《计算机科学导论》课件Unit 6Computer Networks

?计算机科学导论?课件Unit6ComputerNetworks26-1TypesofNetworks

6-2TCP/IPModel

6-3DevicesinNetworks6-4NewDevelopmentinNetworks

6-5ReferencesandRecommendedReading6-6Summary6-7PracticeSetOUTLINE5TypesofNetworks

WeclassifynetworksbytheirspanningdistanceasshowninFigure6.1.Atthetoparethepersonalareanetworksthatsupportsonepersoninsteadofagroup.Beyondthepersonalareanetworksarelonger-rangenetworks.Thesecanbeclassifiedaslocal,metropolitan,andwideareanetworks,eachwithincreasingscale.Finally,theconnectionofmultipleindividualnetworksiscalledaninternetwork.Withoutdoubt,theworldwideInternetisthebest-known(butnottheonly)exampleofaninternetwork.

6TypesofNetworks

TheInternetPlanet10,000kmContinent1000kmWideareanetworkCountry100kmMetropolitanareanetworkCity10kmCampus1kmBuilding100mLocalareanetworkRoom10mPersonalareanetworkSquaremeter1mExampleSystemslocatedinsameSpanningdistanceFigure6.1Classificationofnetworksbyspanningdistance7PersonalareanetworkApersonalareanetwork(PAN)isatechnologythatletdevicescommunicatewithothernearbydevicesnearasingleindividual,overashortrangetypicallywithin10meters.Personalareanetworkscanbeconstructedwithcablesorbewireless.Forexample,acomputercanbeconnectedwithitsperipherals,oralaptopcanbeconnectedwithasmartphoneoraprinter,oraheadsetcanbeconnectedtoamobilephone,usingsomeformofwirelesstechnology.PANscanbewiredorwireless.WiredPANmaybelinkedtogetherusingUSBandFireWire.WirelessPAN(WPAN)isbasedonthestandardIEEE802.15andtypicallyuseANT+,Bluetooth,someinfraredconnections,ZigBee【紫峰技术〔协议〕】,UWB(UltraWideband)【超宽频】,,RFID(RadioFrequencyIdentification)【无线射频识别〔俗称电子标签〕】,6LoWPAN,orNFC(NearFieldCommunication)【近距离无线通信】.8Personalareanetwork

ANT+isproprietarywirelesssensornetworktechnologyusedinthecollectionandtransferofsensordata.Itisusedinsportswatches,workoutmachines,cyclingpowermeters,fitnessequipment,thermometers,speedometers,calorimeters,bloodpressuremonitors,bloodglucosemeters,heartratemonitors,positiontracking,temperaturesensors,andsoon.

9PersonalareanetworkBluetooth,alsoknownasIEEE802.15.1,usesshort-rangeradiowavesandareenergy-efficientforsmalldevices.Infrared(IR)connectionsuseinfraredlighttorepresent0sand1s.IRiswidelyusedinconsumerremotecontrols.ZigBee,alsoknownas,isalow-cost,low-powertechnology,whichmakesitparticularlywell-suitedforremotemonitoringandcontrollingapplicationssuchaswirelesslightingcontrolandfans,homeandbuildingautomationandwirelesssecuritysystems,trafficmanagementsystems,medicalandindustrialsensorapplications,radioremotecontroltoys,games,andsoon.UWBenablesmovementofmassivefilesathighdataratesovershortranges,suchaswirelessmonitors,wirelessprinters,camcorders,andPCperipherals.10PersonalareanetworkRFIDisbasedontheconceptofsendingaqueryoveraradiowaveandreceivingitssubsequentreply.RFIDisusedinsmartcards,trackingsystems,librarybooks,andsupplychainmanagement.6LoWPANmeansIPv6overLow-PowerWirelessPANs.ItisanopenstandarddefinedbytheInternetEngineeringTaskForce(ITEF).ItisanewtechnologythatenablestheintegrationofIPv6andlowpowerdevicesthatconformtoIEEE802.15.4(ZigBee)standardinaPAN,andisprimarilyusedforhomeandbuildingautomation,andhealthcareandenvironmentalmonitoring.

