Basicdefinition
ModemistheabbreviationofModulator(modulator)andDemodulator(demodulator),Chinesecalledmodem,accordingtothehomophonyofModem,itisaffectionatelycalled"cat",Isanelectronicdevicethatcanrealizethemodulationanddemodulationfunctionsrequiredforcommunication.Generallyconsistsofamodulatorandademodulator.Atthesendingend,thedigitalsignalgeneratedbythecomputerserialportismodulatedintoananalogsignalthatcanbetransmittedthroughthetelephoneline;atthereceivingend,themodemconvertstheanalogsignalinputtothecomputerintoacorrespondingdigitalsignalandsendsittothecomputerinterface.Inpersonalcomputers,modemsareoftenusedtoexchangedataandprogramswithothercomputers,andtoaccessonlineinformationserviceprograms,etc.
Externalmodem(4photos)
Theso-calledmodulationistoconvertadigitalsignalintoananalogsignaltransmittedonatelephoneline;demodulationistoconvertananalogsignalIntoadigitalsignal.Collectivelyreferredtoasamodem.ModemisMODEMinEnglish,anditservesasa"translator"foranaloganddigitalsignals.Therearetwotypesofelectronicsignals,oneis"analogsignal"andtheotheris"digitalsignal".Thetelephonelineweusetransmitsanalogsignals,whilethedigitalsignalsaretransmittedbetweenPCs.SowhenyouwanttoconnectyourcomputertotheInternetthroughatelephoneline,youmustuseamodemto"translate"thetwodifferentsignals.AfterconnectingtotheInternet,whenthePCsendsinformationtotheInternet,becausethetelephonelinetransmitsananalogsignal,amodemmustbeusedto"translate"thedigitalsignalintoananalogsignalbeforeitcanbetransmittedtotheInternet.Thisprocessiscalled"modulation"".WhenthePCobtainsinformationfromtheInternet,sincetheinformationtransmittedfromtheInternetthroughthetelephonelineisallanalogsignals,thePCmustalsousethemodem"translation"tounderstandthem.Thisprocessiscalled"demodulation"..Ingeneral,itiscalled"modulationanddemodulation."
Historicaldevelopment
Modemwasoriginallydevelopedforthesemi-automaticgroundairdefensesystem(SAGE)inthe1950s.Itwasusedtoconnectterminals,radarstationsandcommandcontrolcentersatdifferentbasestotheUnitedStatesandSAGECommandCenterinCanada.SAGErunsonadedicatedline,buttheequipmentusedatbothendsisnotthesameastheModematall.IBMisthesupplierofcomputersandmodemsintheSAGEsystem.Afewyearslater,ameetingbetweenthechiefexecutiveofficer(CEO)ofAmericanAirlinesandaregionalmanagerofIBMledtothe"mini-SAGE"airlineautomaticbookingsystem.Inthissystem,aterminallocatedintheticketingcenterisconnectedtothecentralcomputertomanagethevalidityandtimeoftheticket.Thissystem,calledSabre,isanearlyprototypeoftheSABREsystem.
Intheearly1960s,theapplicationofcommercialcomputersgraduallybecamepopular,aswellastheabove-mentionedtechnologicalachievements.In1958,AT&Treleasedthefirstcommercialmodem,Bell103.Usingtwotonestorepresent1and0frequencyshiftkeyingtechnology,103hasbeenabletoachieveatransmissionspeedof300bit/s.ThesuccessorversionBell212wasdevelopedinashorttimeandmovedtoamorestableshiftkeyingtechnologytoincreasethedatarateto1200bit/s.AsystemlikeBell201usesatwo-waysignalsettoachieve2400bit/son4pairsofdedicatedlines.
Varioustypesofmodems(19photos)
HeshiSmartModemisamajoradvancement.HeshiCommunicationwassuccessfullydevelopedin1981.SmartModemisasimple300bpsModem,usingBell103signalingstandard,built-inasmallcontroller,canletthecomputersendcommandstocontrolthetelephoneline,suchasoff-hook,dial,redial,hangupandotherfunctions.Beforesmartmodems,almostallmodemsneedtwostepstocreateaconnection:thefirststepistomanuallydialthenumberoftheotherpartyonthephone,andthenputthereceiverintheacousticcouplerattachedtothemodem,andoneisusedAdevicecomposedoftworubbercupsusedtoswitchbetweensoundsignalsandelectricalsignals.Theuseofsmartmodemsnolongerrequiresacousticcoupler,butdirectlyconnectsthemodemtoastandardtelephonelineorsocket.Thenthecomputercanautomaticallycompletethefunctionofconnectingtothephoneanddialingthenumber.Thischangegreatlysimplifiestheinstallationanduseofbulletinboardsystems(BBS).
