete. MSC reroutes call8 MSC-old-BSS resources released6: Wireless and Mobile Networks6-67home networkHome MSCPSTNcorrespondentMSCanchor MSCMSCMSC(a) before handoffGSM: handoff between MSCsranchor MSC: first MSC visited during calmcall remains routed through anchor MSCrnew MSCs add o
81、n to end of MSC chain as mobile moves to new MSCrIS-41 allows optional path minimization step to shorten multi-MSC chain6: Wireless and Mobile Networks6-68home networkHome MSCPSTNcorrespondentMSCanchor MSCMSCMSC(b) after handoffGSM: handoff between MSCsranchor MSC: first MSC visited during calmcall
82、remains routed through anchor MSCrnew MSCs add on to end of MSC chain as mobile moves to new MSCrIS-41 allows optional path minimization step to shorten multi-MSC chain6: Wireless and Mobile Networks6-69Mobility: GSM versus Mobile IPGSM element Comment on GSM element Mobile IP elementHome systemNetw
83、ork to which mobile users permanent phone number belongsHome networkGateway Mobile Switching Center, or “home MSC”. Home Location Register (HLR)Home MSC: point of contact to obtain routable address of mobile user. HLR: database in home system containing permanent phone number, profile information, c
84、urrent location of mobile user, subscription informationHome agentVisited SystemNetwork other than home system where mobile user is currently residingVisited networkVisited Mobile services Switching Center.Visitor Location Record (VLR)Visited MSC: responsible for setting up calls to/from mobile node
85、s in cells associated with MSC. VLR: temporary database entry in visited system, containing subscription information for each visiting mobile userForeign agentMobile Station Roaming Number (MSRN), or “roaming number”Routable address for telephone call segment between home MSC and visited MSC, visibl
86、e to neither the mobile nor the correspondent.Care-of-address6: Wireless and Mobile Networks6-70Wireless, mobility: impact on higher layer protocolsrlogically, impact should be minimal mbest effort service model remains unchanged mTCP and UDP can (and do) run over wireless, mobiler but performance-w
87、ise:mpacket loss/delay due to bit-errors (discarded packets, delays for link-layer retransmissions), and handoffmTCP interprets loss as congestion, will decrease congestion window un-necessarilymdelay impairments for real-time trafficmlimited bandwidth of wireless links6: Wireless and Mobile Network
88、s6-71Chapter 6 SummaryWirelessrwireless links:mcapacity, distancemchannel impairmentsmCDMArIEEE 802.11 (“wi-fi”)mCSMA/CA reflects wireless channel characteristicsrcellular accessmarchitecturemstandards (e.g., GSM, CDMA-2000, UMTS)Mobilityrprinciples: addressing, routing to mobile usersmhome, visited networksmdirect, indirect routingmcare-of-addressesrcase studiesmmobile IPmmobility in GSMrimpact on higher-layer protocols