NFC(NearFieldCommunication)【近距离无线通信】isanISO/IECsetofstandardsandinvolvesaninitiatorandatarget.ManysmartphonesuseNFCtomakepayment,andtointegratecreditcard.11Localareanetwork Alocalareanetwork(LAN)isdesignedtoshareresourcesbetweencomputers.Itcanbedefinedasacombinationofcomputersandperipheraldevices(e.g.printers)connectedthroughatransmissionmedium(e.g.cable).Figure6.2showsthreecommontopologiesbelongingtoLANs.Figure6.2LANTopologies12LocalareanetworkInthebustopology,terminaldeviceslikecomputersareconnectedthroughacommonmediumcalledabus.Inthistopology,allcomputersreceivetheframeandcheckitsdestinationaddresswhenastationsendsaframetoanothercomputer.Theframeisacceptedandthedatacontainedintheframeisprocessedifthedestinationaddressintheframeheadermatchesthephysicaladdressofthestation;otherwise,itisdiscarded.Thesecondtopologyisthestartopologyinwhichcomputersareconnectedviaaswitch(anintricatehubthatcontrolstheforwardingoftheframe)orahub【集线器】(adevicethatfacilitatesconnection【方便连接】).TheLANworkslogicallylikeabusinanEthernetenvironmentusingahub.Thehubjustsendsdataoutofallitsinterfaces.13Localareanetwork

Butusingaswitch,theaddressintheframeischeckedbytheswitch,andtheframeissentoutonlythroughtheinterfaceofthedestination.Finally,weintroducetheringtopology.Inthistopology,whenacomputerneedstosendaframetoanothercomputer,itsendsittoitslogicalneighborinthering.Heretheframeisregeneratedandsenttothenextneighboruntilitreachesthefinaldestination.Thedestinationopenstheframe,copiesthedata,eitherremovestheframefromtheringoraddsanacknowledgementandsendstheframe(viathering)backtotheoriginalsender.Inthelattercase,theframeisthenremovedbythesender.14Metropolitanareanetwork Ametropolitanareanetwork(MAN)isthecomputercommunicationnetworkbuildacrossacity.ItbelongstobroadbandLAN.MAN’stransmissionmediumisopticalfiber.ThemainfunctionofMANisregardedasinternetbackbone【主〔骨〕干网】whichconnectsdifferentcomputers,databasesandLANs.TheMANhasawiderangeofapplicationsincludinginteractivepersonalityTV,remotemedicine,remoteeducation,remotemonitoring,andsoon.15Wideareanetwork Awideareanetwork(WAN)istheconnectionofdifferentcountriesorareaswhichisalsocalledRemotecomputernetwork(RCN)

【远程网】toreachresourcesharing.Byfar,thebiggestWANcontains180differentcountriesincludingChina.Anditgrowsatarateof15%permonth.WANalsoincludesWirelesswideareanetwork(WWAN)

【无线广域网】,alargeWANthatusescellular(orcell)networktechnologies【蜂窝网络技术】.WWANarecommonlyreferredtoas3G/4G/5Gnetworks.16Internetwork【互联网络】

Asmentionedearlier,youcanconnectindividualLANs,MANsandWANs(usingroutersorgateways)toformanetworkofnetworks,calledaninternetwork【互联网络】oraninternet.Today,therearemanyprivateandpublicinternets.However,themostfamousistheInternet【互联网】.TheInternetwasoriginallyaresearchinternetworkdesignedtoconnectseveraldifferentheterogeneousnetworks.ItwassponsoredbytheDefenseAdvancedResearchProjectAgency(DARPA)【美国国防部高级研究方案署】.Today,theInternetisaninternetworkthatconnectsmillionsofcomputersthroughouttheworld.17LayersofTCP/IPModelIPaddress(IPv4&IPv6)WirelessLANs6-2TCP/IPModel

18TCP/IPModel TheOSI(OpenSystemsInterconnection)【开放式系统互联】referencemodelandtheTCP/IP(TransmissionControlProtocol/InternetProtocol)

