单项训练四.docx
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单项训练四.docx
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单项训练四
单项训练四
四、英译汉
1.Ifweconsiderbinarytransmission,thecompleteinformationaboutaparticularmessagewillalwaysbeobtainedbysimplydetectingthepresenceorabsenceofthepulse.Bycomparison,mostotherformsoftransmissionsystemsconveythemessageinformationusingtheshape,orlevelofthetransmittedsignal;parametersthataremosteasilyaffectedbythenoiseandattenuationintroducedbythetransmissionpath.Consequentlythereisaninherentadvantageforovercomingnoisyenvironmentsbychoosingdigitaltransmission.
2.Thereadermayask,howdoesthedemultiplexerknowwhichgroupof8-digitsrelatestochannel1,2,andsoon?
Clearlythisisimportant!
Theproblemiseasilyovercomebyspecifyingaframeformat,whereatthestartofeachframeauniquesequenceofpulsescalledtheframecode,orsynchronizationwordisplacedsoastoidentifythestartoftheframe.Acircuitofthedemultiplexerisarrangedtodetectthesynchronizationword,andtherebyitknowsthatthenextgroupof8-digitscorrespondstochannel1.
3.Noisecanbeintroducedintotransmissionpathinmanydifferentways;perhapsviaanearbylightningstrike,thesparkingofacarignitionsystem,orthethermallow-levelnoisewithinthecommunicationequipmentitself.Itistherelationshipofthetruesignaltothenoisesignal,knownasthesignal-to-noiseratio,whichisofmostinteresttothecommunicationengineer.
4.Basically,ifthesignalisverylargecomparedtothenoiselevel,thenaperfectmessagecantakeplace;however,thisisnotalwaysthecase.Forexample,thesignalreceivedfromasatellite,locatedinfarouterspace,isveryweekandisatalevelonlyslightlyabovethatofthenoise.Alternativeexamplesmaybefoundwithinterrestrialsystemswhere,althoughthemessagesignalisstrong,soisthenoisepower.
5.Sofarwehaveassumedthateachvoicechannelhasaseparatecoder,theunitthatconvertssampledamplitudevaluestoasetofpulses;anddecoder,theunitthatperformsthereverseoperation.Thisneednotbeso,andsystemsareinoperationwhereasinglecodecissharedbetween24,30,oreven120separatechannels.
6.Ahigh-speedelectronicswitchisusedtopresenttheanaloginformationsignalofeachchannel,takeninturn,tothecodec.Thecodecisthenarrangedtosequentiallysampletheamplitudevalue,andcodethisvalueintothe8-digitsequence.Thustheoutputtothecodecmaybeseenasasequenceof8pulsesrelatingtochannel1,thenchannel2,andsoon.Thisiscalledatimedivisionmultiplexer.
7.TheInternetisaninternationalcollectionofcomputernetworksthatallunderstandastandardsystemofaddressesandcommands,connectedtogetherthroughbackbonesystems.Itwasstartedin1969,whentheU.S.DepartmentofDefenceestablishedanationwidenetworktoconnectahandfulofuniversitiesandcontractors.Theoriginalideawastoincreasecomputingcapacitythatcouldsharedbyusersinmanylocations.
8.PeopleontheARPNETquicklydiscoveredthattheycouldexchangemessageandconductelectronic“conferences”withdistantcolleaguesforpurposesthathadnothingtodowiththemilitaryindustrialcomplex.Ifsomebodyelsehadsomethinginterestingstoredontheircomputer,itwasasimplemattertoobtainacopy.TodaythousandsofnetworksandmillionsofcomputersareconnectedtotheInternet.Itisgrowingsoquicklythatnobodycansayexactlyhowmanyusers“OntheNet”.
9.TheInternetisthelargestrepositoryofinformationwhichcanprovideveryverylargenetworkresources.Thenetworkresourcescanbedividedintonetworkfacilitiesresourcesandnetworkinformationresources.Thenetworkfacilitiesresourcesprovideustheabilityofremotecomputationandcommunication.Thenetworkinformationresourcesprovideusallkindsofinformationservices.
10.ThegoalofyouruseoftheInternetisexchangingmessagesorobtaininginformation.WhatyouneedtoknowisthatyoucanexchangemessagewithothercomputersontheInternetanduseyourcomputerasaremoteterminalondistantcomputers.Buttheinternaldetailsofthelinkarelessimportant,aslongasitworks.
11.Themostcommonlyusednetworkserviceiselectronicmail,orsimplyasmail.Mailpermitsnetworkuserstosendtextualmessagestoeachother.Computersandnetworkshandledeliveringthemail,sothatcommunicatingmailusersdonothavetohandledetailsofdelivery,anddonothavetobepresentatthesametimeorplace.
