1. RRC Connection Re-Establishment Understanding 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: 1) RRC Connection Establishment sequence diagram with explanation. 2) About RRC Connection Re-Establishment with sequence diagram at eNB and UE with module level interaction. 3) Success and Failure case of RRC Connection Re Establishment. 4) Causes for Re Establishment. 5) RRC Connection Re-Establishment Reject procedure with sequence diagram interaction with eNB, UE and MME.
1.2
RRC Connection Establishment Procedure sequence Diagram:
1.2.1 Brief Description: 1) After the cell search is done and when UE initiates the call procedure, RACH procedure is initiated by the UE(if it is contention based RACH) 2) In RACH procedure, UE sends a preamble ID to the eNB asking for the Identity. 3) eNB sends RAR PDU with Uplink Grant on reception of preamble ID. RAR PDU contains the parameters C_RNTI (Cell Radio Network Temporary Identifier), UL grant, Random access channel preamble ID and timing advance. 4) On reception of uplink grant, UE sends the RRC connection request through Msg3. 5) eNB will send back the same msg3 to UE stating that RRC connection request is received on Msg4. 6) UE sends Acknowledgement to eNB on receiving the Msg3. SRB1 is created at eNB. 7) RRC connection setup sent by eNB. On reception of RRC connection setup, SRB1 is created at UE. 8) After SRB1 is created, UE and eNB should communicate through the SRB1 only. 9) eNB on reception of RRC connection setup complete with attach request, sends initial UE message to MME with attach request and S1AP ID. 10)MME communicates with UE by sending the RAND(Random challenge) & AUTN(Authentication Token). UE NAS generates the authentication token and compares. If AUTN comparison fails authentication procedure fails. 11) On successful comparison of AUTN number, UE sends authentication response to MME. 12)On reception of security verification request with Integrity protection, UE derives the integrity algorithm and compares the message with the derived integrity algorithm. If Integrity check is success, UE sends security verification success completed message to MME. 13)If Integrity check fails, security verification fails.
14)MME sends Initial context setup request to eNB which contains the parameters like DRB parameters, security parameters and attach accept message. 15)eNB sends request to UE for its capability information and sends to MME. 16)Security command request is sent by eNB to UE. UE generates the integrity and ciphering key and compare. On success, UE sends security mode complete message to eNB. 17)On reception of security mode complete message by eNB, SRB2 is created at eNB. 18)eNB sends RRC connection reconfiguration message to UE. Then SRB2 is created at UE also on reception of RRC connection configuration message. 19)RRC connection reconfiguration message contains the parameters like SRB2 parameters, DRB parameters and attach accept message. 20)RRC connection reconfiguration completed (Attach completed) message is sent to eNB by UE on SRB2.
1.3
RRC Connection Re-Establishment:
The procedure to re-establish RRC Connection which involves all RBs resumption (except SRB0) and Access Stratum security reactivation can be called as RRC Connection Re-Establishment. When a UE looses sync with the eNB, it tries to regain its connection with the eNB. In simple words, the above process can be interpreted as ReEstablishment. In this procedure, UE initiates RACH procedure and on successful RACH operation, UE requests eNB for RRC connection Re-Establishment setup, which in turn UE sends RRC connection Re-Establishment completed to eNB, to regain back to RRC_Connected state. In this case RRC connection Re-Establishment will be initiated by UE(Contention Based). In this process, there will not be any interaction by UE with MME.
1.3.1 Purpose of Re-Establishment: The purpose of Re-Establishment is to re-establish the RRC connection which involves the re-activation of security process, reconfigure SRB1 and resumption of SRB2 and DRB operations. If security is not enabled or if security procedure has failed, UE does not initiate Re-Establishment. Instead it moves to RRC-IDLE state. The Re-Establishment will be succeeded only if the eNB has a valid UE context. If the eNB does not have a valid UE context, Re-Establishment will not be successful.
1.3.2 Success case of 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 eNB with Module level sequence interaction: 1.3.2.1
RRC connection Re-Establishment sequence diagram at UE with Module level sequence interaction: 1.3.2.2
1.3.3 Failure case of RRC connection Re-Establishment:
Below is a UE and eNB interaction diagram in the case of RRC Connection Re-Establishmet is Failure
UE
eNB
RRC connection Re-Establishment Request
RRC connection Re-Establishment Reject
RRC connection Re-Establishment Reject can happen when eNB rejects the request sent by UE due to Invalid UE context or there is no availability of resources at eNB to accept another UE.
