Section: 4.23.11.3 Xn based handover with re-allocation of intermediate SMF
Content: This procedure is used to hand over a UE from a Source NG-RAN to a Target NG-RAN using Xn interface (in this case the AMF is unchanged) and the AMF decides that the intermediate SMF(I-SMF) is to be changed. This procedure is used for non-roaming or local breakout roaming scenario. In the case of home routed roaming scenario, this procedure is also used except the I-SMF is replaced by V-SMF. The call flow is shown in figure 4.23.11.3 -1. Figure 4.23.11.3-1: Xn based inter NG-RAN handover with intermediate I-SMF re-allocation 1-3. Steps 1-3 are same as steps 1-3 described in clause 4.23.11.2 except that in step 2 the AMF sends Nsmf_PDUSession_UpdateSMContext Request to source I-SMF and then the source I-SMF sends the Nsmf_PDUSession_Update Request to SMF. 4. The target I-SMF sends Nsmf_PDUSession_Context Request to Source I-SMF to retrieve 5G SM Context. 5a-11. Steps 5a-11 are same as steps 5a-11 described in clause 4.23.11.2 with the following difference: In step 6, the target I-SMF invokes Nsmf_PDUSession_Update Request (Secondary RAT usage data, UE Location Information, UE presence in LADN service area, DL CN Tunnel Info of the I-UPF, DNAI list supported by target I-SMF) toward the SMF; In step 9, the SMF respond with Nsmf_PDUSession_Update Response. The SMF may provide the DNAI(s) of interest for this PDU Session to I-SMF as described in step 1 of Figure 4.23.9.1-1. The SMF may update the CN PDB in the response or using a separate PDU Session Modification procedure, based on local configuration. Secondary RAT usage data is extracted from PDU Session To Be Switched with N2 SM Information received from NG RAN. 12a. The AMF sends Nsmf_PDUSession_ReleaseSMContext Request (I-SMF only indication) to source I-SMF. The source I-SMF removes the SM context of this PDU session. An indication is included in this message to avoid invoking resource release in SMF. 12b. The source I-SMF sends N4 Session Release to release the resource in source I-UPF. If the source I-UPF acts as UL CL and is not co-located with local PSA, the source I-SMF also sends N4 Session Release to the local PSA to release the resource for the PDU Session. 13-14. Steps 13-14 are same as steps 12-13 described in clause 4.23.11.2.