Content for  TS 23.502  Word version:  18.5.0

The handover procedure from EPS to 5GC-TNGF is supported as specified in clause 4.11.3.1 for handover procedure from EPS to 5GC-N3IWF with the following differences:

• The untrusted non-3GPP access is substituted by trusted non-3GPP access point (TNAP).
• The N3IWF is substituted by TNGF.
• The registration procedure via trusted non-3GPP access as specified in clause 4.12a.2 shall be performed in step 1.
• The UE requested PDU Session Establishment with Existing PDU Session indication in 5GC via trusted non-3GPP access is performed in step 2 as specified clause 4.12a.5.

The handover procedure from 5GC-TNGF to EPS is supported as specified in clause 4.11.3.2 for handover procedure from 5GC-N3IWF to EPS with the following differences:

• The untrusted non-3GPP access is substituted by trusted non-3GPP access point (TNAP).
• The N3IWF is substituted by TNGF.
• The network requested PDU Session Release via TNAP and TNGF shall be initiated as specified in clause 4.12a.7 from steps 3 to 12.

Figure 4.11.4.2-1 describes the procedure for handing over a PDU Session from 5GS to EPC/ePDG. The UE shall not request a handover of a PDU session with SSC mode 2 or SSC mode 3 to EPC/ePDG.

This clause captures impacts to 5GC procedures in other clauses of this specification to support interworking with EPS. These impacts are applicable to interworking based on N26 and interworking without N26 for PDU Session via 3GPP access and for PDU Session via non-3GPP access. This clause also captures the impact to 5GC if the UE was previously in GERAN/UTRAN as specified in clause 5.17.2.4 of TS 23.501.

The following impacts are applicable to clause 4.2.2.2 (Registration procedure):

• Step 1: If the Registration type is set to "Initial Registration", the UE is not registered in EPS and the UE provides the 5G-GUTI mapped from EPS GUTI as the old GUTI, then the UE includes complete EPS Attach Request in the Registration request message.
• Step 4: If the Registration type is "Initial Registration" as in step 1 of the Registration Procedure captured in clause 4.2.2.2.2, the target AMF may perform Identification Request towards MME along with complete EPS Attach Request message for MME to verify it as in step 3 as specified in clause 5.3.2.1 of TS 23.401.
• Step 14a:
  • If the AMF does not have event subscription information from the UDM, the AMF indicates it in Nudm_UECM_Registration. The UDM then provides event subscriptions (possibly retrieved from UDR) if any.
  • If mobility between GERAN/UTRAN and 5GS is required (as specified in clause 5.17.2.4 of TS 23.501), at Initial Registration, the AMF serving 3GPP access shall also register with the Nudm_UECM_Registration even if the AMF has a valid context.
• Step 14d: If mobility between GERAN/UTRAN and 5GS is required (as specified in clause 5.17.2.4 of TS 23.501), when the UDM stores the associated 3GPP Access Type together with the serving AMF as indicated in step 14a, it will also cause cancellation of any other previously registered serving node (e.g. MME if AMF does not indicate not to cancel or SGSN) via HSS/HLR.
• Step 15: As described in clause 6.3.7.1 of TS 23.501, if the AMF receives PCF Selection Assistance info and PCF ID(s) from the UDM, the AMF checks the PCF Selection Assistance info provided by UDM. If a list of DNN,S-NSSAI combinations are provided in the PCF Selection Info the AMF checks local configuration to determine which DNN,S-NSSAI to use then selects the PCF ID included in the corresponding UE Context in the SMF data. If no PCF ID is received, e.g. EPS interworking is not supported, or no PDN connection and related PCC association exists , the AMF select the PCF by considering other criteria, defined in clause 6.3.7.1 of TS 23.501.
• At the end of registration procedure, the AMF may initiate synchronization of event subscriptions with the UDM if the AMF does not indicate unavailability of event subscription in step 14a.

For PDU Session via 3GPP access the following impacts are applicable to clause 4.2.2.2 (Registration procedure) when the UE has established PDU Session(s): For PDU Session via non-3GPP access, the AMF determines if EPS interworking is supported and sends the indication to the SMF in the same way as for PDU Session via 3GPP access. The SMF makes the final decision on the EPS interworking in the same way as for PDU Session via 3GPP access with the following modification:

For PDU Session via 3GPP, the following impacts are applicable to clause 4.3.2.2 (UE Requested PDU Session Establishment procedure) to support interworking with EPS: For non-emergency PDU Session via non-3GPP, the AMF determines if EPS interworking is supported and sends the indication to the SMF in the same way as for PDU Session via 3GPP. The SMF makes the final decision on the EPS interworking in the same way as for PDU Session via 3GPP with the following modification:

• If the SMF does not receive the interworking indication, the SMF makes its decision based on subscription.

For emergency PDU Session via non-3GPP, the AMF determines if EPS interworking is supported and sends the indication to the SMF in the same way as for emergency PDU Session via 3GPP supporting EPS interworking.