1.3.4 Causes for RRC connection Re-Establishment: Below are some of the reasons for triggering RRC connection Re-Establishment.
Radio link failure RRC connection Re-configuration failure Hand over failure Integrity check failure Upon mobility from E-UTRA failure
1.3.4.1
Radio link failure:
Radio link failure happens because of, 1) On T310 failure, 2) On RACH msg / indication to UE when T311 is not started, 3) On indication to UE that max number of RLC retransmission has been reached. 4) On UL TB CRC failure scenario either in control plane or data plane.
When any of the above causes occurred, the UE will consider that radio link failure happened and should be detected. If the security is activated the UE will initiate the RRC connection re-establishment procedure, else the UE will leave the state RRC_CONNECTED. 1.3.4.2
RRC Connection Re-configuration failure:
On reception of RRC Connection re-configuration message to UE, if the UE is not able to meet the configurations received by 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. On expiry of T300 (RRC Connection reconfiguration request timer), UE goes in to the state RRC_IDLE. 1.3.4.3
Hand over failure:
Hand over failure can happen when any one of the following scenarios were met, leading to triggering of RRC connection ReEstablishment. 1) When TeNB does not have enough resources to admit a new UE. 2) When TeNB does not have that particular UEs valid context. 1.3.4.4
Integrity Check failure:
If the security authentication has failed due to UE / eNB fails at generation / derivation of KeNB key which causes message authentication failure, leads to triggering of RRC connection Re-Establishment procedure. 1.3.4.5 Upon mobility from E-UTRA failure: Mobility from E-UTRA failure can happen when any of the following scenarios were met, leading to triggering of RRC connection ReEstablishment. 1) When the UE is not able to match the configurations included in the mobility from E-UTRA command message. 2) When the UE does not succeed in establishing a connection with the other RAT. 3) Message authentication failure between E-UTRAN and another RAT.
1.3.5 RRC Connection Re-establishment message:
The RRC connection re-establishment message mainly contains the following. 1) UE-Identity
The UE Identity carries the values of C-RNTI, PhysCellId and ShortMAC-I.
For remaining scenarios, same C-RNTI is used from the cell in which the trigger for Re-establishment happened.
2) Cause for Re-establishment
If the Re-establishment procedure is initiated due to reconfiguration failure, the cause for Re-establishment is set as Re-Configuration Failure.
If the Re-establishment procedure is initiated due to hand over failure or inter RAT mobility failure, the cause for Re-establishment is set as Hand over Failure
1.3.6 RRC Connection Re-establishment procedure: When the RRC connection re-establishment procedure is initiated by UE, the functions of UE are, 1) Stop timer T310(UE to get back in sync with eNB) if already running 2) Start timer T311(Cell selection timer) 3) To suspend all Radio bearers except SRB0 a. The reason why we are suspending the PDCP and RLC entities is, after suspending the PDCP and RLC entities, no meta data will be sent to MAC for scheduling. 4) To reset MAC configurations. a. All the HARQ process IDs were released and the state is reset to HARQ_IDLE in this MAC reset process.
b. Despite of receiving ACK / NACK from the UE, when ReEstablishment has been triggered, releasing of HARQ retransmission buffers will happen. 5) To reset and apply default physical channel configurations and MAC main configurations 6) To start cell selection procedure.
At UE after the cell selection is completed the timer T311 is stopped and timer T301 is started to initiate the contention based RACH. If the selected cell is inter RAT cell, the UE will leave the state RRC connected.
The UE will send the RRC connection reestablishment message to lower layers for transmission to eNB.
If the UE receives the RRC connection Re-Establishment setup from eNB, timer T301(RACH initiating timer) is stopped and both PDCP and RLC entities were re-established for SRB1.
UE performs the dedicated radio resource configuration procedure and resumes SRB1.
Updating of security is done. ie., Reactivation of Integrity protection and ciphering algorithm and indicating the successful completion of the security procedure. Thereby submitting the RRC connection Re-establishment complete message to lower layers for sending to eNB.
1.3.7 RRC Connection Re-Establishment Reject: The RRC Connection Re-Establishment reject could happen when the eNB rejects the Re-Establishment request message sent by the UE. The following are few of the reasons for the Re-Establishment reject scenario. 1) When eNB does not have a valid UE context 2) When Security authentication procedure fails (MAC-I calculation fails).
1.3.7.1 RRC Connection Re-Establishment Reject sequence diagram with interaction between eNB, UE and MME:
