《计算机网络——自顶向下方法与Internet特色》幻灯片Lecture12

Chapter1 roadmap 1 1WhatistheInternet 1 2Networkedge1 3Networkcore1 4Networkaccessandphysicalmedia1 5InternetstructureandISPs1 6Delay lossinpacket switchednetworks1 7Protocollayers servicemodels1 8History 1 4 1Accessnetworks Accessnetworkscanbelooselyclassifiedintothreecategories residentialaccesscompanyaccess university company wirelessaccessKeepinmind bandwidth bitspersecond ofaccessnetwork sharedordedicated Accessnetworksarethephysicallinksthatconnectanendsystemtoitsedgerouter whichisthefirstrouteronapathfromtheendsystemtoanyotherdistantendsystem 1 Residentialaccess 1 pointtopointaccess Dialupviamodemupto56Kbpsdirectaccesstorouter oftenless Can tsurfandphoneatsametime can tbe alwayson ADSL asymmetricdigitalsubscriberline 不对称数字用户线 upto1Mbpsupstream todaytypically 256kbps upto8Mbpsdownstream todaytypically 1Mbps AdoptFDM 50kHz 1MHzfordownstream4kHz 50kHzforupstream0kHz 4kHzforordinarytelephoneServiceisalwayson 1 Residentialaccess 2 cablemodems HFC hybridfiber coaxialcableasymmetric upto30Mbpsdownstream 2MbpsupstreamnetworkofcableandfiberattacheshomestoISProuter bycablemodem thru10base TtoPC homesshareaccesstorouterdeployment availableviacableTVcompaniesServiceisalwayson 1 Residentialaccess 2 cablemodems Diagram 3 CableNetworkArchitecture home cableheadend cabledistributionnetwork simplified Typically500to5 000homes 3 CableNetworkArchitecture home cableheadend cabledistributionnetwork 3 CableNetworkArchitecture home cableheadend cabledistributionnetwork simplified 3 CableNetworkArchitecture home cableheadend cabledistributionnetwork FDM 2 Companyaccess localareanetworks company univlocalareanetwork LAN connectsendsystemtoedgerouterEthernet sharedordedicatedlinkconnectsendsystemandrouter10Mbs 100Mbps GigabitEthernetLANs chapter5 3 Wirelessaccessnetworks sharedwirelessaccessnetworkconnectsendsystemtorouterviabasestationalsoknowas accesspoint TherearetwobroadtypesofwirelessInternetaccess wirelessLANs 几十米半径范围 802 11b g WiFi 11or54Mbpswide areawirelessaccess 上万米半径范围 providedbytelcooperator3G 384kbpsWillithappen WAPinEurope 3GinJapen 4 Homenetworks Typicalhomenetworkcomponents ADSLorcablemodemrouter firewall NATEthernetwirelessaccesspoint 1 4 2PhysicalMediawhatliesbetweentransmitter receiver BrieflifeofaBit propagatesbetweentransmitter rcvrpairsthrudifferentphysicalmediaphysicalmediafallintotwocategories guidedmedia 导引型媒体 signalspropagateinsolidmedia copper fiber coaxunguidedmedia 非导引型媒体 signalspropagatefreely e g radio terrestrialradiospectrum satelliteradiospectrum 1 TwistedPair TP twoinsulatedcopperwiresUnshieldedTwistedPair UTP iscommonlyusedforLANs Category3 traditionalphonewires 10MbpsEthernetCategory5 100MbpsEthernet PhysicalMedia coax fiber 2 Coaxialcable twoconcentriccopperconductorsbidirectionalbaseband singlechanneloncablelegacyEthernetbroadband multiplechannelsoncableHFC 3 Fiberopticcable glassfibercarryinglightpulses eachpulseabithigh speedoperation high speedpoint to pointtransmission e g 10 s 100 sGps lowerrorrate repeatersspacedfarapart immunetoelectromagneticnoise Physicalmedia 4 radio signalcarriedinelectromagneticspectrumnophysical wire bidirectionalpropagationenvironmenteffects reflectionobstructionbyobjectsinterference Radiolinktypes terrestrialmicrowavee g upto45MbpschannelsLAN e g Wifi 11Mbps 54Mbpswide area e g cellular e g 3G hundredsofkbpssatelliteKbpsto45Mbpschannel ormultiplesmallerchannels 270msecend enddelayTwotypesofsatellites geosynchronous 同步卫星 lowaltitude 低纬度卫星 Chapter1 roadmap 1 1WhatistheInternet 1 2Networkedge1 3Networkcore1 4Networkaccessandphysicalmedia1 5InternetstructureandISPs1 