4.23.12 N26 based Interworking Procedures with I-SMF

23.5023GPPProcedures for the 5G System (5GS)Release 18TS

4.23.12.1 General

When UE moves from EPS to 5GS, for a each PDU Session, the AMF determines whether I-SMF needs to be inserted based on UE location and service area of the SMF+PGW-C.

This clause describes the N26 based Interworking Procedures with I-SMF insertion when UE move from EPS to 5GS and removal when UE moves from 5GS to EPS.

4.23.12.2 5GS to EPS Idle mode mobility using N26 interface with I-SMF removal

For 5GS to EPS Idle mode mobility using N26 with I-SMF removal, the procedure "5GS to EPS Idle mode mobility using N26 interface" defined in clause 4.11.1.3.2 for the home routed-roaming case are re-used, with the following change:

– The V-SMF is replaced by I-SMF, H-SMF is replaced by SMF and V-UPF is replaced by I-UPF.

– The V-CN Tunnel Info is replaced by Tunnel Info at I-UPF, H-CN Tunnel Info is replaced by Tunnel Info at UPF (PSA).

4.23.12.2A 5GS to EPS Idle mode mobility using N26 interface with Data forwarding and I-SMF removal

For 5GS to EPS Idle mode mobility using N26 with data forwarding and I-SMF removal, the procedure "5GS to EPS Idle mode mobility using N26 interface with data forwarding" defined in clause 4.11.1.3.2A for the home-routed roaming case is re-used, with the following change:

– The V-SMF is replaced by I-SMF, V-UPF is replaced by I-UPF and H-SMF is replaced by SMF.

– Data forwarding tunnel resource is established and the tunnel information is exchanged through N26 interface. Data buffered in I-SMF/I-UPF is forwarded to UE via Serving GW.

4.23.12.3 EPS to 5GS mobility registration procedure (Idle and Connected State) using N26 interface with I-SMF insertion

For EPS to 5GS Mobility registration procedure using N26 with I-SMF insertion, the procedure "EPS to 5GS Mobility Registration Procedure (Idle and Connected State) using N26 interface" defined in clause 4.11.1.3.3 for the home routed-roaming case are re-used, with the following change:

– The V-SMF is replaced by I-SMF and H-SMF is replaced by SMF, V-UPF is replaced by I-UPF.

– The V-SMF selection is replaced by the I-SMF selection.

– The V-CN Tunnel Info is replaced by Tunnel Info at I-UPF, H-CN Tunnel Info is replaced by Tunnel Info at UPF(PSA).

4.23.12.3A EPS to 5GS Idle mode mobility using N26 interface with Data forwarding and I-SMF insertion

For EPS to 5GS Mobility registration procedure using N26 with data forwarding and I-SMF insertion, the procedure "EPS to 5GS Idle Mobility using N26 interface with data forwarding" defined in clause 4.11.1.3.3A for the home routed-roaming case is re-used, with the following change:

– The V-SMF is replaced by I-SMF and H-SMF is replaced by SMF, V-UPF is replaced by I-UPF.

– The V-SMF selection is replaced by the I-SMF selection.

– The V-CN Tunnel Info is replaced by Tunnel Info at I-UPF, H-CN Tunnel Info is replaced by Tunnel Info at UPF(PSA).

– Data forwarding tunnel resource is established and the tunnel information is exchanged through N26 interface. Data buffered in Serving GW is forwarded to UE via I-SMF/I-UPF.

4.23.12.4 Procedures for EPS bearer ID allocation

The EPS Bearer ID allocation procedure, EPS Bearer ID transfer procedure and EPS Bearer ID revocation procedure are perfomed as described for the home-routed roaming case in clause 4.11.1.4, with the following differences:

– H-SMF is replaced by SMF.

– V-SMF is replaced by I-SMF.

– In step 8 of clause 4.11.1.4.1, in addition to home routing roaming scenario, for scenario of deployments topologies with specific SMF Service Areas, if EBI is assigned successfully, the SMF+PGW-C also prepares CN Tunnel Info for each EPS bearer and provides this information to the I-SMF.

4.23.12.5 EPS to 5GS mobility registration procedure (Idle) using N26 interface with AMF reallocation and I-SMF insertion

For EPS to 5GS Mobility registration procedure using N26 with AMF reallocation and I-SMF insertion, the procedure "EPS to 5GS Mobility Registration Procedure (Idle) using N26 interface with AMF reallocation" defined in clause 4.11.1.3.4 are re-used, with the following change:

– The V-SMF is replaced by I-SMF and H-SMF is replaced by SMF, V-UPF is replaced by I-UPF.

– The V-SMF selection is replaced by the I-SMF selection.

– The V-CN Tunnel Info is replaced by Tunnel Info at I-UPF, H-CN Tunnel Info is replaced by Tunnel Info at UPF(PSA).

