2计算机专业英语.ppt

Incomputer storagedevicestypicallyrefertomassstoragedevicesthatarecapableofstoringlargequantityofdata evenafterelectricalpowertothecompteristurnoff Makingordownloadingunauthorizedcopiesofsoftwareisbreakingthelaw nomatterhowmanycopiesorpeopleareinvolved 制作或者下载未经授权的软件复制件都是违法的 无论涉及多少复制件或人员 在计算机中 存储设备通常是指那些即使是在计算机关机断电之后 也可以存储大量数据的大容量存储设备 Computernetworks Today networksareeverywhereandnetworktechnologyisevolvingrapidly Justwhenyouthinkyouhavedial upundercontrol alongcomescableaccessandWi Fi Justasyougetcomfortablewithyourschool scampus widenetwork everyoneisissuednotebookcomputerthatusesomesortofwirlessconnections EvolutionofComputerNetworks Computernetworksemergedinthelate1960s Theyhaveinheritedmanyusefulpropertiesformtheirpredecessors namely olderandmorewidelyadoptedtelephonenetworks Thisisnotsurprising sincebothcomputersandtelephonesareuniversalinstrumentsofcommunications however computernetworkshavebroughtsomethingnewintotheworldofcommunications namely thepracticallyinexhaustiblestoreofinformationaccumulatedbyhumancivilizationduringtheseveralthousandyearsofitsexistence Thisbecameespeciallynoticeableinthemid 1990s whentherapidgrowthoftheInternetclearlydemonstratedthatfreeandanonymousaccesstoinformationandinstant writtencommunicationswerehighlyvaluedbymostindividuals CommunicationBasics Thedatatransmittedcanbevoice text video images sound oracombinationofthem Fortransmittingthedata therearetwotypesofsignals analoganddigital Analogsignals acontinuouselectricalsignalintheformofawavecalledacarrierwave Telephones radios andtelevisions theolderformsofcommunications weredesignedtoworkwithananalogsignal Digitalsignals useson offelectricalpulsesindiscrete ratherthanacontinuouswave Thistwo statesignalrepresentsthetwo statebinarylanguageof0and1thatcomputeruse Directionoftransmission Therearethreetypesoftransmissionmodes Simplex thecommunicationisunidirectional Onlyoneofthetwostationonalinkcantransmit theothercanonlyreceive Forexample aradiostationisasimplexchannelbecauseitalwaystransmitsthesignaltoitslistenersandneverallowsthemtotransmitback Half Duplex isasinglephysicalchannelinwhichthedirectionmaybereversed Messagesmayflowintwodirections butneveratthesametime Full Duplex itallowssimultaneousmessageexchangeinbothdirections Itreallyconsistsoftwosimplexchannels aforwardchannelandareversechannel linkingthesamepoints Communicationschannels Therearemanykindsofcommunicationschannels twisted pairwireitistheordinarycopperwirethatconnectshomeandmanybusinesscomputerstothetelephonecompany coaxialcableitistheprimarytypeofcablingusedbythecabletelevisionindustryandisalsowidelyusedforcomputernetworks suchasEthernet Althoughmoreexpensivethanstandardtelephonewire itismuchlesssusceptibletointerferenceandcancarrymuchmoredata Moreover itcancarryvoiceanddataatafasterrate uptoabout200Mbps comparedto16 100Mbpsfortwisted pairwire broadcastradioitisawirelesstransmissionmediumthatdistributesradiosignalsthroughtheairoverlongdistancessuchasbetweencities regions andcountriesandshortdistancessuchaswithinanofficeorhome Forradiotransmissions youneedatransmittertosendthebroadcastradiosignalandareceivertoacceptit Toreceivethebroadcastradiosignal thereceiverhasanantennathatislocatedintherangeofthesignal Cellularradioitisaformofbroadcastradiothatisusedwidelyformobilecommunications specificallywirelessmodemsandcellulartelephones SomemobileusersconnecttheirnotebookcomputerorothermobilecomputertoacellulartelephonetoaccesstheWeb sendandreceivee mail enterachatroomwhileawayfromastandardtelephoneline MicrowaveCommunicationSatellitesInfrared CommunicationProtocols