计算机网络自顶向下方法第六章(英文版课件).ppt

6:Wireless and Mobile Networks,6-1,Chapter 6Wireless and Mobile Networks,Computer Networking:A Top Down Approach Featuring the Internet,3rd edition.Jim Kurose,Keith RossAddison-Wesley,July 2004.,6:Wireless and Mobile Networks,6-2,Chapter 6:Wireless and Mobile Networks,Background:#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 linkhandling mobile user who changes point of attachment to network,6:Wireless and Mobile Networks,6-3,Chapter 6 outline,6.1 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 Mobility and higher-layer protocols6.9 Summary,6:Wireless and Mobile Networks,6-4,Elements of a wireless network,6:Wireless and Mobile Networks,6-5,Elements of a wireless network,6:Wireless and Mobile Networks,6-6,Elements of a wireless network,wireless linktypically used to connect mobile(s)to base stationalso used as backbone link multiple access protocol coordinates link access various data rates,transmission distance,6:Wireless and Mobile Networks,6-7,Characteristics of selected wireless link standards,384 Kbps,56 Kbps,54 Mbps,5-11 Mbps,1 Mbps,802.15,802.11b,802.11a,g,IS-95 CDMA,GSM,UMTS/WCDMA,CDMA2000,.11 p-to-p link,2G,3G,6:Wireless and Mobile Networks,6-8,Elements of a wireless network,6:Wireless and Mobile Networks,6-9,Elements of a wireless network,Ad hoc modeno base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network:route among themselves,6:Wireless and Mobile Networks,6-10,Wireless Link Characteristics,Differences 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 wellmultipath 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 Networks,6-11,Wireless network characteristics,Multiple wireless senders and receivers create additional problems(beyond multiple access):,Hidden terminal problemB,A hear each otherB,C hear each otherA,C can not hear each othermeans A,C unaware of their interference at B,Signal fading:B,A hear each otherB,C hear each otherA,C can not hear each other interferring at B,6:Wireless and Mobile Networks,6-12,Code Division Multiple 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)decoding: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 Networks,6-13,CDMA Encode/Decode,slot 1,slot 0,Zi,m=di.cm,slot 0channeloutput,slot 1channeloutput,channel output Zi,m,sender,code,databits,slot 1,slot 0,slot 0channeloutput,slot 1channeloutput,receiver,code,receivedinput,6:Wireless and Mobile Networks,6-14,CDMA:two-sender interference,6:Wireless and Mobile Networks,6-15,Chapter 6 outline,6.1 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 Mobility and higher-layer protocols6.9 Summary,6:Wireless and Mobile Networks,6-16,IEEE 802.11 Wireless LAN,802.11b2.4-5 GHz unlicensed radio spectrumup to 11 Mbpsdirect sequence spread spectrum(DSSS)in physical layerall hosts use same chipping codewidely deployed,using base stations,802.11a 5-6 GHz rangeup to 54 Mbps802.11g 2.4-5 GHz rangeup to 54 MbpsAll use CSMA/CA for multiple accessAll have base-station and ad-hoc network versions,6:Wireless and Mobile Networks,6-17,802.11 LAN architecture,wireless host communicates with base stationbase station=access point(AP)Basic Service Set(BSS)(aka“cell”)in infrastructure mode contains:wireless hostsaccess point(AP):base stationad hoc mode:hosts only,BSS 1,BSS 2,hub,switchor router,6:Wireless and Mobile Networks,6-18,802.11:Channels,association,802.11b: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 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,6:Wireless and Mobile Networks,6-19,IEEE 802.11:multiple access,avoid 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 collisions in any case:hidden terminal,fadinggoal:avoid collisions:CSMA/C(ollision)A(voidance),6:Wireless and Mobile Networks,6-20,IEEE 802.11 MAC Protocol:CSMA/CA,802.11 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),sender,receiver,6:Wireless and Mobile Networks,6-21,Avoiding 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 broadcasts clear-to-send CTS in response to RTSRTS heard by all nodessender transmits data frameother stations defer transmissions,Avoid data frame collisions completely using small reservation packets!,6:Wireless and Mobile Networks,6-22,Collision Avoidance:RTS-CTS exchange,AP,A,B,time,defer,6:Wireless and Mobile Networks,6-23,802.11 frame:addressing,Address 2:MAC addressof wireless host or AP transmitting this frame,Address 1:MAC addressof wireless host or AP to receive this frame,Address 3:MAC addressof router interface to which AP is attached,Address 4:used only in ad hoc mode,6:Wireless and Mobile Networks,6-24,H1,R1,802.11 frame:addressing,6:Wireless and Mobile Networks,6-25,802.11 frame:more,duration of reserved transmission time(RTS/CTS),frame seq#(for reliable ARQ),frame type(RTS,CTS,ACK,data),6:Wireless and Mobile Networks,6-26,hub or switch,AP 2,AP 1,H1,BBS 2,BBS 1,802.11:mobility within same subnet,H1 remains in same