Themodemratehasnotchangedmuchuntilthe1980s.TheUnitedStatesgenerallyusesa2400bit/ssystemsimilartoBell212,whiletheEuropeansystemisslightlydifferent.Bythelate1980s,mostmodemscouldsupportallthestandardsatthattime,and2400bit/swasgraduallypopularized.Alargenumberofstandardsforspecificpurposeshavealsobeenadded,usuallyusinghigh-speedchannelstoreceivelow-speedchanneltransmissions.AtypicalexampleistheFrenchMinitelsystem,whereuserterminalsreceiveinformationmostofthetime.TheModemoftheMinitelterminaluses1200bit/storeceivedataat75bit/sandsendcommandsbacktotheserver.
Basicprinciples
Thefrequencyrangeofordinarypeople’svoiceis300-3400Hz.Inordertotransmitvoicesignalsinordinarytelephonesystems,acertainbandwidthisallocatedtoitontheline.Internationalstandardstake4KHzisthefrequencybandwidthoccupiedbyastandardvoicechannel.Inthistransmissionprocess:thevoicesignalisinputatafrequencyof300-3400Hz,andthesender’sphoneconvertsthevoicesignalintoananalogsignal,whichischangedbyafrequencydivisionmultiplexer,sothatthelinecanbesimultaneouslytransmittedAfterthemulti-channelanalogsignalarrivesatthereceivingend,itwillgothroughafrequency-removalprocesstorestoreittotheanalogsignalintheoriginalfrequencyrange,andthentheanalogsignalwillbeconvertedintoasoundsignalbythereceiver'stelephone.
Theinformationinthecomputerisadigitalsignalcomposedof"0"and"1",butonlyanalogelectricalsignalsaretransmittedonthetelephoneline.Usinganalogchannelstotransmitdigitalsignalswithouttakinganymeasureswillinevitablycausegreaterrors(distortion).Therefore,totransmitdataonordinarytelephonenetworks,thedigitalsignalsmustbeconvertedintotheaudiofrequencyspectrumrequiredbytheoriginaldesignofthetelephonenetwork(thatis,300Hz).－3400Hz).
Modulationistousethebasebandpulsetocontrolacertainparameterofthecarrierwaveformtoformasignalsuitableforlinetransmission.
Demodulationiswhenthemodulatedsignalarrivesatthereceivingend,theanalogsignaltransformedbythemodulatorisremovedfromthecarrierandrestoredtotheoriginalbasebanddigitalsignal.
Usingamodemcanalsoconvertaudiosignalsintohigherfrequencysignalsandconverthigherfrequencysignalsintoaudiosignals.Soanotherpurposeofmodulationistofacilitatelinemultiplexinginordertoimprovelineutilization.Basedonthethreemainparametersofthecarriersignal,themodulationmethodscanbedividedintothreetypes:amplitudemodulation,frequencymodulationandphasemodulation.
Classificationmethod
ExternalModem
TheexternalModemisplacedoutsidethechassisandconnectedtothehostthroughtheserialcommunicationport.ThiskindofModemisconvenientanddexterous,easytoinstall,andtheflashingindicatorlightisconvenientformonitoringtheworkingconditionoftheModem.However,externalModemrequiresadditionalpowersupplyandcables.
Built-inModem
Built-inModem(6sheets)
Wheninstallingthebuilt-inModem,thecaseneedstobedisassembledandtheterminalmustbeSettingupwiththeCOMportmakestheinstallationmorecumbersome.ThiskindofModemoccupiestheexpansionslotonthemotherboard,butdoesnotrequireadditionalpowersupplyandcables,andthepriceischeaperthananexternalModem.PCMCIAplug-incardtype
Theplug-incardtypeModemismainlyusedinnotebookcomputersandhasacompactsize.Withmobilephone,mobileofficecanbeeasilyrealized.
RackModem
RackModemisequivalenttoagroupofModemisconcentratedinaboxorshell,andpoweredbyaunifiedpowersupply.RackModemismainlyusedinthecentralcomputerroomofInternet/Intranet,telecommunicationsbureau,campusnetwork,financialinstitutionandothernetworks.
Inadditiontotheabovefourcommonmodems,thereareISDNmodemsandamodemcalledCableModem,aswellasanADSLmodem.CableModemusesthecableofthecableTVtotransmitsignals.Itnotonlyhasthefunctionofmodulationanddemodulation,butalsointegratesrouters,hubs,andbridges.Thetheoreticaltransmissionspeedcanreachmorethan10Mbps.ThroughtheCableModemtosurftheInternet,eachuserhasanindependentIPaddress,whichisequivalenttohavingapersonaldedicatedline.ShenzhenCableTVTianweiNetworkCompanyhaslaunchedthiskindofInternetaccessservicebasedoncableTVnetwork,theaccessrateis2Mbps-10Mbps!