【传输控制协议/互连网协议；网络通讯协议】referencemodelarethetwoimportantnetworkarchitectures.AlthoughtheprotocolsassociatedwiththeOSImodelarenotusedanymore,themodelitselfisactuallyquitegeneralandstillvalid.TheTCP/IPmodelhastheoppositeproperties:themodelitselfisnotofmuchusebuttheprotocolsarewidelyused.Forthisreason,wewillmainlylookattheTCP/IPreferencemodelindetail.19LayersofTCP/IPModel TheTCP/IPisastackofprotocolsthatofficiallycontrolscomputerstoconnecttheInternet.ButduetothewidespreaduseofTCP/IPprotocols,OSImodelisjustatheoreticalmodel.ThestructureofOSImodelisasfollows:(Figure6.3).Figure6.3TCP/IPandOSImodel20(1)Physicallayeranddata-linklayerPhysicallayeranddata-linklayerareresponsibleforspecifyingthephysicalmedium,thesignal,andthebits.Inotherwords,theyjustprovidesservicestotheupperlayerprotocols.(2)Networklayer

Networklayer(ormoreaccurately,theinternetworklayerortheinternetlayer)islocatedinthethirdlayerinTCP/IPmodelwhichisoneofthemostimportantprotocols.Themainfunctionsofnetworkconsistsofrouteselection

congestioncontrol,anderrordetection.ThenetworklayerhasseveralprotocolsincludingInternetProtocol(IP),

AddressResolutionProtocol(ARP)

【地址解析协议】,

ReverseAddressResolutionProtocol(RARP)

【逆地址解析协议】,andInternetControlMessageProtocol(ICMP).Usingtheseprotocols,networklayercanpackthedatafromtheupperlayerandprovidereliablepoint-to-point(PPP)service.LayersofTCP/IPModel21(3)TransportlayerAtthetransportlayer,TCP/IPdefinestwoimportantprotocols:TransmissionControlProtocol(TCP),andUserDatagramProtocol(UDP)【用户报文协议】.Itsmainfunctionsaresegmentationandreassemblyofpackets【数据包分段和重组】,addressingaccordingtoportnumber【根据端口号寻址】,connectionmanagement,andflowcontrol.TCPisdistinguishedfromUDPasfollows:TCPisjustconnection-orientedprotocol;itcanprovidereliableservicebyestablishing“three-wayhandshake〞【在客户端和效劳器之间建立三次握手】betweentheclientandtheserver;afterestablishing“three-wayhandshake〞,itcanensuretheaccuracyofdataandtheorderofthedata.Onthecontrary,UDPisconnectionless-orientedprotocol【无连接协议】;UDPprogrammingisalwayssimplewithout“three-wayhandshake〞soitmaynotprovidereliableserviceandcannotassuretheaccuracyofthedata.BothTCPandUDPareprovidingend-to-endservice.LayersofTCP/IPModel22(4)Applicationlayer TheTCP/IPapplicationlayerisdefinedtotransferdatabetweendifferentapplicationsintwocomputers.ItislocatedatthetopofTCP/IPmodel.Therearemanyprotocolsinapplicationlayer,suchasFTP,SMTP,Telnet,HTTP,andsoon.Thedefinitionoftheseprotocolsisasfollows:FileTransferProtocol(FTP)istheprotocolwhichcanbeusedtotransferdifferentfiles.Ingeneral,theserversendsdatatotheclientcalled“download〞andtheclientsendsdatatotheservercalled“upload〞.ThetransmissionmodeofFTPhastwoways:ASCIItransmissionmodeandbinarydatatransmissionmode.FTPprotocolisoneofthemostimportantprotocolsintheapplicationlayer.LayersofTCP/IPModel23SimpleMailTransferProtocol(SMTP)isaprotocolwhichcanensuretosendandreceiveemail.Itisarelativesimpleprotocol.Whentheuserspecifies【指定】amessagetoareceiverorseveralreceivers,thenthemessageistransmitted.WecanusuallyusetelnetprogramtotesttheSMTPserver.Telnet(TerminalNetwork)【远程登录协议】ismainlyusedforInternetsession.Accordingtothisprotocol,itmaystartasessionwithusernameandpasswordtologinintotheserver.Sotheusercansendinstructiontoserver,asintheconsolecontrollingtheserver【就像操纵本地效劳器一样】.HypertextTransferProtocol(HTTP)isanobject-orientedprotocol.ItisusedtosenddatafromWWWservertolocalbrowser.Itcanhelpthecomputergetthedataincludingtext,multimediafiles,audio,video,images,andsoforthquickly.LayersofTCP/IPModel24AnInternetProtocoladdress(IPaddress)isanumericallabelassignedtoeachdeviceconnectingtoacomputernetworkthatcommunicatesthroughtheuseofInternetProtocol.ItmustbenotedthatIPaddressisuniqueintheInternetandeachdevicealwaysowesoneIPaddress.(1)IPv4IPaddressinIPv4(version4)consistsof4bytes(32bits).ThemaximumnumberofInternetaddressesinIPv4is232(=4,294,967,296).TCP/IPmodelusesanotationthatismoreconvenienttoreadcalleddotted-decimalnotationtakingplaceofthebinaryform.Figure6.4showsanexamplewhichtransfersthebinarytodotted-decimalnotion.