IP subnet:IP address 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 H1,6:Wireless and Mobile Networks,6-27,radius ofcoverage,802.15:personal area network,less than 10 m diameterreplacement 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,6:Wireless and Mobile Networks,6-28,Chapter 6 outline,6.1 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 Mobility and higher-layer protocols6.9 Summary,6:Wireless and Mobile Networks,6-29,Components of cellular network architecture,6:Wireless and Mobile Networks,6-30,Cellular networks:the first hop,Two techniques for sharing mobile-to-BS radio spectrumcombined FDMA/TDMA:divide spectrum in frequency channels,divide each channel into time slotsCDMA:code division multiple access,6:Wireless and Mobile Networks,6-31,Cellular standards:brief survey,2G 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 access,IS-136,GSM,IS-95,GPRS,EDGE,CDMA-2000,UMTS,TDMA/FDMA,Dont drown in a bowlof alphabet soup:use thisoor reference only,6:Wireless and Mobile Networks,6-32,Cellular standards:brief survey,2.5 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 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-95,6:Wireless and Mobile Networks,6-33,Cellular standards:brief survey,3G systems:voice/dataUniversal Mobile 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),6:Wireless and Mobile Networks,6-34,Chapter 6 outline,6.1 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 Mobility and higher-layer protocols6.9 Summary,6:Wireless and Mobile Networks,6-35,What is mobility?,spectrum of mobility,from the network perspective:,mobile wireless user,using same access point,mobile user,passing through multiple access point while maintaining ongoing connections(like cell phone),mobile user,connecting/disconnecting from network using DHCP.,6:Wireless and Mobile Networks,6-36,Mobility:Vocabulary,home 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.,128.119.40.186,home agent:entity that will perform mobility functions on behalf of mobile,when mobile is remote,wide area network,correspondent,6:Wireless and Mobile Networks,6-37,Mobility:more vocabulary,Care-of-address:address in visited network.(e.g.,79,129.13.2),wide area network,visited network:network in which 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 mobile,6:Wireless and Mobile Networks,6-38,How 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?,6:Wireless and Mobile Networks,6-39,Mobility:approaches,Let 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 correspondent to mobile goes through home agent,then forwarded to remotedirect routing:correspondent gets foreign address of mobile,sends directly to mobile,6:Wireless and Mobile Networks,6-40,Mobility:approaches,Let 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 correspondent to mobile goes through home agent,then forwarded to remotedirect routing:correspondent gets foreign address of mobile,sends directly to mobile,not scalable to millions of mobiles,6:Wireless and Mobile Networks,6-41,Mobility:registration,End result:Foreign agent knows about mobileHome agent knows location of mobile,wide area network,home network,visited network,6:Wireless and Mobile Networks,6-42,Mobility via Indirect Routing,wide area network,homenetwork,visitednetwork,6:Wireless and Mobile Networks,6-43,Indirect 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 mobileforeign agent functions may be done by mobile itselftriangle routing:correspondent-home-network-mobileinefficient when correspondent,mobile are in same network,6:Wireless and Mobile Networks,6-44,Indirect Routing:moving between networks,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 connections can be maintained!,6:Wireless and Mobile Networks,6-45,Mobility via Direct Routing,wide area network,homenetwork,visitednetwork,correspondent requests,receives foreign address of mobile,correspondent forwards to foreign agent,6:Wireless and Mobile Networks,6-46,Mobility via Direct Routing:comments,overcome triangle routing problemnon-transparent to correspondent:correspondent must get care-of-address from home agentwhat if mobile changes visited network?,6:Wireless and Mobile Networks,6-47,wide area network,foreign net visited at session start,anchorforeignagent,new foreignagent,correspondentagent,correspondent,new foreignnetwork,Accommodating mobility with direct routing,anchor 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),6:Wireless and Mobile Networks,6-48,Chapter 6 outline,6.1 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 Mobility and higher-layer protocols6.9 Summary,6:Wireless and Mobile Networks,6-49,Mobile IP,RFC 3220has 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 datagramsagent discoveryregistration with home agent,6:Wireless and Mobile Networks,6-50,Mobile IP:indirect routing,Permanent address:128.119.40.186,Care-of address:79.129.13.2,6:Wireless