ModemwithUSBinterface
TheemergenceofUSBtechnologyprovidesafasterandsimplerconnectionmethodforcomputerperipherals.SHARKtakestheleadA56KmodemwithaUSBinterfacewasintroduced.Thismodemonlythesizeofapagerreallybroughtchallengestothetraditionalserialportmodem.JustconnectittotheUSBinterfaceofthehost.Usuallythereare2USBinterfacesonthehost,andtheUSBinterfacecanconnect127devices.Ifyouwanttoconnectmultipledevices,youcanalsobuyaUSBhub.UsuallyUSBmonitorsandprinterscanbeusedasUSBhubs,becausetheyprovide1-2USBinterfacesinadditiontotheUSBinterfaceconnectedtothehost.
TransmissionMode
Modemwasoriginallyonlyusedfordatatransmission.However,withthecontinuousgrowthofuserdemandandfiercecompetitionamongmanufacturers,moreandmore"two-in-one"and"three-in-one"Modemhaveappearedonthemarket.Inadditiontodatatransmission,thesemodemsalsohavefaxandvoicetransmissionfunctions.
1.Faxmode(FaxModem)
FaxingviaModem,inadditiontosavingthecostofadedicatedfax,therearemanybenefits:Youcandirectlyfaxthedocumentsinthecomputertotheotherparty’scomputerorfaxmachinewithoutprintingthedocumentsfirst;youcaneasilysaveoreditthereceivedfax;itcanovercomethehandwritingcausedbyordinaryfaxmachinesduetotheuseofthermalpaperTheproblemisgraduallydisappearing;becauseModemuseserrorcorrectiontechnology,thefaxqualityisbetterthanordinaryfaxmachines,especiallyforgraphicfaxes.FaxModemmostlyfollowsV.29andV.17faxprotocols.V.29supports9600bpsfaxrate,whileV.17cansupport14400bpsfaxrate.
2.VoiceModem
Thevoicemodemainlyprovidestelephonerecordingandmessagerecordingandfull-duplexhands-freecallfunctions,whichtrulymakesthephoneandThecomputerisintegrated.Here,itismainlyanewvoicetransmissionmode-DSVD(DigitalSimultaneousVoiceandData).DSVDisavoicetransmissionstandardproposedbyHayes,Rockwell,U.s.Robotics,Intelandothercompaniesin1995.ItisanextensionoftheexistingV.42errorcorrectionprotocol.DSVDadoptsDigiTalk'sdigitalvoiceanddatasimultaneoustransmissiontechnology,sothattheModemcancarryoutdatatransmissionandtalkontheordinarytelephoneline.
DSVDModemreserves8Kbandwidth(someModemsreserve8.5Kbandwidth)forvoicetransmission,andtheremainingbandwidthisusedfordatatransmission.Thevoicewillbecompressedbeforetransmission,andthenintegratedwiththedatatobetransmitted,andtransmittedtotheotheruserthroughthetelephonecarrier.Atthereceivingend,theModemfirstseparatesthevoicefromthedata,andthendecompressesthevoicesignalandperformsdigital/analogconversion,therebyrealizingthesimultaneousinterpretationofdata/voice.DSVDModemhasbroadapplicationprospectsinremoteteaching,collaborativework,andonlinegames.BecausethepriceofDSVDModemismoreexpensivethanordinaryVoiceModem,andtorealizethedata/voicesimultaneoustransmissionfunction,theotherpartyneedstouseDSVDModem,whichhindersthepopularizationofDSVDModemtoacertainextent.
Transmissionrate
Modem’stransmissionratereferstotheamountofdatatransmittedbytheModempersecond.Generallyspeaking,14.4K,28.8K,33.6K,etc.,refertothetransmissionrateoftheModem.Thetransmissionrateisinbps(bitspersecond).Therefore,a33.6KModemcantransmit33600bitsofdatapersecond.Modemcompressesdataduringtransmission,sothedatathroughputof33.6KModemcantheoreticallyreach115200bps,oreven230400bps.