IPaddress(IPv4&IPv6)

25 (2)IPv6(version6)

TheInternetaddressesinIPv4tendtodeplete【用尽】becauseoflimitedInternetaddressesitcanprovideandunevendistribution,etc.,andcannotsatisfytherapidgrowthofInternetusers.IPv6(version6)wasproposedbyInternetEngineeringTaskForce(IETF)【互联网工程工作组】duetotherapidexhaustionofIPv4addressspace.ThemainfeaturesofIPv6arediscussedbelow.

IPaddress(IPv4&IPv6)

Figure6.4IPv4addressindotted-decimalnotation26Firstandforemost,IPv6hasfourtimeslongeraddressesthanIPv4.Theyare128bitslong,whichsolvestheproblemthatIPv4cannotsolve:providingahugeeffectivenumberofInternetaddresses(2128=232232232232==(232)4=(4,294,967,296)43.4×1038).ThesecondmajorimprovementofIPv6isthesimplificationoftheheader【报头】.Itcontainsonlysevenfields【信息组；字段〔域〕】(versus13inIPv4).Thischangeallowsrouterstoprocesspacketsfasterandthusreduceprocessingtimebyintermediatenodesandimprovesthroughputanddelay.Thethirdmajorimprovementisbettersupportforoptions.Thischangeisessentialwiththenewheaderbecausefieldsthatpreviouslyrequiredarenowoptional(becausetheyarenotusedsooften).IPaddress(IPv4&IPv6)

27Inaddition,eachoftheextensionheadersinIPv6isoptionalandrepresentedindifferentway,makingitsimpleforrouterstobypassoptionswhicharenotintended.Thisfeaturespeedsuppacketprocessingtime.AfourthareainIPv6isinsecurity.IETFhadanewspaperstoryaboutagroupofprecocious【一群早熟的】12-year-olds【12岁大的孩童们】usingtheirpersonalcomputerstobreakintobanksandmilitarybasesallovertheInternet.Finally,IPv6canachievequalityofservice(QoS)asitoffersapacketprioritizationfeaturethatprovidesimprovedresponsetimeofreal-timeapplications.IPaddress(IPv4&IPv6)

28Inthemid-1990s,theindustryconsideredthatawirelessLAN(WLAN)standardmightbeveryhelpfulidea,sotheIEEEcommitteeacceptedataskthatdrewupawirelessLANstandard.Thentheymadethefirstdecision.OtherLANstandardsinclude802.1,802.2,and802.3,upto802.10,thereforethewirelessLANstandardwasgiventhenameof802.11.Ithasacommonname-WiFi.Becausethestandardisveryimportant,westillcallit802.11.Theremainingtaskwasharder.Theinitialtaskwastoseekanappropriatefrequencybandthatwasnotalreadyinuse,preferablyacrossworldwide.Themethodtakenwastheoppositeofthatusedinmobilephonenetworks.LANsWireless29Inplaceofexpensivespectrumlicenses,802.11systemsworkinunlicensedbands,forexample,theISM(Industrial,Scientific,andMedical)bandsdefinedbyITU-R(e.g.,902-928MHz,2.4-2.5GHz,5.725-5.825GHz).Alldevicesarepermittedtoutilizethisspectrumincasetheylimittheirtransmitpowertoallowdifferentdevicescoexist.Withoutdoubt,thisimpliesthat802.11radiosmightbecompetingwithcordlessphones,garagedooropeners,andmicrowaveovens.802.11networksarecomposedofinfrastructureandwirelessclients.InfrastructureusuallyconsistsofwirelessrouterscalledAPs(AccessPoints)

