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ToworkwithmodernwirelessnetworkssuchasUMTSandLTE,itisessential thatthetelecomprofessionalhas fullunderstandingof itsbasicconcepts,suchasthosethatcontrolthecallestablishmentandmaintenance,whetheritisvoice(CS)ordata(PS).

Inthisscenario,RABandRRCaretwoofthemostimportantconceptsbecausetheyareresponsibleforallthenegotiationinvolvedinthosecalls.

Inaddition toRABandRRC,westillhavesomeother termsdirectly involved incontext,asRB,SRB,TRB,amongothers.Thesetermsarealso importantconcepts,sincewithoutthemRABandRRCcouldnotexist.

SoletstrytounderstandtodaythesimplestpossiblewaywhatistheRRCandRABroleinthecallsofthesemobilenetworksinpractice.Asitbecomenecessary,wewillalsotalkaboutotherconcepts.

Note:AlltelecomHallarticlesareoriginallywritteninPortuguese.FollowingwetranslatetoEnglishandSpanish. As our time is short, maybe you find some typos (sometimes we just use the automatictranslator,with only a final and 'quick' review).Weapologize andwehaveanunderstandingof oureffort. If you want to contribute translating / correcting of these languages, or even creating andpublishingyourtutorials,pleasecontactus:contact.

Introduction

To start, we can divide a call into two parts: the signaling (or control) and data (or information).Alreadyaheadofkeyconcepts,wecanunderstandtheRRCasresponsibleforthecontrol,andtheRABasresponsiblefortheinformationpart.

Asmentioned, other auxiliary concepts are involved in calls, but ourgoal today is to learn themostbasicconceptsRRCandRAB,allowingustoevolveinourlearninglater.

Oddlyenough,evenprofessionalswhoalreadyworkwithUMTSWCDMAandLTEnetworkshavetroubleto fullyunderstand theconceptsofRRCandRAB.Andwithout this initialunderstanding,hardly theycanevolvewithclarityandefficiencyintheirdailywork.

Withoutfurtherintroduction, let'sgostraighttothepointandthentrytounderstandonceandforallthesesoimportantconcepts.

What is RRC and RAB?

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Analogy

As always, and as usual the telecomHall, let's make an analogy that helps us to understand thefunctioningoftheRRCandRABinpractice.

Let'sstartimaginingthefollowingscenario:twopeoplearecutoffbyacliff.Ontheleftside,aperson(1)wanttobuysomethingsthatareforsale inastore(2)ordepositontherightside.Intherightside, in addition to the deposit, we also have a seller (3), which will help the buyer to contact(negotiable)withthedeposit.

As additional or auxiliary objects (4),we have some iron barswith different sizes, and some cars someliketrainwagon,otherslikeremotecontrolcars.

Inshort,wehavethesituationoutlinedintheimagebelow.

Andso,howthissituationcanbesolved?

Let'scontinuewithapossiblesolution:thebuyerontheleftwritehisrequestinanote,tieonasmallstonethathefoundonthefloor,andsend(1)ittothesellerontheotherside.So,thestonecarrytheinformationorinitialrequest.

Thesellerreceivestherequest,butsheneedtosendittothedeposit,inorderfortheshoppingtobesent.Shesends the requestona remotecontrolcar (1),which runapreviouslydemarcatedpath tothedeposit.

Sometimelater,thedepositresponsearrivestoseller(1),whichthencheckstoseewhethershewillbeabletosendthedataornot.

So that we can proceedwith our call, let's consider a positive response. That is, what the buyer iswilling,orthe'resources'areavailable.

Seller realizes that to fulfill the request,andbeable tosend thepurchases, shewillneed tobuilda'path' (1)between the twoendsof thecliff, so thewagonscouldcarryoverwith theorders/receiptsandpurchases.Then,thesellerusessomeofitsironbarsandcreatesalinkbetweenthetwosides.

Onceestablishedallthewaybetweenthoseinvolved,requestscanbesentfrombothsidesaswellasthepurchasesoranyotherinformationcanbetransferredbydifferentpathsandwagons/cars!

If you managed to understand how the above problem was solved, congratulations, you justunderstandhowthemostcommonformofUMTSWCDMAandLTEcommunicationhappens!

