Introduction
Thecommunicationcontrollermanagesthedatainputandoutputtothehostorcomputernetwork.Itcanbeacomplexfront-endcomputerinterfaceorsimpledevicessuchasmultiplexers,bridges,androuters.Thesedevicesconverttheparalleldataofthecomputerintoserialdatatransmittedonthecommunicationline,andcompleteallnecessarycontrolfunctions,errordetectionandsynchronization.Modernequipmentalsocompletesdatacompression,routing,securityfunctions,andcollectsmanagementinformation.
Mainfunctions
Themainfunctionsofthecommunicationcontrollerare:①Provideanelectricalinterfacewiththedatacircuitandwiththehost;②Accordingtotheprincipleofserial-parallelconversion,theserialonthedatacircuitThebitstreamisassembledintocharacters,orviceversa,thecharactersaredisassembledintoaserialbitstream;③Theconversionofthedatatransmissionrateonthecircuitandthetransmissionrateofthehost;④Theconversionofthetransmissioncodeandtheinternalcodeofthehost;⑤Theimplementationoftransmissioncontrolprocedures,suchasBasicdatacommunicationcontrolprocedures,advanceddatalinkcontrolprocedures,etc.;⑥Detectionandcorrectionoftransmissionerrors,suchasverticalandhorizontalparitycheck,cycliccodecheck,etc.
Types
Therearemanytypesofcommunicationcontrollers.Usually,theycanbedividedintothreetypes:characterbuffer,codegroupbufferandmessagebufferaccordingtotheirabilitytobufferinformation.①Characterbuffermode:themodeinwhichthecommunicationcontrollertransmitsdatatothehostinunitsofcharacters.Duetothedevelopmentofelectronictechnology,someofthesecommunicationcontrollershavebeenassembledinthemainframecabinetandintegratedwiththemainframe.②Codegroupbuffermode:onthebasisofcharacterbuffermode,thefunctionofassemblingcharactersintocodegroupsanderrorcheckingofcodegroupsisadded,andthecodegroupisusedasaunittotransmittothehost.③Messagebuffermode:expandtobuffertheentiremessageonthebasisofthecodegroupbuffermode.Thefunctionofthiscommunicationcontrollerisquitecomplete,anditsscaleisrelativelylarge,equivalenttoasmallcomputer.Forthesakeofdistinction,thiscommunicationcontrolleriscalledthecommunicationcontrolprocessor,orCCPforshort.Variouscommunicationcontrollersaremainlycomposedoftwoparts:alineconnectionunitandalinecontrolunit.Thenumberoftheformercorrespondstothenumberofconnectedlines,andthelatterrealizeslinecontrolactions.
Thecommunicationcontrollerofcharacterbuffermodeisusedtoconnectasmallnumberofcommunicationlines;andthecommunicationcontrolprocessorofmessagebuffermodeisusedtoconnectmultiplecommunicationlines.ItcanbeusedasadataswitchandlinecentralizedAndfrontprocessoretc.Whatkindofcommunicationcontrollertouse,orhowtoallocatethefunctionsbetweenitandthehost,shouldbeconsideredbythedatacommunicationsystemdesignerbasedonrationalityandeconomy.
Anexampleisasfollows
Theterminalserverisusedtoconnectalargenumberofterminalstothehostsystem.Allterminalsareconnectedintoabox,whichisconnectedtothehostthroughanetworkoraremotelink.
Thefront-endprocessorprovidestheconnectionoftheterminalandnetworktothehostsystem.
Themultiplexermergesdatastreamsfrommultipledevicesintoonelineandusesvariousmediafortransmission.
Repeaters,bridges,androutersareusedtointerconnectthelocalareanetwork.
IntheIBMmainframecomputerenvironment,thecommunicationcontrollersareIBM3750,IBM3720,IBM3725andIBM3745.Thesedevicesareusedtoconnectremotegroupcontrollerstothehost.
Therearetwopossibleconnectionmethods
Inthefirstmethod,theremotegroupcontrollerisconnectedtothecommunicationcontrollerconnectedtothehostbytheremotecommunicationlink;
Inthesecondmethod,thecommunicationcontrollerisremoteandprovidesaconnectionpointformultiplegroupcontrollers.Thenthiscommunicationcontrollermanagestheinformationflowfromthegroupcontroller,andthenpassesthroughtheremotecommunicationlinkItistransmittedtothecommunicationcontrollerconnectedtothehostonthehostside.
Thecommunicationcontrollerisoftencalledtheforegroundprocessor,becauseitisaseparatedeviceofthehostsystemandhandlesallcommunicationswithexternaldevices,suchasterminals.Thispreventsthehostfrombeingcontinuouslyinterruptedbyexternaldevices,allowingittoprocessapplicationsmoreefficiently.
Thecommunicationcontrollerimplementsthefollowingfunctions
Establishacommunicationsessionbetweentheterminalandthenetworknode.
Managethedatacommunicationonthecommunicationlinkandcontroltheflowofdata.
Centralizetheconnectionofthegroupcontrollerandpollthegroupcontrollertoseeiftheyhavedatatotransmit.
Providebufferfordatainputoroutput.
Errordetectionandcorrection.
Providetheroutingfunctionofdatatransmissiontothedestination.
Thecommunicationcontrollerisactuallyacomputerwithabus,memoryandcentralprocessingunit(CPU)inside,andalsoincludesanadapterconnectedtothehostchannelandaninterfacecomponentconnectedtothegroupcontroller.TheCPUmanagesthedataflowbetweenthechanneladapterandtheinterfacecomponent.
TheIBMcommunicationcontrollerincludestheearlierdiscontinued3705andanewmodelwithenhancedfunctions:
IBM3705provides128to353communicationsatarateof23.4KbpsLink,canhandleBISYNC.SDLCorASCIIprotocol,itsupportsahostconnectedby8channels,anditreliesonthehostforcontrolanddiagnosis.
IBM3725isdesignedasareplacementforIBM3705.Therearetwomodels:Type1supportsupto256full-duplexcommunicationlinksand8hosts;Type2supports80full-duplexlinksand4hosts.
IBM3720isaninexpensivecontroller.Itisidealforlower-pricedmainframessuchastheIBM9370mainframe.Differentmodelsprovidedirectorremoteconnectionswith16to28communicationlinks,andalsohaveatokenringinterface.
IBM3745wasacommunicationcontrollerfirstintroducedin1988.Therearetwomodels:Model210hasonlyonecontrolunit;Model410hastwocontrolunits,whichcanworkastwoindependentcommunicationcontrollersoroneasabackuporbackupsupporttopreventsoftwareorhardwarefailures.Model210canbeupgradedtoModel410tosupportupto16hostsand8tokenringadapters.Thesedevicesalsoincludeaharddiskcontainingarestartsoftwaremodule.