【接入点】thatareinstalledinbuildings,andwirelessclientscanbelaptops,smartphones,orworkstationsequippedwithwirelessNICs(NetworkInterfaceCards)

【网卡】.Accesspointsalsoreferredtoasbasestations【基站】.LANsWireless30Theaccesspointsplugsintothewirednetwork,andenablesallclientstoconnectthroughanaccesspoint.ThereisanothermodeofoperationforIEEE802.11whichiscalledanadhocnetwork.Thismodeallowsdirectcommunicationbetweentheclientsthatareinradiorange,forexample,communicationbetweentwocomputersinabuildingwithoutanaccesspoint.However,comparedwithaccesspointmode,itisusedmuchless.BothmodesareshowninFigure6.5.LANsWirelessFigure6.5(a)Wirelessnetworkwithanaccesspoint(b)Adhocnetwork【自组网】31Mobilitybecomesanotherissue.Ifamobileclientismovedawayfromtheaccesspointitisusingtoanother;theremustbesomewayofhandingitoff.Thesolutionisthatan802.11networkmayincludemultiplecells,eachwithitsownaccesspoint,andadistributionsystemthatlinksallthecells.WeoftenuseswitchedEthernet【交互式以太网】torealizethedistributionsystem,butothertechnologymaybeusedforit.Whenthemobileclientsmove,theycanuseotheraccesspointwithabettersignaltoreplacetheoriginalaccesspointandshifttheirassociation.Asawhole,wecanconsidertheentiresystemasasinglewiredLAN【单一的有限局域网】.Finally,therecomesecurityissues.Sincewirelessnetworksbroadcasttransmissions,sonearbydeviceuserswhowanttogettheinformationcaneasilyreceivethepacketsnotintendedforthem.LANsWireless32Inordertostopthissituation,awell-knownencryptionschemenamedasWEP(WiredEquivalentPrivacy)

【有线等效保密】wasassignedtothe802.11standard.Theaimofthiswastomakethewirelesssecuritysameasthewiredsecurity.Withthedevelopmentoftechnology,WiFiProtectedAccess【无线保护访问】,initiallycalledWPA,butnowupgradedtoWPA2,hasreplacedtheoldschemes.The802.11standardisarevolutioninwirelessnetworking.Beyondbuildings,itcanbeusedintrains,planes,boats,andautomobiles.PeopleareabletosurftheInternetwhenevertheywantorwherevertheygo.ZigBeetechnology

【紫蜂技术】isoneofthemostimportantwirelessnetworkstandard.ThecharacteristicsofZigBeeareshorttransmissiondistance,lowpowerconsumption,lowspeed,lowcomplexity,andlowcost.Itcanhelpmobilephonesandallmannerofconsumerelectronics,fromgameconsolestodigitalcameras,tocommunicatemoreeasily.LANsWireless33RepeatersBridgesSwitchesRoutersGateways6-3DevicesinNetworks

34DevicesinNetworks Nowadaystherearealotofconnectingdevicestoconnectdifferentnetworks.However,connectingdevicescanbedividedintofourtypesbasedontheirfunctionality:repeaters,bridges,routers,andgateways.RepeatersandbridgesaredevicesthatconnectsdifferentLANs.RoutersandgatewaysaredevicesthatconnectsdifferentWANandMAN.35Repeaters

Arepeater【中继器】isadevicethatisusedtoforwardphysicalsignalsbetweentwodifferentnetworknodes.Repeatermainlyworksatthephysicallayertomakeastrong,cleancopyofthesignal,byadjustingtheoriginalsignalandregeneratingit.Themainfeaturesofrepeaterareasfollows:expandingtheconnectingdistance;increasingthemaximumnumberofnetworknodes;usingdifferenttransmissionratesbetweendifferentnetworksegments;improvingthereliabilityandperformanceofLAN.36Bridges

Bridge【网桥】isanequipmentwhichworksatthedatalinklayertoachieveLANinterconnection.ItcandivideabigLANintoseveralnetworksegments.Ingeneral,bridgereceivesMACdataframefromLAN.Afteropening,andvalidatingtheframe,bridgewillsendittothephysicallayeraccordingtoanotherLANreassemblingdataframe【重组数据帧】.Becausethebridgeisadatalinklayerdevice,itmaynotdealwiththedataheaderthatupperlayerprotocolsputin.

