外文翻译计算机网络和数据库Word下载.docx
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外文翻译计算机网络和数据库Word下载.docx
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Somereasonsarecausingcentralizedcomputersystemstogivewaytonetworks.
Thefirstoneisthatmanyorganizationsalreadyhaveasubstantialnumberofcomputersinoperation,oftenlocatedfarapart.Initially,eachofthesecomputersmayhaveworkedinisolationfromtheotherones,butatacertaintime,managementmayhavedecidedtoconnectthemtobeabletocorrelateinformationabouttheentireorganization.Generallyspeaking,thisgoalistomakeallprograms,data,andotherresourcesavailabletoanyoneonthenetworkwithoutregardtothephysicallocationoftheresourceandtheuser.
Thesecondoneistoprovidehighreliabilitybyhavingalternativesourcesofsupply.Withanetwork,thetemporarylossofasinglecomputerismuchlessserious,becauseitsuserscanoftenbeaccommodatedelsewhereuntiltheserviceisrestored.
Yetanotherreasonofsettingupacomputernetworkiscomputernetworkcanprovideapowerfulcommunicationmediumamongwidelyseparatedpeople.
ApplicationofNetworks
Oneofthemainareasofpotentialnetworksueisaccesstoremotedatabase.Itmaysomedaybeeasyforpeoplesittingattheirterminalsathometomakereservationsforairplanestrains,buses,boats,restaurants,theaters,hotels,andsoon,atanywhereintheworldwithinstantconfirmation.Homebanking,automatednewspaperandfullyautomatedlibraryalsofallinthiscategory.Computeraidededucationisanotherpossiblefieldforusingnetwork,withmanydifferentcoursesbeingoffered.
Teleconferencingisawholenewformcommunication.Withitwidelyseparatedpeoplecanconductameetingbytypingmessagesattheirterminals.Attendeesmayleaveatwillandfindoutwhattheymissedwhentheycomeback.Internationalcontactsbyhumanbeginmaybegreatlyenhancedbynetworkbasedcommunicationfacilities.
NetworkStructure
Broadlyspeaking,therearetwogeneraltypesofdesignsforthecommunicationsubnet:
(1)Point–to–pointchannels
(2)Broadcastchannels
Inthefirstone,thenetworkcontainsnumerouscablesorlesasedtelephonelines,eachoneconnectingapairofnodes.Iftwonodesthatdonotshareacablewishtocommunicate,theymustdothisindirectlyviaothernodes.Whenamessageissentfromnodetoanotherviaoneormoreintermediatemodes,eachintermediatenodewillreceivethemessageandstoreituntiltherequiredoutputlineisfreesothatitcantransmitthemessageforward.Thesubnetusingthisprincipleiscalledapoint–to–piontorstore–and–forwardsubnet.
Whenapoint–to–pointsubnetisused,theimportantproblemishowtodesigntheconnectedtopologybetweenthenodes.
Thesecondkindofcommunicationarchitectureusesbroadcasting.Inthisdesignthereisasinglecommunicationchannelsharedbyallnodes.Theinherenceinbroadcastsystemsisthatmessagessentbyanynodearereceivedbyallothernodes.
TheISOReferenceModel
TheReferenceModelofOpenSystemInterconnection(OSI),asOSIcallsit,hassevenlayers.Themajoronesoftheprinciples,fromwhichOSIappliedtogetthesevenlayers,areasfollows:
(1)Alayershouldbecreatedwhereadifferentlevelofabstractionisneeded.
(2)Eachlayershouldperformawelldefinedfunction.
(3)Thefunctionofeachlayershouldbechosenwithaneyetowarddefininginternationallystandardizedprotocols.
(4)Thelayerboundariesshouldbechosentominimizetheinformationflowacrosstheinterfaces.
(5)Thenumberoflayersshouldbelargeenoughsothatdistinctneednotbeputtogetherinthesamelayerwithoutnecessity,andsmallenoughsothatthearchitecturewillnotbecomeoutcontrol.
ThePhysicalLayer
Thephysicallayerisconcernedwithtransmittingrawbitsoveracommunicationchannel.Typicalquestionsherearehowmanyvoltsshoulebeusedtorepresentan1andhowmanya0,howmanymicrosecondsabitoccupies,whethertransmissionmayproceedsimultaneouslyinbotharefinished,howtoestablishtheinitialconnectionandwhatkindoffunctioneachpinhas.Thedesignissuesherelargelydealwithmechanical,electricalandproceduralinterfacingtothesubnet.
Thedatalinklayer
Thetaskofthedatalinklayeristoobtainarawtransmissionfacilityandtotransformitintoalinethatappearsfreeoftransmissionerrorstothenetworklayer.Itaccomplishesthistaskbybreadingtheinputdataupintodataframes,transmittingtheframessequentially,andprocessingtheacknowledgmentframessentbackthereceiver.