Although analogies are not perfect, it help us a lot to understand the complex functioning of thesenetworks,especiallyinrelationtonewconceptssuchasRRCandRAB,butalsoaveryoftenused,the'bearer'somuchthatit'sworthtalkingalittlebitaboutit.

What is Bearer?

Ifwesearchtheword'bearer' inthedictionary,we'll findsomethingliketrasnporter,orcarrier.Inasimpleway:onewhocarriesorconveyssomethingfromsomepointtoanotherpoint.Inarestaurant,wecancomparethe'bearer'toawaiter.

Butfromthetelecommunicationspointofview,'bearer'isbestunderstoodasa'pipe'thatconnectstwoormorepointsinacommunicationsystem,throughwhichthedataflows.

Technicallyspeaking, it isachannelthatcarriesVoiceorData,alogicalconnectionbetweendifferentpoints (nodes) that ensures that the packets that are traveling have the same QoS attributes.Explaining better: for each 'bearer' we have several associated parameters, such as the maximumdelay and packet loss limit and these attributes that make sure each packet going in the samechannelhavethesameQoSattributes.

General Flowchart RRC, RAB and Others

Nowthatweknowwhatisbearer,let'sgobacktotheanalogypresentedearlier,butnowbringingittothereal,moretechnicalside.

All thatwe'll talkcanbesummarized inasingle figure,havingall theconceptsseentoday,andthatwillbedetailedfromnowon.

Note:Ifyoumanagetounderstandtheconceptsthatwillbeexplainedinthefigurebelow,youwillbewith a great base for bothWCDMA and LTE networks. This is because, in order to facilitate we useWCDMA nomenclatures, but the principle is pretty much the same in LTE. Just do the equivalentreplaces,likeNodeBforeNB.

On that ficticious scenario, the seller is the UTRAN, responsible for creating and maintaining thecommunication between the UE (buyer) and CN (deposit) so that theQoS requirements of each aremet.

UTRAN:UMTSTerrestrialRadioAccessNetworkNodeBRNC

UE:UserEquipmentCN:CoreNetwork

MSC:forswitchedvoiceservicesSGSN:forpacketswitchedservices

ThecliffistheUuInterfacebetweentheUEandtheUTRAN,andtheroadthroughtheremotecontrolcargoesuntilthedepositistheIuInterface,betweentheUTRANandCN.

Sendingrequestsandreceipts ispartofsignaling,ortheRRC.Theshipmentofpurchases isthedatapart,or theRAB. Inour scenario, theRRCare theRails, andRAB is the full serviceof sendingdatabetweentheUEandtheCN.

RRC:RadioResourceControlRAB:RadioAccessBearer

Note:theRRCisinLayer3controlplane,whiletheRABoccursbetweentheUEandCN,intheuserplane.

TherailcarsaretheRBs,andconveytheinformationintheradiopath.Thesewagonsdefinewhattypeofthingwillbetransported,andinwhatquantity.Similarly,theRBsdefinewhattypeofdatawillintheRRC,whichcanbeDataorSignaling.When theQoSattributeschange, then theRbsassociatedwiththatRRCconnectionneedtobereconfigured.

TheremotecontrolcarsaretheIubearer,andcarryinformationonIuInterface(betweentheUTRANandtheCN),eitherCSorPS.

RB:RadioBearerIubearer:IuBearerInterface

Note:RABisthecombinationofRBandIubearer.

AsexamplesofRABforsomeservicesanddifferentrateswehave:

TheConversationalRABandtheInteractiveRABcanbeusedtogether,andinthiscasewehaveacaseofMultiRAB.

TheRBisalayer2connectionbetweentheUEandtheRNC,andcanbeusedforSignallingandcontrolUserData.When it isused forSignallingorControlMessages is calledSRB.Andwhen it isused foruserdataiscalledTRB.

SRB:SignallingRadioBearer(ControlPlane)TRB:TrafficRadioBearer(UserPlane)

Note: inanoptimizednetwork,wecanfindmuchof thetrafficbeinghandledbyHSPAbearers,evenMultiRAB.ThisoptionfreesresourcesfromCE(ChannelElements),relievingtheloadonR99(thatcanonlyusetheseresources).However,itshouldbedonewithcaution,becauseifimproperlyconfigureditcandegradethePerformanceIndicatorswithBlockage(Congestion)andFailures.