Inorderfordatapacketstotravelfromasourcetoadestinationonanetwork itisimportantthatallthedevicesonthenetworkspeakthesamelanguageorprotocol Aprotocolisasetofrulesthatmakecommunicationonanetworkmoreefficient Somecommonexamplesareasfollows Whiledrivingacar driversshouldsignalwhentheywishtomakeaturn iftheydon t thentheroadswillbechaos Whileflyinganairplane pilotsobeyveryspecificrulesforcommunicationwithotherairplanesandwithairtrafficcontrol Theprotocolincommunicationssoftwarewillspecifyhowreceivingdevicesacknowledgesendingdevices aprocesscalledhandshaking OSIReferenceModel IntheOSIreferencemodel therearesevennumberedlayers eachofwhichillustratesaparticularnetworkfunction Thisseparationofnetworkingfunctioniscalledlayering EachindividualOSIlayerhasasetoffunctionthatitmustperforminorderfordatapacketstotravelfromasourcetoadestinationonanetwork AlthoughtheOSIreferencemodelisuniversallyrecognized thehistoricalandtechnicalopenstandardoftheInternetisTransmissionControlProtocol InternetProtocol TCP IP TheTCP IPreferencemodelandtheTCP IPprotocolmakedatacommunicationpossiblebetweenany twocomputers anywhereintheworld LetusnowturnfromtheOSIreferencemodeltothereferencemodelusedinthegrandparentofallcomputernetworks theARPANET anditssuccessor theworldwideInternet TheARPANETwasaresearchnetworksponsoredbytheU SDepartmentofDefense Iteventuallyconnectedhundredsofuniversitiesandgovernmentusingleasedtelephonelines Whensatelliteandradionetworkswereaddedlater theexistingprotocolshadtroubleworkingwiththem soanewreferencearchitecturewasneeded ThisarchitecturelaterbecameknownastheTCP IPReferenceModel Theseapplicationsincludethefollowings TCP IP TransmissioncontrolprotocolFTP filetransferprotocolSMTP simplemailtransferprotocolDNS domainnamesystemPOP3 postofficeprotocol3 TheTCP IPmodelemphasizesmaximumflexiblity attheapplicationlayer fordevelopersofsoftware Thetransportlayerinvolvestransmissioncontrolprotocolanduserdatagramprotocol Thelowestlayer thenetworkaccesslayer referstotheparticularLANorWANtechnologythatisbeingused TheBluetoothTechnology BluetoothhasbeenregardedasanewwaveintheITsectorfromthedayofitsorigin Bluetoothiscontinuingitssuccessstory ThisnewtechnologyissupportedbytheSpecialInterestGroup SIG TheSIG foundedin1998 workstopromoteBluetoothtechnologyandconsistsofmembersoflikeIBM Intel Microsoft andNokia Whenputinsimpletermsabluetoothdeviceiscomprisedofabase bandprocessor aradioandanantenna Thebase bandprocessorconvertsthedataintosignalswhichradioscandecipher Thesesignalsarechangedintoafrequencyof2 4GHz whichtheradiotransmitsthroughtheantenna TheantennaofanotherBluetoothdevice withinatleast30feet receivesatransmittedsignalintheair HaraldBlatand CommunicationDevices Anetworkcanstandalone butitcanalsoconnecttoothernetworks eithersimilarordifferentintechnology Hardwareandsoftwareconbinationsareusedasinterfacestomaketheseconnections ThefollowingcommunicationdevicesaremainlyusedinLANS Modem Computersusedigitalsignals However manyofourpresentcommunicationslines suchastelephoneandmicrowave arestillanalog Togetaroundthisproblem youneedamodemtoconvertthedigitalsignalofacomputertoanalogandviceversa Thewordmodemisderivedfromacombinationofthewordsmodulate tochangeintoananalogsignal anddemodulate toconvertananalogsignalintoadigitalsignal Amodemcanbeanexternaloraninternaldevice Anexternalmodemisastand alonedevice Thisdevicemaybelargeoritmaybeportable evenpocketsize Aninternalmodemisacircuitboardthatplugsintoaslotinsidethesystemcabinet orisintegratedintothemotherboard Nowadays nostmicrocomputerscomewithaninternalmodemalreadyinstalled Twoadvantagesofaninternalmodemare itdoesnottakeupextraspaceonyourdesk