12.BeforeyoucanusetheInternet,youmustchooseawaytomovedatabetweentheInternetandyourPC.Thislinkmaybeahigh-speeddatacommunicationcircuit,alocalareanetwork,atelephonelineoraradiochannel.Mostlikely,youwilluseaModemattachedtoyourtelephonelinetotalktotheInternet.Naturally,thequalityofyourInternetconnectionandserviceisdictatedbytheamountofmoneywhichyouarewillingtospend.
13.Althoughalltheseservicescanwellsatisfytheneedsoftheusersforinformationexchange,adefiniterequirementisneededfortheusers.Notonlyshouldtheusersknowwheretheresourceslocate,butalsoheshouldknowsomeoperatingcommandsconcerned.Toeasethesearchingburdenoftheusers,recentlysomeconvenientsearchingtoolsappear,suchasGopher,andWWWNetscape.
14.AmoreandmoresystemsjointheInternet,andasmoreandmoreformsofinformationcanbeconvertedtodigitalform,theamountofstuffavailabletoInternetuserscontinuestogrow.AtsomepointsverysoonafterthenationwideInternetstartedtogrow,peoplebegantotreattheNetasacommunity,withitsowntraditionandcustoms.
15.WorldWideWeb(WWW)isanetworkedhypertextprotocolanduserinterface.ItprovidesaccesstomultipleservicesanddocumentslikeGopherdoesbutismoreambitiousinitsmethod.AjumptootherInternetservicecanbetriggeredbyamouseclickona“hotlinked”word,image,oricononthewebpage.
16.Oneofmanyreasonsfordevelopingacellularmobiletelephonesystemanddeployingitinmanycitiesistheoperationallimitationsofconventionalmobiletelephonesystem:
limitedservicecapability,poorserviceperformance,andinefficientfrequencyspectrumutilization.
17.Amajorproblemfacingtheradiocommunicationindustryisthelimitationoftheavailableradiofrequencyspectrum.Insettingallocationpolicy,theFederalCommunicationsCommissionseekssystemswhichneedminimalbandwidthbutprovidehighusageandconsumersatisfaction.
18.Theidealmobiletelephonesystemwouldoperatewithinalimitedassignedfrequencybandandwouldserveanalmostunlimitednumberofusersinunlimitedareas.Threemajorapproachestoachievetheidealare:
1)Single-sideband,whichdividestheallocatedfrequencybandintomaximumnumbersofchannels.
2)Cellular,whichreusestheallocatedfrequencybandindifferentgeographiclocations.
3)Spreadspectrum,frequency-hopped,whichgeneratesmanycodesoverawidefrequencyband.
19.In1971,thecomputerindustryenteredanewera.Microprocessorsandminicomputersarenowusedforcontrollingmanycomplicatedfeaturesandfunctionswithlesspowerandsizethanwaspreviouslypossible.Large-scaleintegratedcircuittechnologyreducedthesizeofmobiletransceiverssothattheyeasilyfitintothestandardautomobile.Theseachievementswereafewoftherequirementsfordevelopingadvancedmobilephonesystemsandencouragingengineerstopursuethisdirection.
20.Why800MHz?
TheFCC’sdecisiontochoose800MHzwasmadebecauseofseverespectrumlimitationsatlowerfrequencybands.Forexample,FMbroadcastingservicesoperateinthevicinityof100MHz.Thetelevisionbroadcastingservicestartsat41MHzandextendsupto960MHz.Therefore,itwashardfortheFCCtoallocateaspectruminthelowerportionsofthe30to400MHzbandsincetheservicesofthisbandhadbecomesocrowded.Ontheotherhand,mobileradiotransmissioncannotbeappliedat10GHzorabovebecauseseverepropagationpathloss,multipathfading,andrainactivitymakethemediumimproperformobilecommunications.
21.Abasiccellularsystemconsistsofthreeparts:
amobileunit,acellsite,andamobiletelephoneswitchingoffice.Themobileunitcontainsacontrolunit,atransceiver,andanantennasystem.ThecellsiteprovidesinterfacebetweentheMTSOandthemobileunits.Ithasacontrolunit,radiocabinets,antennas,apowerplant,anddataterminals.TheMTSOisthecentralcoordinatingelementforallcellsites,containingthecellularprocessorandcellularswitch.Itinterfaceswithtelephonecompanyzoneoffices,controlscallprocessingandhandlesbillingactivities.
22.Theradioandhigh-speeddatalinksconnectthethreesubsystemsofthecellularmobiletelephonesystem.Eachmobileunitcanonlyuseonechannelatatimeforitscommunicationlink.Butthechannelisnotfixed;itcanbeanyoneintheentirebandassignedbytheservingarea,witheachsitehavingmultichannelcapabilitiesthatcanconnectsimultaneouslytomanymobileunits.Thecellularswitchswitchescallstoconnectmobilesubscriberstoothermobilesubscribersandtothenationwidetelephonenetwork.
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