ThetransmissionrateoftheModemisactuallydeterminedbythemodulationprotocolsupportedbytheModem.TheV.32.V.32bis,V.34.V.34+,V.fc,etc.seenonthemodemboxormanualrefertothemodulationprotocoladoptedbythemodem.V.32isanasynchronous/synchronous4800/9600bpsfull-duplexstandardprotocol;V.32bisisanenhancedversionofV.32thatsupportsatransmissionrateof14400bps;V.34isasynchronous28800bpsfull-duplexstandardprotocol;andV.34+Itisthestandardprotocolofsynchronousfull-duplex33600bps.TheabovestandardsareallformulatedbytheITU(InternationalTelecommunicationsUnion),whileV.fcisa28800bpsmodulationprotocolproposedbyRockwell,butithasnotbeenwidelysupported.
WhenitcomestothetransferrateofModem,youcan'thelpbutmentionthe56KModem,whichissohotnowadays.Infact,the56Kstandardhasbeenproposedformanyyears,butbecauseofthelong-standingexistenceoftwoincompatiblestandards,namelyK56flexledbyRockwellandX2ledbyU.S.Robotics,56KModemhasnotbeenpopularizedforalongtime.InFebruary1998,withtheeffortsoftheInternationalTelecommunicationUnion,the56KstandardwasfinallyunifiedintoITUV9.0.Manymodemmanufacturershavealsointroducedupgrademeasures,andmodemsthattrulysupportV9.0havealsoblossomedeverywhere.56Kisexpectedtobecomethemainstreamofthemarketwithinonetotwoyears.SincemanydomesticISPsdonotprovide56Kaccessservices,itisbesttocheckwithyourserviceproviderbeforepurchasing56KModemtoavoidwaste.
Thetransmissionratesmentionedaboveareallderivedunderidealconditions.Inactualuse,themodemspeedoftencannotreachthenominalvalue.Theactualtransmissionratemainlydependsonthefollowingfactors:
1.Thequalityofthetelephoneline
Becausethemodulatedsignalistransmittedthroughthetelephoneline,ifthequalityofthetelephonelineisnotgood,themodemwillreducetheratetoensureaccuracy.Forthisreason,whenconnectingtheModem,trytoreducethelengthofthecable,cutofftheexcesscable,anddonotstackitinacircle.Inaddition,itisbestnottouseanextension,andtheconnectionshouldalsoavoidpassingthroughinterferencesourcessuchasTVsets.
2.Isthereenoughbandwidth?
Iftherearealotofpeoplegoingonlineatthesametime,itwillcausecongestionandcongestionoftheline,andthetransmissionrateoftheModemwillnaturallydecrease.Therefore,whethertheISPcanprovideenoughbandwidthisverycritical.Inaddition,avoidingtheInternetduringbusyhoursisalsoasolution.Especiallywhendownloadingfiles,thetimespentdownloadingduringbusyhoursandnon-peakhourswillbeseveraltimesdifferent.
3.Themodemrateoftheotherparty
Themodulationprotocolsupportedbythemodemisbackwardcompatible,andtheactualconnectionratedependsonthepartywiththelowerrate.Therefore,iftheotherparty’sModemis14.4K,evenifthe56KModemisused,itcanonlybeconnectedatarateof14400bps.
TransmissionProtocol
ModemtransmissionprotocolsincludeModulationProtocols,ErrorControlProtocols,DataCompressionProtocolsandFileTransmissionProtocols.Themodulationprotocolhasbeenintroducedabove,andtheremainingthreetransmissionprotocolsareintroduced.
ErrorProtocol
AsthetransmissionrateofModemcontinuestoincrease,noiseonthetelephoneline,abnormalsuddenchangesincurrent,etc.,willcausedatatransmissionerrors.Whattheerrorcontrolprotocolneedstosolveishowtoensuretheaccuracyofdatainhigh-speedtransmission.Therearetwoindustrystandardsforerrorcontrolprotocols:MNP4andV4.2.Amongthem,MNP(MicrocomNetworkProtocols)isatransmissionprotocolformulatedbyMicrocom,includingMNP1-MNP10.Forcommercialreasons,MicrocomonlyannouncedMNP1-MNP5,ofwhichMNP4isoneofthewidelyusederrorcontrolprotocols.V4.2isanimprovedversionofMNP4formulatedbytheInternationalTelecommunicationUnion,whichincludestwocontrolalgorithms,MNP4andLAP-M.Therefore,aModemthatusestheV4.2protocolcanestablishanerror-freecontrolconnectionwithaModemthatonlysupportstheMNP4protocol,butnotviceversa.Therefore,whenpurchasingaModem,itisbesttochooseaModemthatsupportstheV4.2protocol.
Inaddition,somecheapmodemcardsonthemarketdonothaveharderrorcorrectionfunctionstoreducecosts,butusesoftwareerrorcorrectionmethods.Everyoneshouldpayattentiontothedistinctionwhenbuying,anddon'tbeconfusedbythewords"witherrorcorrectionfunction"onthebox.
