8.7.8 SgNB Change
36.4233GPPEvolved Universal Terrestrial Radio Access Network (E-UTRAN)Release 17TSX2 Application Protocol (X2AP)
8.7.8.1 General
This procedure is used by the en-gNB to change to another en-gNB.
The procedure uses UE-associated signalling.
8.7.8.2 Successful Operation
Figure 8.7.8.2-1: SgNB Change, successful operation.
The en-gNB initiates the procedure by sending the SGNB CHANGE REQUIRED message to the MeNB including the Target SgNB ID Information IE. When the en-gNB sends the SGNB CHANGE REQUIRED message, it shall start the timer TDCoverall.
The SGNB CHANGE REQUIRED message may contain
– the SgNB to MeNB Container IE.
If the MeNB is able to perform the change requested by the en-gNB, the MeNB shall send the SGNB CHANGE CONFIRM message to the en-gNB. For each E-RAB configured with the PDCP entity in the en-gNB, the MeNB may include the DL Forwarding GTP Tunnel Endpoint IE and the UL Forwarding GTP Tunnel Endpoint IE within the E-RABs To Be Released Item IE to indicate that it requests data forwarding of uplink and downlink packets to be performed for that bearer.
The en-gNB may start data forwarding and stop providing user data to the UE and shall stop the timer TDCoverall upon reception of the SGNB CHANGE CONFIRM message.
If the SGNB CHANGE REQUIRED message includes the SCG UE History Information IE, the MeNB shall, if supported, use the information to update UE History Information with PSCell history.
If the Conditional PSCell Change Information Required IE is included in the SGNB CHANGE REQUIRED message, the MeNB shall, if supported, consider that the requirement concerns CPAC, as described in TS 37.340 [32]. The SgNB to MeNB Container IE within the Conditional PSCell Change Information Required IE contains at least the suggested PSCell list for each candidate target SgNB. Accordingly, the MeNB may include the Conditional PSCell Change Information Confirm IE in the SGNB CHANGE CONFIRM message.
If the Estimated Arrival Probability IE is contained in the Conditional PSCell Change Information Required IE included in the SGNB CHANGE REQUIRED message, the MeNB shall, if supported, forward this information to the candidate target en-gNB, then the candidate target en-gNB may use the information to allocate necessary resources for the incoming CPAC procedure.
Interaction with MeNB initiated SgNB Release:
If the MeNB receives the SGNB CHANGE REQUIRED message releasing target en-gNB and cancelling all the prepared PSCells in the target en-gNB(s), the MeNB shall, if supported, trigger the MeNB initiated en-gNB release procedure to the target en-gNB(s) and cancel all the prepared PSCells at the target en-gNB(s).
8.7.8.3 Unsuccessful Operation
Figure 8.7.8.3-1: SgNB Change, unsuccessful operation.
In case the request change cannot be performed successfully the MeNB shall respond with the SGNB CHANGE REFUSE message to the en-gNB with an appropriate cause value in the Cause IE.
8.7.8.4 Abnormal Conditions
If the timer TDCoverall expires before the en-gNB has received the SGNB CHANGE CONFIRM or the SGNB CHANGE REFUSE message, the en-gNB shall regard the requested change as failed and may take further actions like triggering the SgNB initiated SgNB Release procedure to release all en-gNB resources allocated for the UE.
Interaction with the MeNB initiated handover procedure:
If the MeNB, after having initiated the handover procedure, receives the SGNB CHANGE REQUIRED message, the MeNB shall refuse the SgNB change procedure with an appropriate cause value in the Cause IE.