8.7.9 MeNB initiated SgNB Release

36.4233GPPEvolved Universal Terrestrial Radio Access Network (E-UTRAN)Release 17TSX2 Application Protocol (X2AP)

8.7.9.1 General

The MeNB initiated SgNB Release procedure is triggered by the MeNB to initiate the release of the resources for a specific UE.

The procedure uses UE-associated signalling.

8.7.9.2 Successful Operation

Figure 8.7.9.2-1: MeNB initiated SgNB Release, successful operation

The MeNB initiates the procedure by sending the SGNB RELEASE REQUEST message. Upon reception of the SGNB RELEASE REQUEST message the en-gNB shall stop providing user data to the UE. The SgNB UE X2AP ID IE shall be included if it has been obtained from the en-gNB.

If the bearer context in the en-gNB was configured with the PDCP entity in the en-gNB, for E-RAB for which the MeNB requests forwarding of uplink/downlink data, the MeNB includes the UL Forwarding GTP Tunnel Endpoint/ DL Forwarding GTP Tunnel Endpoint IE within the E-RABs To Be Released Item IE of the SGNB RELEASE REQUEST message to indicate that the en-gNB should perform data forwarding of uplink/downlink packets for that E-RAB.

Upon reception of the SGNB RELEASE REQUEST message containing UE Context Kept Indicator IE set to "True", the en-gNB shall, if supported, only initiate the release of the resources related to the UE-associated signalling connection between the MeNB and the en-gNB.

If the en-gNB confirms the request to release en-gNB resources it shall send the SGNB RELEASE REQUEST ACKNOWLEDGE message to the MeNB.

If the RLC Mode IE is included for an E-RAB within the E-RABs Admitted To Be Released List IE (for E-RABs hosted at the en-gNB) in the SGNB RELEASE REQUEST ACKNOWLEDGE message, it indicates the mode that the en-gNB used for the E-RAB when it was hosted at the en-gNB.

If the MeNB did not include the SgNB UE X2AP ID IE in the SGNB RELEASE REQUEST message, the MeNB shall ignore the SgNB UE X2AP ID IE in the SGNB RELEASE REQUEST ACKNOWLEDGE message.

Upon successful completion of the procedure, the MeNB shall start counting time, so that information regarding time since Secondary Node Release may be transferred towards the MME as specified in TS 36.413 [4].

If the SGNB RELEASE REQUEST ACKNOWLEDGE message includes the SCG UE History Information IE, the MeNB shall, if supported, use the information to update UE History Information with PSCell history.

Interaction with SN Status Transfer procedure:

If the UE Context Kept Indicator IE set to "True" and the E-RABs transferred to MeNB IE are included in the SGNB RELEASE REQUEST message, then the en-gNB shall, if supported, include the uplink/downlink PDCP SN and HFN status for the listed E-RABs, as specified in TS 37.340 [32].

Interaction with Data Forwarding Address Indication procedure:

If the SGNB RELEASE REQUEST message concerns UE for which a CPC has been triggered, the en-gNB shall, if supported, consider that the triggered Conditional PSCell Change has been executed, and act as specified in TS 37.340 [32].

8.7.9.3 Unsuccessful Operation

Figure 8.7.9.3-1: MeNB initiated SgNB Release, unsuccessful operation

If the en-gNB cannot confirm the request to release en-gNB resources it shall send the SGNB RELEASE REQUEST REJECT message to the MeNB with an appropriate cause indicated in the Cause IE.

If the MeNB did not include the SgNB UE X2AP ID IE in the SGNB RELEASE REQUEST message, the MeNB shall ignore the SgNB UE X2AP ID IE in the SGNB RELEASE REQUEST REJECT message.

8.7.9.4 Abnormal Conditions

If the SGNB RELEASE REQUEST message refer to a context that does not exist, the en-gNB shall ignore the message.

When the MeNB has initiated the procedure and did not include the SgNB UE X2AP ID IE the MeNB shall regard the resources for the UE at the en-gNB as being fully released.

Interactions with the UE Context Release procedure:

If the MeNB does not receive the reply from the en-gNB before it has to relase the EN-DC connection, or it receives SGNB RELEASE REQUEST REJECT, it may trigger the UE Context Release procedure. If the en-gNB received the UE CONTEXT RELEASE right after receiving the SGNB RELEASE REQUEST (and before or after responding to it), the en-gNB shall consider the related MeNB initiated SgNB Release procedure as being the resolution of abnormal conditions and release the related UE context immediately.