4.23.12.6 5GS to EPS handover using N26 interface with I-SMF removal

The procedure "5GS to EPS handover using N26 interface" in clause 4.11.1.2.1 for the home routed-roaming case is applied to handover from 5GS to EPS using N26 interface with I-SMF removal with the following changes:

– The V-SMF is replaced by I-SMF and H-SMF is replaced by SMF, V-UPF is replaced by I-UPF.

4.23.12.7 EPS to 5GS handover using N26 interface with I-SMF insertion

4.23.12.7.1 Preparation phase

For handover from EPS to 5GS using N26 interface, if I-SMF needs to be inserted, the preparation procedure defined in clause 4.11.1.2.2.1 for the home routed-roaming case are re-used, with the following changes:

– The V-SMF is replaced by I-SMF and H-SMF is replaced by SMF, V-UPF is replaced by I-UPF.

– The V-SMF selection is replaced by the I-SMF selection.

– The V-CN Tunnel Info is replaced by Tunnel Info at I-UPF, H-CN Tunnel Info is replaced by Tunnel Info at UPF(PSA).

4.23.12.7.2 Execution phase

For handover from EPS to 5GS using N26 interface, if I-SMF needs to be inserted, the execution procedure defined in clause 4.11.1.2.2.2 for the home routed-roaming case are re-used, with the following change:

– The V-SMF is replaced by I-SMF and H-SMF is replaced by SMF, V-UPF is replaced by I-UPF.

– The V-SMF selection is replaced by the I-SMF selection.

– The V-CN Tunnel Info is replaced by Tunnel Info at I-UPF, H-CN Tunnel Info is replaced by Tunnel Info at UPF(PSA).

4.23.12.8 Impact to 5GC procedures

4.23.12.8.1 General

This clause captures impact to 5GC procedures in other clauses of clause 4.23 to support interworking with N26.

4.23.12.8.2 UE Triggered Service Request with I-SMF insertion/change/removal

The following impact is applicable for UE triggered Service Request with I-SMF insertion in clause 4.23.4.3:

– In step 8a, when an I-SMF is inserted, if EBI(s) have been allocated before but the SMF+PGW-C has not prepared the CN Tunnel Info for each EPS bearer, the SMF+PGW-C requests the PGW-U+UPF to allocate the CN Tunnel for each EPS bearer for PDU Session(s). PGW-U+UPF allocates the PGW-U tunnel info for the EPS bearer and sends it to the SMF+PGW-C.

– In step 8c, the SMF+PGW-C provides also CN Tunnel Info for each EPS bearer to the I-SMF.

NOTE: The CN Tunnel Info for each EPS bearer provided to the I-SMF is to prepare for UE mobility to EPC network so that the I-SMF does not need interact with the SMF+PGW-C to get the EPS bearer context(s) at mobility to EPC.

4.23.12.8.3 PDU Session establishment procedure

The following impact is applicable for PDU Session establishment in clause 4.23.5.1:

– If I-SMF is involved in the PDU Session establishment, after EBI has been successfully allocated, the SMF+PGW-C also prepares CN tunnel info for each EPS bearer and provides it to the I-SMF, see clause 4.23.12.4.

4.23.12.8.4 PDU Session modification procedure

The following impact is applicable for PDU Session modification in clause 4.23.5.3:

– If I-SMF is involved in the PDU Session modification, if EBI needs to be allocated and the allocation is successful, the SMF+PGW-C also prepares CN tunnel info for each EPS bearer and provides it to the I-SMF, see clause 4.23.12.4.

4.23.12.8.5 Inter NG-RAN node N2 based handover with I-SMF insertion/change/removal

The following impact is applicable for Inter NG-RAN node N2 based handover with I-SMF insertion in clause 4.23.7.3.2:

– In step 7c, at I-SMF insertion, if EBI(s) have been allocated before but the SMF+PGW-C has not prepared the CN Tunnel Info for each EPS bearer, the SMF+PGW-C requests the PGW-U+UPF to allocate the CN Tunnel for each EPS bearer for PDU Session(s). PGW-U+UPF allocates the PGW-U tunnel info for the EPS bearer and sends it to the SMF+PGW-C.

– In step 7f, the SMF+PGW-C provides also the CN Tunnel Info for each EPS bearer to the I-SMF.

4.23.12.8.6 Xn based handover with insertion of I-SMF

The following impact is applicable Xn based handover with insertion of I-SMF in clause 4.23.11.2:

– In step 6, at I-SMF insertion, if EBI(s) have been allocated before but the SMF+PGW-C has not prepared the CN Tunnel Info for each EPS bearer, the SMF+PGW-C requests the PGW-U+UPF to allocate the CN Tunnel for each EPS bearer for PDU Session(s). PGW-U+UPF allocates the PGW-U tunnel info for the EPS bearer and sends it to the SMF+PGW-C.

– In step 9, the SMF+PGW-C provides also the CN Tunnel Info for each EPS bearer to the I-SMF.