RRC Connection ReEstablishment

8/10/2019 RRC Connection ReEstablishment

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1. RRC Connection Re-Establishment Understandingdocument

1.1 Objective

The main objective of this document is to understand the RRC connectionEstablishment procedure and RRC connection Re-Establishment procedure at botheNB and UE perspective.

This document deals with the following sections

!" RRC Connection Establishment se#uence diagram with e$planation.

%" &bout RRC Connection Re-Establishment with se#uence diagram at eNB

and UE with module level interaction.

'" (uccess and )ailure case of RRC Connection Re Establishment.

*" Causes for Re Establishment.

+" RRC Connection Re-Establishment Reject procedure with se#uence

diagram interaction with eNB, UE and E.


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1.2 RRC Connection Establishment Procedure sequence Diagram


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1.2.1 !rie" Descri#tion:

!" &fter the cell search is done and when UE initiates the call procedure, R&C

procedure is initiated b/ the UE0if it is contention based R&C"

%" 1n R&C procedure, UE sends a preamble 12 to the eNB as3ing for the

1dentit/.

'" eNB sends R&R 42U with Uplin3 5rant on reception of preamble 12. R&R

42U contains the parameters C6RNT1 0Cell Radio Networ3 Temporar/

1dentifier", U7 grant, Random access channel preamble 12 and timing

advance.

*" 8n reception of uplin3 grant, UE sends the RRC connection re#uest through

sg'.

+" eNB will send bac3 the same msg' to UE stating that RRC connection

re#uest is received on sg*.

9" UE sends &c3nowledgement to eNB on receiving the sg'. (RB! is created

at eNB.

:" RRC connection setup sent b/ eNB. 8n reception of RRC connection setup,

(RB! is created at UE.

;" &fter (RB! is created, UE and eNB should communicate through the (RB!

onl/.


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!*"E sends 1nitial conte$t setup re#uest to eNB which contains the

parameters li3e 2RB parameters, securit/ parameters and attach accept

message.

!+"eNB sends re#uest to UE for its capabilit/ information and sends to E.

!9"(ecurit/ command re#uest is sent b/ eNB to UE. UE generates the integrit/and ciphering 3e/ and compare. 8n success, UE sends securit/ mode

complete message to eNB.

!:"8n reception of securit/ mode complete message b/ eNB, (RB% is created

at eNB.

!;"eNB sends RRC connection reconfiguration message to UE. Then (RB% is

created at UE also on reception of RRC connection configuration message.

!


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1.$.1 Pur#ose o" Re-Establishment

The purpose of Re-Establishment is to re-establish the RRC connectionwhich involves the re-activation of securit/ process, reconfigure (RB! andresumption of (RB% and 2RB operations.

1f securit/ is not enabled or if securit/ procedure has failed, UE does notinitiate Re-Establishment. 1nstead it moves to RRC-127E state.

The Re-Establishment will be succeeded onl/ if the eNB has a valid UEconte$t. 1f the eNB does not have a valid UE conte$t, Re-Establishment will not besuccessful.

1.$.2 %uccess case o" RRC connection Re-Establishment

Below is a UE and eNB interaction diagram in the case of RRC ConnectionRe-Establishmet is success.

RRC connection Re-Establishment Request

RRC connection Re-Establishment set-up

RRC connection Re-Establishment Complete

UE eNB


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1.$.2.1 RRC connection Re-Establishment sequence diagram at e&!'ith (odule level sequence interaction


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1.$.2.2 RRC connection Re-Establishment sequence diagram at UE'ith (odule level sequence interaction


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1.$.$ )ailurecase o" RRC connection Re-Establishment


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Below is a UE and eNB interaction diagram in the case of RRC ConnectionRe-Establishmet is )ailure

RRC connection Re-Establishment Request

RRC connection Re-Establishment Reject

RRC connection Re-Establishment Reject can happen when eNB rejectsthe re#uest sent b/ UE due to 1nvalid UE conte$t or there is no availabilit/ ofresources at eNB to accept another UE.

1.$.* Causes "or RRC connection Re-Establishment

Below are some of the reasons for triggering RRC connection Re-Establishment.

Radio lin3 failure

RRC connection Re-configuration failure and over failure

1ntegrit/ chec3 failure

Upon mobilit/ from E-UTR& failure

1.$.*.1 Radio lin+ "ailure

Radio lin3 failure happensbecause of,

!" 8n T'!= failure,%" 8n R&C msg @ indication to UE when T'!! is not started,'" 8n indication to UE that ma$ number of R7C retransmission has been

reached.*" 8n U7 TB CRC failure scenario either in control plane or data plane.