anditdoesnothaveaseparatepowercord Forlaptopcomputer modemsareonPC PCMCIA cards ThePCMCIA personalcomputermemorycardinternatioanlassociation isaninternationalstandardsandtradeassociationwithover200membercompanies Itwasfoundedin1989toestablishstandardsforIntegratedCircuitCardsandtopromoteinterchangeabilityamongmobilecomputers Astheneedsofmobilecomputerusershavechanged sohasthePCcardstandard Today thePCMCIApromotestheinteroperabilityofPCcardsnotonlyinmobilecomputers butinsuchdiverseproductsasdigitalcameras cableTV set topboxes andautomobiles NetworkInterfaceCard Anetworkinterfacecard NIC isaprintedcircuitboardthatfitsintotheexpansionslotonacomputer smotherboard Itisalsocalledanetworkadapterandprovidesaportforconnectingtothenetwork NICisconsideredLayertwodevicesbecauseeachindividualNICthroughouttheworldcarriesauniquecode calledaMediaAccessControl MAC address Thisaddressisusedtocontroldatacommunicationforthehostonthenetwork hub Hub Ahubjoinsmultiplecomputerstogethertoformasinglenetworksegment Onthisnetworksegment allcomputerscancommunicatedirectlywitheachother Thepurposeofahubistoregenerateandretimenetworksignals Repeater Arepeaterregenerateselectrical wirelessoropticalsignals WithphysicalmedialikeEthernet datatransmissionscanonlyspanalimiteddistancebeforethequalityofthesignaldegrades Repeatersattempttopreservesignalintegrityandextendthedistanceoverwhichdatacansafetytravel RepeaterareclassifiedasLayer1devicesintheOSImodel becausetheyactonlyonthebitlevel switch Switch AswitchisaLayer2device Infactaswitchiscalledamulti portbridge justlikeahubiscalledamulti portrepeater ThedifferencebetweenthehubandswitchisthatswitchesmakedecisionsbasedonMACaddressandhubsdonotmakedecisionsatall Becauseofthedecisionsthatswitchesmake theymakeaLANmuchmoreefficient router Router ArouterisahighlyintelligentdevicethatsupportsconnectivitybetweenbothlikeandunlikeLANs andbetweenLANsandWANsorMANs Thepurposeofarouteristoexamineincomingpackets choosethebestpathforthemthroughthenetwork andthenswitchthemtotheproperoutgoingport Theyenablevirtuallyanytypeofcomputertocommunicatewithanyothercomputeranywhereintheworld ClassificationofComputerNetworks Computernetworkscanbeclassifiedbasedonseveralfactors suchasbandwidth connectionmethodandfunctionalrelationship Themostknownclassificationisbasedonthephysicalsizeofthenetwork Belowisalistofthemostcommontypesofcomputernetworks Apersonalareanetwork PAN PANisacomputernetworkusedforcommunicationamongcomputerdevices telephonesandPDA closetooneperson ThereachofaPANistypicallyafewmeters PANsmaybewiredwithcomputerbusessuchasUSB AwirelesspersonalareanetworkcanalsomadepossiblewithnetworktechnologiessuchasIrDAandBluetooth LocalAreaNetwork LAN CampusAreaNetwork CAN MetropolitanAreaNetwork MAN WideAreaNetwork WAN IntranetExtranet Topologyofcomputernetworks Networktopologyisthestudyofthearrangementormappingofthenodesofanetwork especiallythephysicalandlogicalinterconnectionsbetweennodes Thearrangementormappingoftheelementsofanetworkgiverisetocertainbasictopologieswhichmaythenbecombinedtoformmorecomplextopologies hybridtopology Themostcommonofthesebasictypesoftopologiesare Bus Linear Star Ring Mesh Tree Hybrid Allthesetypesarephysicaltopologies whichrefertothemappingofthenodesofanetworkandthephyscialconnectionsbetweenthem harddiskwriteinternalfilesharedownloadaccessWebapplicationserver ALANisagroupofcomputersandassociateddevicesthatshareacommoncommunicationslineandtypically theresourceofasingleprocessoror withinasmallgeographicarea Typically asuiteof programscanbekeptontheLANserver Userswhoneedanapplicationfrequentlycan