Thebasicfeaturesofbridgeareasfollows:bridgecanconnectdifferentLANsatdatalinklayer;bridgecanconnectLANswhichhavedifferentdatalinklayerprotocol,differenttransmissionmediumanddifferenttransmissionspeed;bridgeconnecttheinternetandtransmit/receivedata,givesinformationaboutthestorage,filterMACaddress,andretransmitdatapackets.37Switches

Switches【交换机】arewidelyusedinourdailylifelikelaboratoryincollege,officeincompany,orordinaryhouse.Switchisadevicethatcanamplifytheinternetthroughprovidingmoreconnectingportforsubnet.Themainfeaturesofswitcharehighflexibility,highperformance-to-pricerate,relativesimplicityandeasyimplementation.SinceEthernetnetworkingtechnologieshasbecomethemostpopularinLANs,sonetworkswitchhasbecomethemostpopularswitch.Onthewhole,switchconsistsofWANswitchandLANswitch.38RoutersRouter【路由器】isthemostimportantdeviceintheinternetbecauseitcanbeusedtoconnectbiggerinternetlikeMANorWAN.Routersmainlyusetwotypesofroutingprotocolsnamelystaticroutingprotocols,anddynamicroutingprotocols.Themainfunctionsofrouterarepathselectionaccordingtoroutingprotocol,packetswitching,buildingthemapofthenetworkintheformofroutingtable,routingdatabetweennetworks,accessingcontrollists,restrictbroadcaststotheLAN,andsoon.Itmainlyworksatnetworklayer(OSIModel’slayer3).Routerhasfivetypesincludingaccessrouter,enterpriserouter,backbonerouter,

terabitrouter,andmulti-homingrouter.Andwecanchooseappropriaterouteraccordingtodifferentdemands.Figure6.6showsanexampleofarouterthatconnectsdifferentnetworks.39RoutersFigure6.6Networkdevicesinaninternet40Gateways Gateway【网关】isalsocalledprotocolconvertersthatconnectsnetworkabovethenetworklayer.Itisthemostcomplexofallinternetconnectingdevice.Afirewallandaproxyserveractsasagateway【防火墙和代理效劳器都能作为网关】.Somepeopleregardthegatewayasarouterthatconsistsofprotocoltranslators,andothersregardarouterasagatewaybecausearoutercancontrolthepaththroughwhichpacketsofdataaresentinandout.Thedistinctionbetweenthetwotermsisdisappearing.41Third&fourthgenerationofmobilenetworksGRIDRFIDandSensorNetworksSoftwareDefinedNetworkMicrowavesignaltransmissiontechnologyandthegigabitopticalfiber6-4

NewDevelopmentinNetworks42Third&fourth&fifthgenerationofmobilenetworksOverthepast40years,thearchitectureofthemobilephonenetworkhasrealizedtremendousgrowth,andthisgrowthstillgoesonrapidly.Mobilenetworkisalsocalledcellularnetwork.Inthefirst-generationmobilephonesystems,analogtrafficchannelswereusedtotransmitvoicecallsinsteadofdigitalbits.In1982,AMPS(AdvancedMobilePhoneSystem)【高级移动系统】waswidelyusedintheUnitedStatesanditbelongstothefirstgenerationsystem.Inordertoincreasecapacity,improvesecurity,andprovidetextmessaging,inthesecond-generationmobilephonesystemsdigitaltechnologywasusedtotransmitvoicecalls.Since1991,GSM(GlobalSystemforMobilecommunications)【全球移动通信系统】isthemostwidelydeployedmobilephonesystemdesignedasasecondgeneration(2G)wirelesstelephonetechnology.43Third&fourth&fifthgenerationofmobilenetworksThethirdgeneration(3G)systemswereinitiallyimplementedin2001andprovidebothbroadbanddigitaldataservicesanddigitalvoice.Theyalsocomewithanumberofdifferenttechnologiesandmanydifferentstandardstochoosefrom.3GisanITUspecificationprovidingbandwidthofatleast2Mbps(megabitpersecond)forstationaryorwalkingusersand384kbpsinamovingvehicle.UMTS(UniversalMobileTelecommunicationsSystem)【通用移动通信系统】isa3GwirelessstandardthatiswidelydeployedworldwideandusesWCDMA(WidebandCodeDivisionMultipleAccess)【宽带码分多址移动通信系统】astheradiotransmissionstandard.Itoffersmaximum14Mbpsinthedownlink【(中继卫星向地面发送信息等用)下行链路】andalmost6Mbpsontheuplink【(地面向卫星发送信息等用)上行链路】.4Gand5Gtechnologiesaretheextendedonesof3Gwhichcanincreasethedatatransmissionspeedandaddsomenewcommunicationtechnology.44Third&fourth&fifthgenerationofmobilenetworks5Gtechnologyistheproposednextmajorphaseofmobiletelecommunicationsstandardsbeyondthecurrent