UE eNB


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?hen an/ of the above causes occurred, the UE will consider that radio lin3failure happened and should be detected. 1f the securit/ is activated the UE willinitiate the RRC connection re-establishment procedure, else the UE will leave thestate RRC6C8NNECTE2.

1.$.*.2 RRC Connection Re-con"iguration "ailure

8n reception of RRC Connection re-configuration message to UE, if the UEis not able to meet the configurations received b/ the eNB in the RRC connectionreconfiguration message, the UE will continue with the configurations used beforereception of RRC connection reconfiguration message. This is one of the cause fortriggering ReEstablishment.

8n e$pir/ of T'== 0RRC Connection reconfiguration re#uest timer", UE goesin to the state RRC6127E.

1.$.*.$ ,and over "ailure

and over failure can happen when an/ one of the following scenarios weremet, leading to triggering of RRC connection ReEstablishment.

!" ?hen TeNB does not have enough resources to admit a new UE.%" ?hen TeNB does not have that particular UEs valid conte$t.

1.$.*.* ntegrit Chec+ "ailure

1f the securit/ authentication has failed due to UE @ eNB fails at generation @

derivation of AeNB 3e/ which causes message authentication failure, leads totriggering of RRC connection Re-Establishment procedure.

1.$.*./ U#on mobilit "rom E-U0R "ailureobilit/ from E-UTR& failure can happen when an/ of the following

scenarios were met, leading to triggering of RRC connection ReEstablishment.!" ?hen the UE is not able to match the configurations included

in the mobility from E-UTRA commandmessage.%" ?hen the UE does not succeed in establishing a connection

with the other R&T.

'" essage authentication failure between E-UTR&N and anotherR&T.

1.$./ RRC Connection Re-establishment message


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The RRC connection re-establishment message mainl/ contains thefollowing.

!" UE-1dentit/

The UE 1dentit/ carries the values of C-RNT1, 4h/sCell1d and

(hort&C-1.

)or remaining scenarios, same C-RNT1 is used from the cell in which

the trigger for Re-establishment happened.

%" Cause for Re-establishment

1f the Re-establishment procedure is initiated due to reconfiguration

failure, the cause for Re-establishment is set as Re-Configuration

Failure.

1f the Re-establishment procedure is initiated due to hand over failure or

inter R&T mobilit/ failure, the cause for Re-establishment is set as Hand

over Failure

1.$. RRC Connection Re-establishment #rocedure

?hen the RRC connection re-establishment procedure is initiated b/ UE, thefunctions of UE are,

!" (top timer T'!=0UE to get bac3 in s/nc with eNB" if alread/ running

%" (tart timer T'!!0Cell selection timer"

'" To suspend all Radio bearers e$cept (RB=

a. The reason wh/ we are suspending the 42C4 and R7C entities is,

after suspending the 42C4 and R7C entities, no meta data will be

sent to &C for scheduling.

*" To reset &C configurations.

a. &ll the &R process 12s were released and the state is reset to

&R6127E in this &C reset process.


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b. 2espite of receiving &CA @ N&CA from the UE, when

ReEstablishment has been triggered, releasing of &R

retransmission buffers will happen.

+" To reset and appl/ default ph/sical channel configurations and &C main

configurations

9" To start cell selection procedure.

&t UE after the cell selection is completed the timer T'!! is stopped and

timer T'=! is started to initiate the contention based R&C. 1f the selected

cell is inter R&T cell, the UE will leave the state RRC connected.

The UE will send the RRC connection reestablishment message to lower

la/ers for transmission to eNB.

1f the UE receives the RRC connection Re-Establishment setu#from

eNB, timer T'=!0R&C initiating timer" is stopped and both 42C4 and R7C

entities were re-established for (RB!.

UE performs the dedicated radio resource configuration procedure and

resumes (RB!.

Updating of securit/ is done. ie., Reactivation of 1ntegrit/ protection and

ciphering algorithm and indicating the successful completion of the securit/

procedure. Thereb/ submitting the RRC connection Re-establishment complete

message to lower la/ers for sending to eNB.

1.$.3 RRC Connection Re-Establishment Reject

The RRC Connection Re-Establishment reject could happen when the eNBrejects the Re-Establishment re#uest message sent b/ the UE. The following arefew of the reasons for the Re-Establishment reject scenario.

!" ?hen eNB does not have a valid UE conte$t%" ?hen (ecurit/ authentication procedure fails 0&C-1 calculation fails".


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1.$.3.1 RRC Connection Re-Establishment Reject sequence diagram'ith interaction bet'een e&!4 UE and ((E

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RRC Connection ReEstablishment