itonceandthenrunitfromtheirlocal UserscanorderprintingandotherservicesasneededthroughapplicationsrunontheLANserver Ausercanshare withothersattheLANserver share server application harddisk download files readingand accessismaintainedbyaLANadministrator ALANservermayalsobeusedasa serverifsafeguardsaretakentosecure applicationsanddatafromoutside writing Web internal access TheRiseofBaidu BaiduisaChinesesearchenginewhichcansearchtextandimages TheChineseword Baidu translatesto hundredsoftimes inEnglish Baiduprovidesanindexof740millionwebpages 80millionimagesand10millionmultimediafiles ThenameofBaiduwasinspiredbyapoemconstructedintheciformbythepoetXinQiji writtenmorethan800yearsagoduringtheSongDynasty Thepoemcomparesthesearchfora Beautyamidchaoticglamourwiththesearchforone sdreamwhileconfrontedbylife smanyobstacles Hundredsandthousandsoftimes forherIsearchedinchaos suddenly Iturnedbychance towherehelightswerewaning andthereshestood Baidu literallymeaninghundredsfotimes representpersistentsearchfortheideal Baidu smostpopularfeatureisitssupportformultimediasearch called MP3Search ItcanlocatefileformatssuchasMP3 WMAandSWF Themultimediasearchfeatureismainly UsedinsearchesforChinesepopmusic Otherthanimagesandtextsearch Baiduprovidesseveralotherservices BaiduZhidao AQ AserviceBaiduPost AdisussiongroupservicebasedonkeywordsBaiduBaike AwikilikeencyclopediaBaiduSpace AfreeSocialNetworkingplatformBaiduMap SimilartoGoogleMapsbutdetailedmapsareavailableonlyforbigcities BhaditsinitialpublicofferingthemorningofFriday 5August2005 Bopenedat 27 share AtthecloseofNasdaqtradingontharFriday Bsharesclosedat 122 54 up95 54fromitsopeningprice againof353 Bhasbeencalledthe GoogleofChina Nowadays BaiduhasbecomeChina sbiggestindependentInternetsearchengineandisoneofGoogle sstrongestrivalsinChina Yahoo spelledwithanexclamationmark isshortfor YetAnotherHierachicalOfficiousOracle Thisoddandratherlongnamewascoinedin1994bytwoElectricalEngineeringPhDcandidatesatStanfordUniversity DavidFiloandJerryYang Theoriginalname David sandJerry sGuidetotheWorldWideWeb wasappropriate butnotexactlycatchy Theyusedthedictionarytocomeupwith yahoo atermthatanyonecanrememberandsaywithease Moreimportantly JerrryandDavidsaidtheylikedthedefinationofayahoo rude unsophisticated uncouth Intheend thewordYahoo didroughlydescribeitasawebsearchdirectory Theterm hierachical describedhowtheYahoo databasewasarrangedindirectorylayers Theterm oracle wasintendedtomean sourceoftruthandwisdom And officious described themanyofficeworkerswhoshouldusetheYahoo Databasewhilesurfingfromwork JerryandDavidlovedtosurftheWeb TheWebwasonly5yearsoldandstillrelatively small in1994 butthousandsofwebsitesbeingcreateddaily itwasratherdifficulttofindanythingfast So thetwostudentsbuiltYahoo astheirownguidetotheWorldWideWeb Intheirownwords theywere justtryingtotakeallthatstuffandorganizeittomakeituseful JerryandDavidspentmanynightscollatingthelistoftheirfavoritewebsitesintotheYahoo database Atfirst thelistwasmanageablebut soon itbecametoolargetonavigatewithease Thisiswhenthehugelistdividedintocategories Afterawhile thecategoriesalsobecametoofullandhadtobesplitintosubcategories This ofcourse becamewhatisknownasthe contextbased searchconceptbehindYahoo Subsequently theYahoo audiencegrewquickly Withinayear theStanfordnetworkbecamesocloggedwithYahoo websearchtraffic JerryandDavidhadtomovetheirYahoo DatabasetotheNetscapeoffices DavidandJerryhadrecognizedYahoo spotentialasacorporatebusiness andincorporatedinMarchof 95 TheybothlefttheirgraduatestudiestoworkonYahoo fulltime NowYahoo hasbecomealeadingglobalInternetcommunication