Asyou'veprobablynoticed,we'retalkingaboutseveralnewtechnicalterms,butthesetermsarewhatyou'llfindforexamplewhenreadingUMTSorLTEcallflowcharts.Butifyoucanunderstandatleastinparttheconceptspresentedtoday,everythingwillbemucheasier.

Letusthentakealookagainonourfigure,andcontinueouranalogy.

Aswesaw,intelecomweworkwiththeconceptoflayers.Andthiswayofseeingthenetworkbringsus many advantages, mainly because we were able to 'wrap' physical access. In this way, anymodificationorreplacementcanbemadewithlesscomplexity.

We don't need to tell you howmuch the radio path is complex, continuously changing, right? Thisstructureusingbeares ensures this simplification: theRNCandCNbotherwithQoS requirements inthe path between them (Iu Interface) and only the RNC have toworry aboutmeeting the complexradiopathQoS.

Sure,butwhywehavetwotypesofcarrierswagonsandremotecontrolcars?Theanswertothisisinthe very characteristic of the two existing paths. Being the Iu a more robust interface, and alsobecausewehavemajorchanges inRABsduringconnections, it isnormal that thesebearersarealsodifferent for the paths. it's like using a 4x4 pickup truck to climb amountain, and a race car to an

asphaltrace.

Regardlessthecarriers,withtheRABtheelementsoftheCNhastheimpressionofaphysicalpathtotheUE,sodon'tneedtobeworryingaboutthecomplexaspectsofradiocommunication.

Forexample,aUEcanhave3RABsbetweenheandtheRNC,andtheseRABsmaybechanging,asinthecaseofsofthandovers,whiletheRNChasonly1Iubearerforthisconnection.

Fromthepointofviewofthecarriers,themaintaskoftheUTRANismanagingtheseservicesontheseinterfaces.ShecontrolstheUuinterface,andalongwiththeCN,controlstheprovisionofservices intheIuinterface.

RememberthatinacommunicationbetweentheUEandtheCN,severalotherelementsareinvolved,mainly to negotiate QoS requirements between both parties. These requirements aremapped in theRABs, that are visible to both (UE and CN), where the UTRAN is responsible for creating andmaintainingtheseRABssothatallofthisisservedinallaspects.

Alittlebitmoredetails...

ARRCconnectionexistswhenanUEperformsthecallestablishmentprocedure,andgetresourcesfromtheUTRAN.WhenaRRCconnectionisestablished,theUEwillalsogetsomeSRBs.(Ifforsomereasontheinitialrequestisnotaccepted,theUEcanmakeanewrequestaftersometime).

SincetheSRBwasestablishedbetweentheUEandtheCN,theRNCchecksaseriesofinformationsuchas the UE identity, what is the reason for the request and whether the UE is able to handle therequestedservice.

The RNC that maps the requested RABs into RBs, to transfer between the UE and the UTRAN. InadditionitisalsochecktheattributesoftheRABs:iftheycanbemetbytheavailableresources,andevenwhether toactivateor reset radiochannels (reconfigurationof lower layersservices )basedonthenumberofSignalingConnectionsandRABstobetransferred.

This way, it creates the impression that there is a physical path between the UE and the CN.Rememberingagain thatnomatterhowmanysignalingandRABsconnections therearebetweentheueandtheCNthereisonlyasingleRRCconnectionusedbytheRNCtocontrolandtransferbetweentheUEandtheUTRAN.

NowthatwehaveseenalotaboutRRCandRAB,let'slearnonlyafewmoreconceptstodayafterall,wealreadyhaveenoughinformationpresented.Let'stalkabouttheASandNAS.

ASAccessStratumisagroupofspecificprotocolsofaccessnetworkNASNONAccessStratum:so,aretheotherprotocols,orthosethatarenotaccessnetwork

Atthispointofview,theASprovidestheRABtotheNAS,orinformationtransferservice.