Datacompressionprotocol
Inordertoincreasethedatatransmissionvolumeandshortenthetransmissiontime,mostmodemsnowcompressthedatafirstduringtransmission.Similartotheerrorcontrolprotocol,therearealsotwoindustrystandardsforthedatacompressionprotocol:MNP5andV4.2bis.MNP5usestwocompressionalgorithms,Run-LengthcodingandHuffmancoding,withamaximumcompressionratioof2:1.WhileV4.2bisusesLempel-Zivcompressiontechnology,themaximumcompressionratiocanreach4:1.ThisiswhyV4.2bisisfasterthanMNP5.Itshouldbenotedthatthedatacompressionprotocolisbasedontheerrorcontrolprotocol,MNP5needsthesupportofMNP4,andV4.2bisalsoneedsthesupportofV4.2.And,althoughV4.2includesMNP4,V4.2bisdoesnotincludeMNP5.
File
Filetransferisthemainformofdataexchange.Duringfiletransfer,inordertoenablethefiletobecorrectlyidentifiedandtransferred,aunifiedtransferprotocolneedstobeestablishedbetweenthetwocomputers.Thisagreementincludestheidentificationofdocuments,thestartandendtimeoftransmission,andthejudgmentandcorrectionoferrors.Commontransferprotocolsareasfollows:
ASCII:Thisisthefastesttransferprotocol,butitcanonlytransfertextfiles.
Xmodem:Thisancienttransmissionprotocolisslower,butduetotheCRCerrordetectionmethod,thetransmissionaccuracycanbeashighas99.6%.
Ymodem:ThisisanimprovedversionofXmodem.Ituses1024-bitsectiontransmission,whichisfasterthanXmodem.
Zmodem:Zmodemusesastreamingtransmissionmethod,whichhasafastertransmissionspeed.Italsohasfunctionssuchasautomaticchangeofsectionsize,resumabletransmissionofbreakpoints,andfasterrordetection.Thisisthemostpopularfiletransferprotocol.
Inadditiontotheabove,thereareprotocolssuchasImodem,Jmodem,Bimodem,Kermit,andLynx.
Installationmethod
TheinstallationprocessofModemcanbedividedintotwosteps:hardwareinstallationandsoftwareinstallation:hardwareinstallationandsoftwareinstallation.
Modeminstallation
1.ExternalModeminstallation:
Thefirststep:connectthetelephoneline.InserttheRJ11plugofthetelephonelineintotheLineinterfaceoftheModem,andthenusethetelephonelinetoconnectthePhoneinterfaceoftheModemtothetelephone.
Step2:Turnoffthepowerofthecomputer,andconnectoneendofthecable(25-pinmaleend)ofthemodemtothemodem,andtheotherend(9-pinor25-pinplug)totheCOMportonthehostconnect.
Step3:ConnectthepowertransformertothePOWERorACinterfaceoftheModem.Afterthepoweristurnedon,theMRindicatoroftheModemshouldbeonforalongtime.IftheMRlightdoesnotturnonorflashescontinuously,itmeansthatthemodemhasnotbeeninstalledcorrectlyorthemodemitselfismalfunctioning.ForaModemwithvoicefunction,theSPKinterfaceoftheModemshouldalsobeconnectedtotheLineIninterfaceonthesoundcard.Ofcourse,itcanalsobedirectlyconnectedtoanoutputdevicesuchasheadphones.
Inaddition,theMICinterfaceoftheModemisusedtoconnectanelectretmicrophone,butitisbettertoconnectthemicrophonetothesoundcard.
2.Installationofbuilt-inModem:
Thefirststep:Accordingtotheinstructionsofthemanual,settherelevantjumpers.SinceCOM1andCOM3,COM2andCOM4shareaninterrupt,itcanusuallybesettoCOM3/IRQ4orCOM4/IRQ3.
Step2:Turnoffthepowerofthecomputerandopenthecase,inserttheModemcardintoanyvacantexpansionslotonthemotherboard.
Step3:Connectthephoneline.InserttheRJ11plugofthetelephonelineintotheLineinterfaceontheModemcard,andthenusethetelephonelinetoconnectthePhoneinterfaceontheModemcardtothetelephone.Pickupthephoneatthistimeandyoushouldbeabletomakeacallnormally.
