RRC Connection Reestablishment Request

The purpose of RRC CONNECTION RE-ESTABLISHMENT procedure is to re-establish the RRC connection, which involves the resumption of SRB1 operation and the re-activation of security (without changing algorithms). The UE shall only initiate this procedure when AS security has been activated.

 The connection re-establishment succeeds only if the concerned cell is prepared i.e. has a valid UE context. In case E-UTRAN accepts the re-establishment, SRB1 operation resumes while the operation of other radio bearers remains suspended. If AS security has not been activated, the UE doesn’t initiate this procedure, instead, it moves to RRC_IDLE directly

The UE initiates the RRC CONNECTION RE-ESTABLISHMENT procedure when one of the following conditions is met:

• Upon detecting radio link failure; or
• Upon handover failure; or
• Upon mobility from E-UTRA failure; or
• Upon integrity check failure indication from lower layers; or
• Upon an RRC CONNECTION RECONFIGURATION failure

RRC Connection Reestablishment Request Message

Direction: UE => E-UTRAN
Signalling Radio Bearer: SRB0
RLC Mode: TM
Logical Channel: CCCH
Transport Channel: UL-SCH

IEs in RRC CONNECTION REESTABLISHMENT REQUEST message are given below:

ue-Identity: UE identity is included to retrieve UE context and to facilitate contention resolution by lower layers. The UE Identity shall be set as follows:

1. Set the C-RNTI to the C-RNTI used in the source PCell (In case of handover and mobility from E-UTRA failure) or used in the PCell in which the trigger for the re-establishment occurred (other cases);
2. Set the physCellId to the physical cell identity of the source PCell (handover and mobility from E-UTRA failure) or of the PCell in which the trigger for the re-establishment occurred (other cases);
3. Set the shortMAC-I to the 16 least significant bits of the calculated MAC-I

 reestablishmentCause: This IE indicates the failure cause that triggered the re-establishment procedure and shall be set as follows:

1. If the re-establishment procedure was initiated due to reconfiguration failure (the UE is unable to comply with the reconfiguration sent in RRC CONNECTION RECONFIGURATION), then set the reestablishmentCause to the value 'reconfigurationFailure';
2. If the re-establishment procedure was initiated due to handover failure (intra-LTE handover failure or inter-RAT mobility from EUTRA failure) then, set the reestablishmentCause to the value 'handoverFailure'
3. Set the reestablishmentCause to the value 'otherFailure' if the re-establishment procedure was triggered due other causes than indicated in cases 1 and 2

Example: RRC CONNECTION REESTABLISHMENT REQUEST

Reference: 3GPP TS 36.331