commerceandmediacompanythatoffersnetworkservicestomorethan345millionindividualseachmonthworldwide Exercises Ascomputerusagegrows eventhesmallestbusinessescantakeadvantageofnetworkingtosharethecostofperipherals suchasprintersandscanners andprovideaccesstoshareddata Wirelessnetworkingproductshavebecomemorepopularinthelastfewyearsduetoanincreaseincompetitionamongmanufacturersandtheemergenceofamoredominantwirlesstechnologystandard 随着计算机使用的增长 即使最小规模的企业也可以利用联网来共享诸如打印机和扫描仪这样的外围设备以及访问共享数据 由于各个生产厂商的竞争以及更多主要无线技术标准的出现 无线网络产品在最近几年越来越受欢迎 TCP IPconsistofmanyotherassociatednetworkprotocolsandowesitexistencetoISO OSIwhichnowservesasreferencetoallotherprotocols TCP IP由很多其他相互关联的网络协议组成 它的存在应该感谢现在已经作为所有其他协议的参考模型的ISO OSI DatabaseandInformationSystems Thedatabaseisthecentralcomponentofamoderninformationsystemforbusiness governmentorscientificorganizations Forexample goingtothebanktodepositorwithdrawfunds makingahotelorairlinereservation accessingacomputerizedlibrarycatalogtosearchforabibliographicitem ororderingamagazinesubscriptionfromapublisher chancesarethattheseactivitieswillinvolvesomeoneaccessingadatabase DataandInformation Datacanbedefinedasacollectionofrawunprocessedfacts figuresandsymbols Inthepast datawaslimitedtonumbers lettersandsymbolsrecordedbykeyboards Nowdataismuchricherandinclueds audio music photographsandvideo Informationisdatathatisorganized meaningful anduseful Typicalexamplesofinformationstoredforsomepracticalpurposeare informationcollectedforthesakeofmakingastatisticalanalysis e g thenationalcensus orasurveyofcracksinastretchofmotorway DataIntegrity Foracomputertoproducecorrectinformation thedatathatisinputintoadatabasemusthaveintegrity Dataintegrityisthedegreetowhichdataiscorrect Amisspelledmovietitleinamoviedatabaseisanexampleofincorrectdata Whenadatabasecontainsthesetypesoferrors itlosesintegrity Themoreerrorsthedatacontains thelessitsintegrity Dataintegrityisveryimportantbecausecomputersandpeopleuseinformationtomakedecisionsandtakeactions Forexample whenyouorderaproduct suchasabook aprocessbeginsthateventuallydeliversyouthebook Beforeyoureceivethebook theretailerusuallychargestheordertoyourcreditcard Theretailerwillbillanincorrectamounttoyourcreditcardifthebook spriceisnotcorrectinitsdatabase Thistypeoferrorcausesbothyouandtheretailerextratimeandefforttoremedy Database Databaseanddatabasetechnologyarehavingmajorimpactonthegrowinguseofcomputer Itisfairtosaythatdatabasesplayacriticalroleinalmostareaswherecomputersareused includingbusiness engineering medicine law educationandlibraryscience ADatabase DB isanintegratedcollectionoflogicallyrelatedfiles Itincludesacollectionofdataorganizedsoyoucanaccess retrieve and usethedata Databasearedesignedtoofferanorganizedmechanismforstoring managingandretrievinginformation Theydosothroughtheuseoftables Databasetablesconsistofcolumnsandrows Eachcolumncontainsadifferenttypeofattributeandeachrowcorrespondstoasinglerecord Forexample imagineadatabasetablethatcontainsnamesandtelephonenumbers Thecolumnswouldprobablybetitled FirstName LastName and PhoneNumber Rowswouldbeadded underneaththosecolumnscontainingthedatatobestored Atthispoint theobviousquestion ifadatabaseissomuchlikeaspeadsheet whynotjustuseaspreadsheet Databaseareactuallymuchmorepowerfulthanspreadsheetsinthewaysinwhichdatacanbemanipulated Herearejustafewoftheactionswhichcanbeperformedonadatabasethatwouldbedifficulttoperformonaspeadsheet Retrieveallrecordsthatmatchcertaincritera UpdaterecordsinbulkCross referencerecordsindifferenttablesPerformcomplexaggregatecalculations RelationalDatabase ItisadatabasebasedontheretionalmodeldevelopedbyE F Codd therelationaldatabasetak