Softwareinstallation
Whenthehardwareinstallationiscomplete,turnonthecomputer,theexternalmodemshouldalsoturnonthemodemswitch.Formostmodems,Windows98willreport"foundanewhardwaredevice".Atthistime,youonlyneedtoselect"driversprovidedbythehardwaremanufacturer"andinsertthemodeminstallationdisk.IfthemodemfailstobedetectedafterWindows98isstarted,youcanalsocompletetheinstallationaccordingtothefollowingsteps:
Thefirststep:entertheWindows95"ControlPanel",double-clickthe"Modem"icon,andinthepropertieswindowClickthe"Add"buttoninthe
Step2:Check"Donotdetectthemodem,butwillselectonefromthelist",andthenclick"Next"
ThirdStep:SelectthecorrespondingmanufacturerandmodelintheModemlist,andthenclick"Next".OrafterinsertingtheinstallationdiskoftheModem,select"InstallfromDisk".ToprovewhethertheModemisinstalledsuccessfully,youcanusethephonedialerintheWindows98accessorytomakearandomcall.Ifitissuccessful,itmeansthattheModemhasbeeninstalledcorrectly.ForInternetusers,dial-upnetworkingandprotocolsneedtobeinstalled.
ThemeaningoftheModemindicatorlight:
POWER:Powerindicatorlight
DSL(ADSL-LINK):Signallight,flashesrapidlyafterbeingturnedon,andthenalwayslightsgreen.Underworkingconditions,conditionsotherthanconstantlightareabnormal
ADSL-ACT:Signaldatalight,whichshineswhenthereisdatatransmission,andisalwaysdimmedatalltimes
ETH(ETHNET)(LAN-LINK):LANlight,itwillalwaysberedwhenturnedon,indicatingthattheconnectionbetweenyournetworkcardandthemodemisnormal,otherwisepleasecheckyournetworkcardandnetworkcardcable(thethickerone)
LAN-ACT:LANdatalight,flasheswhenthereisdatatransmission,alwaysdim
MR:Modemisreadyandsuccessfullypassedself-test.
TR:Theterminalisready.
SD:Modemissendingdata.
RD:Modemisreceivingdata.
OH:Off-hookindication,Modemisoccupyingthetelephoneline.
CD:Carrierdetection,themodemissuccessfullyconnectedtotheotherparty.
RI:Modemisinautomaticresponsestate.SomemodemsarerepresentedbyAA.
HS:High-speedindication,therateisgreaterthan9600.
PS:Ifallthelightsonthemodemarealwaysonandthebroadbandcannotbeconnected,pleaseturnoffthemodempowerandtryagainlaterandextendtheinterval.Ifallthelightsarestillonaftertheabovemethodistried,pleaseContactthelocaltelecommunicationsdepartmenttoreportmodemproblems.
Chip-related
Modemchipsareliketheprocessor’sbrand.Thereareproductsfromdifferentmanufacturers.Amongthem,theROCKWELLchiphasthelargestshare,whichaccountsforTheglobalmarketshareisabout70%,anditsstatusissimilartothatofIntelintheprocessormarket.MostofthedomesticexternalModemproductsuseROCKWELLchips.ThesecondisTIchips.ThefamousUSR"BigBlackCat"usesTIchips.Inaddition,thereareCirrusLogicproducts,buttherearefewerexternalModemsusingthischip.Ingeneral,theperformanceandstabilityoftheModemusingtheROCKWELLchiparebetterthanthoseusingotherchips,butthe"bigblackcat"ofUSRisaspecialcase.Intheinternationalmarket,thetransactionpriceofTIchipsislowerthanthatofROCKWELLchips.
Modulationmethod
ThedigitaldebuggingmethodsincludeFSK,ASK,PSK,TCM,QAM,etc.
Performanceandprice
AlthoughAllROCKWELLchipsareused,butsome"cats"lovetodroptheconnection,andsome"cats"cannotreachthenominalperformance.Thedifferenceinmodemperformanceiscausedbyavarietyofreasons,firstisthemodemchipused,andsecondisthematerialselectionandcircuitdesign.Generally,thecircuitoftheModemadoptsthepublicboarddesignrecommendedbythechipmanufacturer,andthedifferenceinmaterialselectionisbig.Somemodemsproducedbysmallfactoriesusesecond-handcomponentsorlow-qualitycomponents,whichcausesthemodemtobecomeunstableduetoheatandotherreasonsafterworkingforalongtime.Inordertomaintaintheirreputation,well-knownmanufacturersgenerallyusehigh-reliabilitycomponents,whichsurpasssmallmanufacturersinperformanceandstability.Becauseofthis,theexternalModemofROCKWELLisalsoused.Themostexpensiveonecostsmorethan800yuan,whilethecheapestoneonlycoststensofyuan.
