1. RRC Conne Connection ction Re-Es Re-Establi tablishmen shmentt Understa Understanding nding document 1.1
Objective The main objective of this document is to understand the RRC connection Establishment procedure and RRC connection Re-Establishment procedure at both eNB and UE perspective.
This document deals with the following sections !" RRC Connecti Connection on Establishment Establishment se#uence se#uence diagram diagram with e$planation. e$planation. %" &bout RRC RRC Connection Connection Re-Establishment Re-Establishment with se#uence se#uence diagram diagram at eNB and UE with module level interaction. '" (uccess and )ailure )ailure case of RRC Connection Connection Re Establish Establishment. ment. *" Causes Causes for Re Establis Establishmen hment. t. +" RRC Connection Connection Re-Establishment Re-Establishment Reject procedur procedure e with se#uence se#uence diagram interaction with eNB, UE and E.
1.2
RRC Connection Establishment Procedure sequence Diagram
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/. <" eNB on reception of RRC connection setup complete with attach re#uest, sends initial UE message to E with attach re#uest and (!&4 12. !="E communicates with UE b/ sending the R&N20Random challenge" > &UTN0&uthentication To3en". UE N&( generates the authentication to3en and compares. 1f &UTN comparison fails authentication procedure fails. !!" 8n successful comparison of &UTN number, UE sends authentication response to E. !%"8n reception of securit/ verification re#uest with 1ntegrit/ protection, UE derives the integrit/ algorithm and compares the message with the derived integrit/ algorithm. 1f 1ntegrit/ chec3 is success, UE sends securit/ verification success completed message to E. !'"1f 1ntegrit/ chec3 fails, securit/ verification fails.
!*"E sends 1nitial conte$t setup re#uest to eNB which contains the parameters li3e 2RB parameters, securit/ parameters and attach accept message. !+"eNB sends re#uest to UE for its capabilit/ information and sends to E. !9"(ecurit/ command re#uest is sent b/ eNB to UE. UE generates the integrit/ and ciphering 3e/ and compare. 8n success, UE sends securit/ mode complete message to eNB. !:"8n reception of securit/ mode complete message b/ eNB, (RB% is created at eNB. !;"eNB sends RRC connection reconfiguration message to UE. Then (RB% is created at UE also on reception of RRC connection configuration message. !<"RRC connection reconfiguration message contains the parameters li3e (RB% parameters, 2RB parameters and attach accept message. %="RRC connection reconfiguration completed 0&ttach completed" message is sent to eNB b/ UE on (RB%.
1.$
RRC Connection Re-Establishment
The procedure to re-establish RRC Connection which involves all RBs resumption 0e$cept (RB=" and &ccess (tratum securit/ reactivation can be called as RRC Connection Re-Establishment. ?hen a UE looses s/nc with the eNB, it tries to regain its connection with the eNB. 1n simple words, the above process can be interpreted as ReEstablishment. 1n this procedure, UE initiates R&C procedure and on successful R&C operation, UE re#uests eNB for RRC connection Re-Establishment setup, which in turn UE sends RRC connection Re-Establishment completed to eNB, to regain bac3 to RRC6Connected state. 1n this case RRC connection Re-Establishment will be initiated b/ UE0Contention Based". 1n this process, there will not be an/ interaction b/ UE with E.
1.$.1 Pur#ose o" Re-Establishment
The purpose of Re-Establishment is to re-establish the RRC connection which involves the re-activation of securit/ process, reconfigure (RB! and resumption of (RB% and 2RB operations. 1f securit/ is not enabled or if securit/ procedure has failed, UE does not initiate Re-Establishment. 1nstead it moves to RRC-127E state. The Re-Establishment will be succeeded onl/ if the eNB has a valid UE conte$t. 1f the eNB does not have a valid UE conte$t, Re-Establishment will not be successful.
1.$.2 %uccess case o" RRC connection Re-Establishment
Below is a UE and eNB interaction diagram in the case of RRC Connection Re-Establishmet is success.
UE
eNB
RRC connection Re-Establishment Request
RRC connection Re-Establishment set-up
RRC connection Re-Establishment Complete
RRC connection Re-Establishment sequence diagram at e&! 'ith (odule level sequence interaction 1.$.2.1
RRC connection Re-Establishment sequence diagram at UE 'ith (odule level sequence interaction 1.$.2.2
1.$.$ )ailure case o" RRC connection Re-Establishment
Below is a UE and eNB interaction diagram in the case of RRC Connection Re-Establishmet is )ailure
UE
eNB
RRC connection Re-Establishment Request
RRC connection Re-Establishment Reject
RRC connection Re-Establishment Reject can happen when eNB rejects the re#uest sent b/ UE due to 1nvalid UE conte$t or there is no availabilit/ of resources at eNB to accept another UE.
1.$.* 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.$.*.1
Radio lin+ "ailure
Radio lin3 failure happens because of, !" 8n T'!= failure, %" 8n R&C msg @ indication to UE when T'!! is not started, '" 8n indication to UE that ma$ number of R7C retransmission has been reached. *" 8n U7 TB CRC failure scenario either in control plane or data plane.
?hen an/ of the above causes occurred, the UE will consider that radio lin3 failure happened and should be detected. 1f the securit/ is activated the UE will initiate the RRC connection re-establishment procedure, else the UE will leave the state RRC6C8NNECTE2.
1.$.*.2
RRC Connection Re-con"iguration "ailure
8n reception of RRC Connection re-configuration message to UE, if the UE is not able to meet the configurations received b/ the eNB in the RRC connection reconfiguration message, the UE will continue with the configurations used before reception of RRC connection reconfiguration message. This is one of the cause for triggering ReEstablishment. 8n e$pir/ of T'== 0RRC Connection reconfiguration re#uest timer", UE goes in to the state RRC6127E.
1.$.*.$
,and over "ailure
and over failure can happen when an/ one of the following scenarios were met, 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 to triggering of RRC connection Re-Establishment procedure. 1.$.*./ U#on mobilit "rom E-U0R "ailure obilit/ from E-UTR& failure can happen when an/ of the following scenarios were met, leading to triggering of RRC connection ReEstablishment. !" ?hen the UE is not able to match the configurations included in the mobility from E-UTRA command message. %" ?hen the UE does not succeed in establishing a connection with the other R&T. '" essage authentication failure between E-UTR&N and another R&T.
1.$./ RRC Connection Re-establishment message
The RRC connection re-establishment message mainl/ contains the following. !" 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.$. RRC Connection Re-establishment #rocedure
?hen the RRC connection re-establishment procedure is initiated b/ UE, the functions of UE are, !" (top timer T'!=0UE to get bac3 in s/nc with eNB" if alread/ running %" (tart timer T'!!0Cell selection timer" '" To suspend all Radio bearers e$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.
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.$.3 RRC Connection Re-Establishment Reject
The RRC Connection Re-Establishment reject could happen when the eNB rejects the Re-Establishment re#uest message sent b/ the UE. The following are few of the reasons for the Re-Establishment reject scenario. !" ?hen eNB does not have a valid UE conte$t %" ?hen (ecurit/ authentication procedure fails 0&C-1 calculation fails".
1.$.3.1 RRC Connection Re-Establishment Reject sequence diagram 'ith interaction bet'een e&!4 UE and ((E
