8.3.5 S-NG-RAN node initiated S-NG-RAN node Change

38.4233GPPNG-RANRelease 17TSXn Application Protocol (XnAP)

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

8.3.5.1 General

This procedure is used by the S-NG-RAN node to trigger the change of the S-NG-RAN node.

The procedure uses UE-associated signalling.

8.3.5.2 Successful Operation

Figure 8.3.5.2-1: S-NG-RAN node initiated S-NG-RAN node Change, successful operation.

The S-NG-RAN node initiates the procedure by sending the S-NODE CHANGE REQUIRED message to the M-NG-RAN node including the Target S-NG-RAN node ID IE. When the S-NG-RAN node sends the S-NODE CHANGE REQUIRED message, it shall start the timer TXn_DCoverall.

The S-NODE CHANGE REQUIRED message may contain

– the S-NG-RAN node to S-NG-RAN node Container IE.

If the M-NG-RAN node is able to perform the change requested by the S-NG-RAN node, the M-NG-RAN node shall send the S-NODE CHANGE CONFIRM message to the S-NG-RAN node. For DRBs configured with the PDCP entity in the S-NG-RAN node, the M-NG-RAN node may include data forwarding related information in the Data Forwarding Info from target NG-RAN node IE.

If the S-NODE CHANGE CONFIRM message includes the DRB IDs taken into use IE, the S-NG-RAN node shall, if applicable, act as specified in TS 37.340 [8].

The S-NG-RAN node may start data forwarding and stop providing user data to the UE and shall stop the timer TXn_DCoverall upon reception of the S-NODE CHANGE CONFIRM message.

If the S-NODE CHANGE REQUIRED message includes the SCG UE History Information IE, the M-NG-RAN node shall, if supported, use the information to update UE History Information with PSCell history.

If the S-NODE CHANGE REQUIRED message includes the SN Mobility Information IE, the M-NG-RAN node shall, if supported, store this information and use it as defined in TS 38.300 [9].

If the S-NODE CHANGE REQUIRED message includes the Source PSCell ID IE, the M-NG-RAN node shall, if supported, store the information and act as specified in TS 38.300 [9].

The M-NG-RAN node may also provide configuration information in the M-NG-RAN node to S-NG-RAN node Container IE.

If the Conditional PSCell Change Information Required IE is included in the S-NODE CHANGE REQUIRED message, the M-NG-RAN node shall, if supported, consider that the requirement concerns CPAC, as described in TS 37.340 [8]. The S-NG-RAN node to M-NG-RAN node Container IE within the Conditional PSCell Change Information Required IE contains at least the suggested PSCell list for each candidate target S-NG-RAN node. Accordingly, the M-NG-RAN node may include the Conditional PSCell Change Information Confirm IE in the S-NODE CHANGE CONFIRM message.

If the Estimated Arrival Probability IE is contained in the Conditional PSCell Change Information Required IE included in the S-NODE CHANGE REQUIRED message, the M-NG-RAN node shall, if supported, forward this information to the candidate target S-NG-RAN node, then the candidate target S-NG-RAN node may use the information to allocate necessary resources for the incoming CPAC procedure.

Interaction with M-NG-RAN node initiated S-NG-RAN node Release:

If the M-NG-RAN node receives the S-NODE CHANGE REQUIRED message indicating releasing target S-NG-RAN node(s) and cancelling all prepared PSCells in the target S-NG-RAN node(s), the M-NG-RAN shall, if supported, trigger the M-NG-RAN node initiated S-NG-RAN node release procedure to the target S-NG-RAN node(s) and cancel all the prepared PSCells at the target S-NG-RAN node(s).

8.3.5.3 Unsuccessful Operation

Figure 8.3.5.3-1: S-NG-RAN node initiated S-NG-RAN node Change, unsuccessful operation.

In case the request modification cannot accept the request to change the S-NG-RAN node the M-NG-RAN node shall respond with the S-NODE CHANGE REFUSE message to the S-NG-RAN node with an appropriate cause value in the Cause IE.

8.3.5.4 Abnormal Conditions

If the timer TXn_DCoverall expires before the S-NG-RAN node has received the S-NODE CHANGE CONFIRM or the S-NODE CHANGE REFUSE message, the S-NG-RAN node shall regard the requested change as failed and may take further actions like triggering the S-NG-RAN node initiated S-NG-RAN node Release procedure to release all S-NG-RAN node resources allocated for the UE.

If the M-NG-RAN node receives an S-NODE CHANGE REQUIRED message including a PDU Session SN Change Required Item IE, not containing the PDU Session Resource Change Required Info – SN terminated IE, the M-NG-RAN node shall fail the S-NG-RAN node initiated S-NG-RAN node Change procedure indicating an appropriate cause.

Interaction with the M-NG-RAN node initiated Handover Preparation procedure:

If the M-NG-RAN node, after having initiated the Handover Preparation procedure, receives the S-NODE CHANGE REQUIRED message, the M-NG-RAN node shall refuse the S-NG-RAN node initiated S-NG-RAN node Change procedure with an appropriate cause value in the Cause IE.

Interaction with the S-NG-RAN node initiated S-NG-RAN node Release procedure:

If the S-NG-RAN node receives an S-NODE CHANGE CONFIRM message including a PDU Session SN Change Confirm Item IE, not containing the PDU Session Resource Change Confirm Info – SN terminated IE, the S-NG-RAN node shall trigger the S-NG-RAN node initiated S-NG-RAN node Release procedure indicating an appropriate cause.