Mainbrands
TherearemanyforeignbrandsofModem,themorewell-knownonesareHeshi,USR,Diamond,Americantanksandsoon.ThebigblackcatofUSRisconsideredthebestintheheartsofmanyenthusiasts.AlthoughitusesTIchips,itissecondtononeintermsofperformanceandstability,andthevolumeofitsbuilt-inspeakerscanbeadjustedfreely,makingyoulikestayingupallnight.Thewormsdon’thavetoworryaboutthestrangenoisesoftheModemmakingtheneighborsnoisy.Oneofthetricksofthebigblackcatisnottodroptheconnection,unlessitisturnedoff,itwillalwayshangontheInternet.Diamondisawell-knownmanufacturerofmultimediaequipmentabroad.Diamond'sModemhasasmallappearance,exquisitematerialselection,andmoderateprice.Itisafineproductamongcats.
Domesticbrands"cats"alsohaveaconsiderablemarketshare.Themorewell-knowndomesticbrandsincludeOmni,Shida,Lenovo,etc.TheytakeChina'snationalconditionsintoconsiderationintheirdesign,andtheirpricesarerelativelyapproachable.ItisalsonecessarytomentionTaiwaneseproductshere.Taiwanisthemainproductionplaceofmodemproductsintheworld.Infact,althoughmanymodemsonthemarkethavedifferentbrands,theyareallOEM-producedbyTaiwanesemanufacturers,includingUSR's"BigBlackCat"andDiamond'sproductsareallinTaiwanOEMproduction.InadditiontoOEM,Taiwan'swell-knownmodembrandsincludeAcer,Kao,GVC,WISECOM,etc.DuetotherelativelylowtechnicalcontentofModem,themarketisstillfloodedwithalargenumberofModemproductsproducedbysmallfactoriesinsouthernChina.MostoftheseModemproductsarethree-noproducts,withpoorquality,butlowprices.
Purchasenotice
Foreignmodemsarenotnecessarilybetterthandomesticones.Manypeopleliketobuymodemsfromforeignbrands.Infact,mostofthosemodemsareOEMproducedinTaiwanProvince,buttheyaremoreparticularabouttheselectionofmaterials.Thedomesticmodemproductsaredesignedtotakeintoaccounttheactualdomesticsituation,thepriceisrelativelycheap,andtheperformanceisnotinferiortoforeignproducts.Don'tpayattentiontoappearance,thekeyistolookatinnerquality.Modemshellsonthemarketaregettingmoreandmoreflowery,andthecolorsaregettingmoreandmoregorgeous,butwhenyoubuyit,don'tbeconfusedbytheappearance.Theso-calledseriesproductslaunchedbymanymanufacturersdiffergreatlyinprice,butinfacttheyusethesamecircuitboard,buttheshellisdifferent.ThereisnoneedtospendafewhundredyuanfortheshelltobuyaModem.
TheexternalModemoftheUSBinterface,mostoftheexternalModemoftheUSBinterfacearesoft"cats",itissmallinsize,doesnotrequireanexternalpowersupply,andcanbehot-plugged,whichhasmanyadvantages.However,theconnectionrateofmanymodemswithUSBinterfaceistoolow,andmanycannotreacheven50K.AndsomemodemswithUSBinterfacewillhaveproblemswhendialing,soitisbettertobuyanexternalmodemwithserialinterface.Modemperformanceatalowpriceisnotnecessarilypoor.Thecheapest56KexternalModemonthemarketcostsonlyabout350yuan,andSupermanhimselfusesthisTP-LINK56K"cat"withapriceofonly300yuan.Afterusingitforaboutayear,thedisconnectionisdefinitelynotasfrequentasinthelegend.Theonlyproblemisthatthemaximumconnectionspeedisonly48K.
Relatedterms
Communicationprotocol
Thecommunicationprotocolisusuallycalledthe"datatransmissionstandard".Thegeneral56KbpsdatatransmissionstandardistheV.90protocolspecifiedbytheITU,whichallowsmodemstoachieveadatatransmissionrateof56Kpsonastandardtelephoneswitchingnetwork.ModemprotocolsareallloadedintheBIOS,solimitedupgradescanbeachievedbyrefreshingthecontentintheBIOS.