TheUEandCNneedtocommunicate(events/messages)witheachothertoperformseveralprocedureswithmanypurposes.Andthe'language'ofthisconversationbetweenthemiscalledprotocols.

Theprotocolsare then responsible forallowing this conversationbetween theUEandCN,andcausetheCNdonotworryaboutthemethodofaccess(beitGSM/GPRS,UTRAN,LTE).InourcasetheRNCactsasaprotocolbetweentheUTRANandCN.

Accordingtowhatwelearnedtoday,theRABiscarried:

Between the UE and the UTRAN: within the RRC connection. The RRC Protocol is responsible for negotiating the(logical)channelsofUuandIuBinterfaces,andfortheestablishmentofsignalingdedicatedchannelsasSRBsandRBsamongtheseinterfaces.Between the RNC and the CN: after being negotiated andmapped, in the RANAP protocol connection, through Iuinterface(CS/PS).

RANAP:RadioAccessNetworkApplicationPart

Aswehaveseenabove,theRNCmapsrequestedRABsintoRBsusingcurrentradionetworkresourcesinformation,andcontrolstheservicesoflowerlayers.Tooptimizetheuseoftheseresources,aswellas the network band and physical resource sharing between different entities, the UTRAN can alsoperformthefunctionofCNmessagesdistribution.

Forthis,theRRCProtocoltransparentlytransfersmessagesfromCNtotheaccessnetworkthroughadirect transferprocedure.When thisoccurs,a specific indicatorofCN is inserted in thesemessages,andtheentitieswiththedistributionfunctioninRNCusethissameindicatorfordirectmessagestotheappropriateCN,andviceversa.

But now it started to getmore complex, andwehave already reachedour goal today,whichwas tolearnthebasicsofRRCandRAB.

Everythingwejusttalkedaboutabovecanbeseenagaininthesamefigurebelow,thesamefromthebeginningoftheexplanations.

RRC and RAB in GSM?

Okay,weunderstandhowRRCandRABworksinUMTSWCDMAandLTEnetworks.ButinGSM,doeswehavetheseconceptsaswell?

Atfirst,theanswerisNO.However,withwhatwelearnedtoday,wecanmakeacomparisonwithsomeGSM'equivalent'parameters.

WecancomparetheSDCCHphaseandTCHphaseofaGSMcallwithRRCandRABinUMTS.

RRC is the Radio Resource Control that works as Control Plane in Layer 3. Is used primarily forSignaling in UMTS. Then we can compare with the signaling in GSM, as the Immediate AssignmentprocessforSDCCHresourceallocation.

RAB is the radio access 'transporter' that works as the User Plane to provide data for the servicesrequestedbytheuser.ThenwecancomparewiththeuserpartinGSM,astheTCHAssignment.

Foreachservicerequestedbytheuserwehaveonly1RAB.Forexample,iftherequestedserviceisaVoiceCall(CSAMR),then1CSRABwillbegeneratedandprovidedtotheuser.ThesameistrueforPS.

Soourequivalencetablewouldbe:

UMTS/LTE GSM

Control RRCConnection ImmediateAssignment

User RABAssignment(RNCCN) Assignment(BSCMSC)

RRC Connection and RAB example

Tocompletefortoday,let'ssee(alwaysinsimplifiedform)asimpleRRCconnectionandRAB.

Whenever the UE needs the UTRAN resources, he asks. So that these resources are allocated, itestablishesaRRCconnectionwithsomeSRBs.

Inthiscase,aRABconnectioniscreatedtoenablethetransferofuserdata.WeremindyouthattheRABconsistsofRB+Iubearer.TheRABiscreatedbyCN,withaspecificQoSrequest.

ForasingleUE,theremaybemultipleRABforNASservice(CSorPS).

But let's juststicktothe initialprocedure,that is,howisperformedthe 'RRCSetup'procedure, fromtheUE'srequest.

Thefollowingfigureshowsthismorestraightforward.

TheRRChasalways3steps:

1. TheUErequestsanewconnectionintheUplink(RRCCONNECTIONREQUEST)2. Withsufficientresourcesavailable, the 'RRCDownlinkCONNECTIONSETUP'message issent, includingthereason,

alongwiththeSRBconfiguration(Note:otherwise,iftheRRCconnectioncannotbeestablished,themessagesentis'RRCCONNECTIONSETUPREJECT').