6Delay lossinpacket switchednetworks1 7Protocollayers servicemodels1 8History 1 5Internetstructure networkofnetworks roughlyhierarchicalatcenter tier 1 ISPsTier 1ISPsareInternetbackbonenetworks e g MCI Sprint AT T Level3 Qwest CableandWireless Directlyconnectedtoeachoftheothertier 1ISPsConnectedtoalargenumberoftier 2ISPsandothercustomernetworksNational internationalincoverage Tier1ISP Tier1ISP Tier1ISP Tier 1ISP e g Sprint SprintUSbackbonenetwork 1 5Internetstructure networkofnetworks A Tier 2 ISPtypicallyhasregionalornationalcoverage andimportantlyconnecttoonlyafewofthetier 1ISPsConnectpossiblyothertier 2ISPsPOP Point of Presence thepointsatwhichtheISPconnecttootherISPs whetherbelow above oratthesamelevel areknownasPOPs Tier1ISP Tier1ISP Tier1ISP 1 5Internetstructure networkofnetworks Tier 3 ISPsandlocalISPslastaccessnetwork closesttoendsystems Tier1ISP Tier1ISP Tier1ISP 1 5Internetstructure networkofnetworks apacketpassesthroughmanynetworks Tier1ISP Tier1ISP Tier1ISP Chapter1 roadmap 1 1WhatistheInternet 1 2Networkedge1 3Networkcore1 4Networkaccessandphysicalmedia1 5InternetstructureandISPs1 6Delay lossinpacket switchednetworks1 7Protocollayers servicemodels1 8History 1 Howdolossanddelayoccur packetsqueueinrouterbufferspacketarrivalratetolinkexceedsoutputlinkcapacitypacketsqueue waitforturn A B 2 Foursourcesofpacketdelay 1 nodalprocessing checkbiterrorsdetermineoutputlink 2 queueingtimewaitingatoutputlinkfortransmissiondependsoncongestionlevelofrouter 2 Foursourcesofpacketdelay 3 Transmissiondelay R linkbandwidth bps L packetlength bits timetosendbitsintolink L R 4 Propagationdelay d lengthofphysicallinks propagationspeedinmedium 2x108m sec propagationdelay d s Note sandRareverydifferentquantities 3 Caravananalogy Cars propagate at100km hrTollboothtakes12sectoserviceacar transmissiontime car bit caravan packetQ Howlonguntilcaravanislinedupbefore2ndtollbooth Timeto push entirecaravanthroughtollboothontohighway 12 10 120secTimeforlastcartopropagatefrom1stto2ndtollbooth 100km 100km hr 1hrA 62minutes ten carcaravan 100km 100km 3 Caravananalogy more Carsnow propagate at1000km hrTollboothnowtakes1mintoserviceacarQ Willcarsarriveto2ndboothbeforeallcarsservicedat1stbooth Yes After7min 1stcarat2ndboothand3carsstillat1stbooth 1stbitofpacketcanarriveat2ndrouterbeforepacketisfullytransmittedat1strouter ten carcaravan 100km 100km 4 Nodaldelay dproc processingdelaytypicallyafewmicrosecsorlessdqueue queuingdelaydependsoncongestion discussedinnextsection dtrans transmissiondelay L R significantforlow speedlinksdprop propagationdelayafewmicrosecstohundredsofmsecs 5 Queuingdelay R linkbandwidth bps L packetlength bits a averagepacketarrivalrate trafficintensity La R La R 0 averagequeueingdelaysmallLa Riscloseto1 delaysbecomelargeLa R 1 more work arrivingthancanbeserviced averagedelayinfinite 6 Real Internetdelaysandroutes Whatdo real Internetdelay losslooklike Tracerouteprogram providesdelaymeasurementfromsourcetorouteralongend endInternetpathtowardsdestination Foralli sendsthreepacketsthatwillreachrouterionpathtowardsdestinationrouteriwillreturnpacketstosendersendertimesintervalbetweentransmissionandreply 3probes 3probes 3probes 6 Real Internetdelaysandroutes 1cs gw 128 119 240 254 1ms1ms2ms2border1 rt fa5 1 0 gw umass edu 128 119 3 145 1ms1ms2ms3cht vbns gw umass edu 128 119 3 130 6ms5ms5ms4jn1 at1 0 0 204 147 132 129 16ms11ms13ms5jn1 so7 0 0 204 147 136 136 21ms18ms18ms6abilene vbns abilene ucaid edu 198 32 11 9 22ms18ms22ms7nycm wash abilene ucaid edu 198 32 8 46 22ms22ms22ms862 40 103 253 62 40 103 253 104ms109ms106ms9de2 62 40 96 129 62 40 96 50 113ms