ErrorCorrectionCompressionProtocol
Innetworkcommunication,dataissentintheformofdatapackets,becauseofsignalattenuationandpoorlinequality,orinterference,etc.,oftenThereisaphenomenonthatdatapacketsarelostordamagedduringtransmission.Thefunctionoftheerrorcorrectionprotocolistodetectwhetherthereceiveddatapackethaserrors.Onceanerrorisfound,theerrorcorrectionprotocolwilltrytoregainthecorrectdatapacketortrytorepairthedamageddatapacketthroughanalgorithm.CommonerrorcorrectionprotocolsareV.42andMNPseries.V.42isanerrorcorrectionprotocollaunchedbyITU-T(InternationalTelecommunicationsUnion).Itsfunctionisthatoncethedatapacketsentbythesenderislost,thereceivercanimmediatelyrequesttheotherpartytoresendthedatapacket.MNPisaseriesofprotocolsproposedbyMicrosoft.Itisdividedinto10levelsfromMNP1toMNP10.Thehigherthelevel,thestrongerthefunctionandthebackwardcompatibility.ThefunctionofMNPisthatonceV.42failstocompletetheapplicationerrorpacketRe-sentthetask,itwilltrytocorrecttheerror.BothoftheseerrorcorrectionprotocolsareuniversallysupportedbyModem.TheV.42protocoladditionallybearsthetaskofdatacompression.
ATcommands
(ATCommands)
InventedbyHayes,ithasbecomethedefactostandardandismanufacturedbyallmodemsAmodemcommandlanguageusedbyvendors.Eachcommandstartswiththeletter"AT",hencethename.ATfollowedbylettersandnumbersindicatespecificfunctions.Forexample,"ATDT"isadial-upcommand,andothercommandsinclude"initializethemodem","controlspeakervolume","specifythenumberofringingtimesthemodemstartstoanswer",and"selecttheformatforerrorcorrection"Wait,theATcommandsofdifferentbrandsofmodemsarenotexactlythesame.PleasereadtheMODEMusermanualcarefullytouseATcommandscorrectly.
Baudrate
(BaudRate)
Therateofanaloglinesignal,alsocalledmodulationrate,expressedinwaveformpersecondMeasuredbythenumberofoscillations.Ifthedataisnotcompressed,thebaudrateisequaltothenumberofdatabitstransmittedpersecond.Ifthedataiscompressed,thenthenumberofdatabitstransmittedpersecondisusuallygreaterthanthemodulationrate,sothattheexchangeofbaudandbitspersecondoccasionallyproducesmistake.
DCE
Theacronymfor"DataCommunicationEquipment".DCEprovidesfunctionstoestablish,maintain,andterminateconnections,andamodemisakindofDCE.
DTE
Theacronymfor"DataTerminalEquipment".DTEprovidesorreceivesdata.ThecomputerconnectedtothemodemisatypeofDTE.
Others
LineRate
AlsoknownasDCErate,theunitisbit/s(bps).Referstothedatatransferrateonthetelephoneline(ordedicatedline)connectingtwomodems.Commonratesare56000bps,334bps,28800bpsandsoon.
PortRate
AlsoknownasDTErateormaximumthroughput.Referstothetransferratefromthecomputerserialporttothemodem.SincealmostallmodemstodaysupportthisrateofV.42bisandMNP5compressionstandards(compressionratiosareboth4:1),thisrateisgenerallymuchhigherthanthelinerate.
DedicatedLine/Dialup
Dedicatedlinereferstotwoordinarypassive(oractive)wires.Thereisnodialtonefordialingonadedicatedline,sospecialhardwaresupportisrequired.Thedial-uplineisanordinarytelephoneline,whichisdialedthroughthetelephonesystem.Commonmodemsallsupportdial-uplines,butnotnecessarilydedicatedlines.
RemoteSetup
Afterthelocalmodemisconnectedtotheremotemodem,theremoteusercansettheparametersofthelocalmodem.
Databitsandflowcontrol
WhentheModemtransmitsdata,everytimeagroupofdataistransmitted,thedatapacketmustcontainthecorrespondingcontroldata.Therearedifferentdatabitandendbitstandardsunderdifferentcommunicationenvironments.FlowcontrolisusedtocoordinatethedataflowtransmissionbetweentheModemandthecomputer.ItcanpreventdatalosscausedbythemismatchofthecommunicationprocessingspeedbetweenthecomputerandtheModem.Flowcontrolisdividedintotwoforms:hardwareflowcontrol(RTS/CTS)andsoftwareflow(XON/XOFF)control.
Data/VoiceSimultaneousInterpretation(SVD)
Theso-calleddata/voicesimultaneousinterpretationmeansthatyoucanuseordinarytelephoneswhileMODEMisconductingdatacommunication.call.Accordingtodifferentimplementationmethods,therearetwotypesofdata/voicesimultaneousinterpretation(ASVD:AnalogSimultaneousVoiceandData)anddigitaldata/voicesimultaneousinterpretation(DSVD:DigitalSimultaneousVoiceandData).