3. Ifallgoeswell,theUEsendsthemessageintheUplink:RRCCONNECTIONSETUPCOMPLETE.

And after this, the MEASUREMENT CONTROL message are being sent in the Downlink, for thecommunicationcontinuity.

AftertheRRCconnectionisestablished,theUTRANmakesthechecksbetweentheCNandtheUE,forexampletheauthenticationandsecurityoperations.

Andso,theCNinformstheRABtoUTRANinaccordancewithrequirementsoftheservicerequestedbytheUE. Aswe have seen, RAB occurs after theRRC, andwithout a RRC connection noRABmay beestablished.

Conclusion

We have seen today a simplified explanation that covers a number of concepts involved in thecommunicationofthemostmodernexistingmobilenetworks,primarilyrelatedtoRRCandRAB.

Withthisconceptualbase,wewillcontinuetoevolveinthenexttutorialswithexamplesthatmaketheassimilationofthesecomplexconceptsinataskfarlessexhaustivethannormal.

We hope that you have enjoyed, and we count on your participation, which can be for examplesuggestingnewtopics,orsharingoursitewithyourfriends.Ifpossible,leavealsoyourcommentsjustbelow.

Untilthenexttutorial.

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40Comments Sortby

IshratGulCoreNetworkEngineeratKabul,Afghanistan

REallyappreciatedLikeReplyJul26,201511:34am

Newest

Addacomment...

MukeshJatJakharRFEngineeratBhartiAirtel

veryeasyexplanation....LikeReplyJul18,20151:02pm

MohsinAliRawalpindi,Pakistan

MultiRABmeansweareusingmultipleservicesCall,Videoetc??andwhataboutRABCongestion??

LikeReply 1Mar31,20158:11pm

WellafellaGutaUniversityofKwaZuluNatal(HowardCollege)

NumberoffailedRRCconnectionpreparation,DueToTrans(Times).........HavingthisalarmontheZTEnRNC.Transmissioncapacityisenough.Whatcouldbethepossiblecauses?


SibroSibroSibroThankyoufortheexplanation.Thishelpmetohavebetterunderstandingaboutcellularnetwork.


HustiucAndreiColegiulNationaldeMedicinasiFarmaciedinChisinau

whichmaybethecauseofRRCconnectionreject?


TatoRosalesHiyoucanfindthecausesforRRCconnectionrejectinthefollowingpdfhttp://www.etsi.org/.../03.06.00_60/ts_125331v030600p.pdf

LikeReply 1Mar22,20157:28am

RajeevYadavSr.EngineeratHuaweiTechnologiesCo.,Ltd

GreatArticle

LikeReply 1Feb13,20151:08pm

AzizMirzojonov

Great.Readingthisarticlewasinteresting.Onequestion.NumberofRRCandRABmustbeequalorRABgreaterorlessthenRRC?

LikeReply 1Jan16,20159:32pm

TatoRosalesHiAziz,RABcanbegreaterthantheRRCbecausetheRRCisthesignallingandyoucanhavemultipleRABsfromoneUE,butyouwillonlyhaveoneRRC

LikeReply 4Jan29,20155:26am

ZainAbidiUETTaxila

@TatoRosalesiobservedthatwhenmymobilereceivescallduringHSPAsessionthenHSPAsessionstopsduringcall.heremultiRABisnotworking.PSRABstopsandCSRABrunsandaftercallPSRABrunswhy?????


TatoRosalesHiZainAbidiifitisonlyyourphonethatishavingthisproblemitcouldbethatthefirmwarethatyouareusinginyourphoneisnothomologatedbyyourserviceprovider,trycheckingoutwiththemwichonesarehomologated,iftheproblemisforeveryonethenineedmoreinformation

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theproblemisforeveryonethenineedmoreinformationLikeReplyMar22,20157:22am

MakhloufiHocineGREAT!

LikeReply 1Dec27,20142:50pm

MiCasaPropiaGREAT!

LikeReply 1Dec9,20141:51am

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what is rrc and rab_ - telecomhall

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