Sincethephysicallayermerelyacceptsandtransmitsastreamofbitswithoutanyregardtomeaningorstructure,itcancreateandrecognizeframeboundariesuntilthedatalinklayer.Thiscanbeaccomplishedbyattachingspecialbitspatternstothebeginningandtheendoftheframe.Butitproducetwoproblems:
oneisanoiseburstonthelinecandestroyaframecompletely.Inthiscase,thesoftwareinthesourcemachinemustretransmittheframe.Theotheristhatsomemechanismmustbeemployedtoletthetransmitterknowhowmuchbufferspacethereceiverhasatthemoment.
Thenetworklayer
Thenetworklayercontrolstheoperationofsubnet.Itdeterminesthechiefcharacteristicsofthenode-hostinterface,andhowpackets,theunitsofinformationexchangedinthislayer,areroutedwithinthesubnet.
Whatthislayerifsoftwaredoes,basically,istoacceptmessagesfromthesourcehost,convertthemtopackets,andobservethepacketstogetthedestination.Thekeydesignissueishowtherouteisdetermined.Itcouldnotonlybaseonstatictable,eitherare“wiredinto”thenetworkandrarelychanged,byalsoadopthighlydynamicmanner,whichcandeterminepacketagaintoreflectthecurrentnetworkload.
Thetransportlayer
Thebasicfunctionoftransportlayeristoacceptdatafromthesessionlayer,splititupintosmallerunits,ifnecessary,passthesetothenetworklayer,andensurethatthepiecesallarrivecorrectlyattheotherend.
Thislayerisatrueend-to-endlayer.Inotherwords,aprogramonthesourcemachinecarriesonaconvenestationwithassimilarprogramonthedestinationmachine,usingthemessageheaderandcontrolmessages.
Thesessionlayer
Withthesessionlayer,theusermustnegotiatetoestablishaconnectionwithaprocessonanothermachine.Theconnectionisusuallycalledasession.Asessionmightbeusedtoallowausertologintoaremotetime-sharingsystemortotransferafilebetweentwomachines.Theoperationofsettingupasessionbetweentwoprocessesisoftencalledbinding.
Anotherfunctionofthesessionlayeristomanagethesessiononceithasbeensetup.
Thepresentationlayer
ThepresentationlayercouldbedesignedtoacceptASCⅡstringsasinputandproducecompressedbitpatternsasoutput.Thisfunctionofthepresentationlayeriscalledtextcompression.
Inaddition,thislayercanalsoperformothertransformations.Encryptionprovidesecurityisonepossibility.Conversionbetweencharactercodes,suchasASCⅡtoEBCDIC,mightoftenbeuseful.Moregenerally,differentcomputersusuallyhaveincompatiblefileformats,soafileconversionoptionmightbeusefulattimes.
Theapplicationlayer
Manyissuesoccurhere.Forexample,alltheissuesofnetworktransparency,hidingthephysicaldistributionofresourcesfromuser.Anotherissueisproblempartitioning:
howtodividetheproblemamongthevariousmachineinordertotakemaximumadvantageofthenetwork.
2.Databasesystem
Theconceptionusedfordescribingfilesanddatabaseshasvariedsubstantiallyinthesameorganization.
Adatabasemaybedefinedasacollectionofinterrelateddatastoredtogetherwithaslittleredundancyaspossibletoserveoneormoreapplicationsinanoptimalfashion;
thedataarestoredsothattheyareindependentofprogramswhichusethedata;
acommonandretrievingexistingdatawithinthedatabasesiftheyareentirelyseparateinstructure.
Adatabasemaybedesignedforbatchprocessing,real-timeprocessing,orin-lineprocessing.Adatabasesysteminvolveapplicationprogram,DBMS,anddatabase.
Oneofthemostimportantcharacteristicsofmostdatabasesisthattheywillconstantlyneedtochangeandgrow.Easyrestructuringofthedatabasemustbepossibleasnewdatatypesandnewapplicationsareadded.Therestructuringshouldbepossiblewithouthavingtorewritetheap0plicationprogramandingeneralshouldcauseaslittleupheavalaspossible.Theeasewithwhichadatabasecanbechangedwillhaveamajoreffectontherateatwhichdata-processingcanbedevelopedinacorporation.
Thetemdataindependenceisoftenquotedasbeingoneofthemainattributesofadatabase.Itimpliesthatthedataandtheapplicationprogramswhichusethemareindependentsothateithermaybechangedwithoutchangingtheother.Whenasinglesetofdataitemsservesavarietyofapplications,differentapplicationprogramsperceivedifferentrelationshipsbetweenthedataitems.
Toalargeextent,data-baseorganizationisconcernedwiththerepresentationbetweenthedataitemaboutwhichwestoreinformationreferredtoasentities.Anentitymaybeatangibleobjectornontangible.Ithasvariouspropertieswhichwemaywishtorecord.Itcandescribestherealworld.Thedataitemrepresentsanattribute,andtheattributemustbeassociatedwiththerelevantentity.Wedesignvaluestotheattributes,oneattributehasaspecialsignificanceinthatitidentifiestheentity.
Anattributeorsetofattributeswhichthecomputerusestoidentifyarecordortupleisreferredtoasakey.Theprimarykeyisdefinedasthatkeyusedtouniquelyidentifyonerecordortuple.Theentityidentifierconsis
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