4G/IMT-Advanced

standards.5Gplanningincludes

somesurprisingcapabilitiessuchasultra-lowlatency【等待时间】,broadercoverage,higherefficiency,morefrequencyspectrum,highbandwidth,Internetconnectionspeedsfasterthancurrent4G,networkedrobotsconnectedtothecloudinrealtime,deliveringuninterruptedcommunicationflowtoself-drivingcars,InternetofThings(IoT),smartcities,remotesurgeries,andotherimprovements.TheNextGenerationMobileNetworks(NGMN)Alliance

feelsthat5Gwillberolledoutby2021tomeetbusinessandconsumerdemands.45Third&fourth&fifthgenerationofmobilenetworksChinahasgonethroughtheperiodsof'2Gtracking,3Gbreakthrough,4Gsynchronization.Topromote5G,ChinatakestheleadinsettinguptheIMT-2021(5G)PromotionGroupintheAsia-PacificRegion.Chinaalsointegratestheeliteforcefromindustry,universities,researchinstitutions,andapplicationcircle【整合产、学、研、用精锐力量】toactivelycommunicatewiththeInternationalTelecommunicationUnion(ITU).Inaddition,sometelecommunicationoperatorsandindustrymanufacturersfromChinahavefullyparticipatedintheglobalmainstreamR&Dteamsincluding5GPPP,NGMNAlliance,etc.Insummary,Chinaaimstogainsuperiorityof5Gtechnology【占据5G技术制高点】andleadthedevelopmentoftelecomindustry.Futurewirelessnetworkswillemploymultipleantennasandradiostoofferfasterconnectionspeedsforusers.46Third&fourth&fifthgenerationofmobilenetworks2021年11月17日凌晨0点45分，在3GPPRAN1#87次会议的5G短码方案讨论中：Huawei‘sPolarCoding【极化码】isadoptedforthecontrolinformationforeMBB(EnhancedMobileBroadband)【增强移动宽带】【极化码被采纳为5GeMBB场景短码的控制信道编码方案】Qualcomm’【美国高通公司】LDPC(Low-densityparity-checkcode)【低密度奇偶校验码】whichisanerrorcorrectingcodeisadoptedforeMBBdatachannels【被采纳为5GeMBB场景长码和短码的编码信道编码方案】47Third&fourth&fifthgenerationofmobilenetworksTable6.1showsthecomparisonofseveralmobilenetworks.Table6.1ComparisonofseveralmobilenetworksItem/networks2G3GGSMTD-SCDMACDMA2000Speed5kbps384kbps2.4MbpsBandwidth200KHZ1.6MHZ2.5MHZSecurityLowHighHighAdvantagesGoodcompatibility;Stronganti-interference;Completecoverage;Avoidbreathingeffect【避免呼吸效应】;Highspectrumefficiency;lowconstructioncost;updatefromCDMA1Xdirectly;DisadvantagesPoorbusinessperformance;Donotsupport3Gservices;Interferenceproblem;Highsynchronizationrequirements;Near-fareffect【远-近效应】;Multipleaccessinterference【多址干扰】;StandardContributorEuropeChinaAmerica48Third&fourth&fifthgenerationofmobilenetworksTable6.1showsthecomparisonofseveralmobilenetworks.Table6.1ComparisonofseveralmobilenetworksItem/networks4G5GTD-LTE-AdvancedSpeed1Gbps1GbpsBandwidth100MHZ28GHZSecurityHig