For PDU Session via 3GPP, the following impacts are applicable to clause 4.3.3.2 (UE or network requested PDU Session Modification (non-roaming and roaming with local breakout)) to support interworking with EPS:

• Step 1: In addition to the triggers listed in step 1 of clause 4.3.3.2, the procedure may be also triggered by the following event:
  • AMF initiated modification: If the support of EPS Interworking for this PDU Session has changed, e.g. the change of the UE's subscription data (e.g. Core Network Type Restriction to EPS), or change of 5GMM capability (e.g. "S1 mode supported"), the AMF invokes Nsmf_PDUSession_UpdateSMContext update the status of EPS interworking support in the to SMF.
• Step 3a: This step also applies to AMF initiated modification. For AMF initiated modification, the SMF may determines whether the PDU session supports EPS interworking need be changed. If it need be changed, the SMF invokes Nudm_UECM_Update service operation to add or remove the PGW-C FQDN for S5/S8 interface from the UE context in SMF data stored at the UDM,

For PDU Session via 3GPP, the following impacts are applicable to clause 4.3.3.3 (UE or network requested PDU Session Modification (home-routed roaming)) to support interworking with EPS:

• Step 1a (AMF to V-SMF): Same impact as for step 1 of clause 4.3.3.2 above.
• Step 1a (V-SMF to H-SMF): The V-SMF pass the status of EPS interworking support to the H-SMF.
• Step 1a (H-SMF to V-SMF): Same impact as for step 3a of clause 4.3.3.2 above.

For interworking with the N26 interface, if status of interworking with EPS for a PDU session is changed at SMF+PGW-C, the SMF+PGW-C invokes EBI allocation or revocation as described in clause 4.11.1.4.1 and clause 4.11.1.4.2 respectively. For PDU Session via non-3GPP access, the AMF determines if EPS interworking is supported and sends the indication to the SMF in the same way as for PDU Session via 3GPP access. The SMF makes the final decision on the EPS interworking in the same way as for PDU Session via 3GPP access with the following modification:

The following impacts are applicable to clause 4.9.1.2.1 (General) to support interworking with EPS:

• If there is Mapping between EBI(s) and QFI(s) for the UE in the source NG-RAN, the source NG-RAN sends the Mapping to target NG-RAN during handover.

The following impacts are applicable to clause 4.9.1.3.2 (Preparation phase) to support interworking with EPS:

• Step 7: If the PDU session supports EPS interworking, the N2 SM information contains the Mapping between EBI(s) and QFI(s).

The following impacts are applicable to clause 4.3.4.2 (UE or network requested PDU Session Release for Non-Roaming and Roaming with Local Breakout) to support interworking with EPS:

• Step 2b: If the released PDU Session used APN Rate Control in EPC then the PGW-U+UPF provides the APN Rate Control Status to the SMF if the released PDU Session supported interworking with EPC and it is the last PDU Session to the DNN that is the same as the APN identified in the APN Rate Control Status.
• Step 3: If the PGW-U+UPF provided APN Rate Control Status to the SMF then the SMF provides the APN Rate Control Status to the AMF.

The following impacts are applicable to clause 4.3.4.3 (UE or network requested PDU Session Release for Home-routed Roaming) to support interworking with EPS:

• Step 2b: Same impact as for step 2b for the Non-roaming and Roaming with Local Breakout case above.
• Step 3: Same impact as for step 3 for the Non-roaming and Roaming with Local Breakout case above.

To avoid the need for identifier coordination between AMFs and MMEs, the AMF provides to the NG-RAN its served GUAMIs by separating the values between native AMF values and the values mapped from MME.

Support of NSAC in conjunction with interworking with EPC is described in clause 5.15.11.5 of TS 23.501. If EPS counting is required for a network slice, the SMF+PGW-C performs NSACF discovery that supports controlling the maximum number of PDU sessions per network slice as described in clause 6.3.22 of TS 23.501 and in clause 5.2.7.3.2. The following impacts are applicable to clause 4.2.11.4 (Number of PDU Sessions per network slice availability check and update procedure):

• The SMF+PGW-C invokes this procedure to perform network slice availability check on the number of PDU Sessions and update for S-NSSAI associated with the PDN connection during PDN connection establishment. In this case the SMF in Figure 4.2.11.2-1 is replaced with SMF+PGW-C.
• Step 3, The NSACF determines whether or not to accept the request as described in clause 5.15.11.5 of TS 23.501.

The SMF+PGW-C performs NSACF discovery that supports controlling the maximum number of UEs per network slice as described in clause 6.3.22 of TS 23.501 and in clause 5.2.7.3.2. If non-Hierarchical NSAC or Centralized NSAC architecture is deployed at the network, the following impacts are applicable to clause 4.2.11.4 (Number of PDU Sessions per network slice availability check and update procedure):

• The SMF+PGW-C invokes this procedure to perform network slice availability check on the number of PDU Sessions and update for S-NSSAI associated with the PDN connection during PDN connection establishment and PDN connection release. In this case the SMF in Figure 4.2.11.4-1 is replaced with SMF+PGW-C.
• Step 3: The NSACF determines whether or not to accept the request as described in clause 5.15.11.5 of TS 23.501.

If Hierarchical NSAC architecture is deployed at the network, the following impacts are applicable to clause 4.2.11.4a:

• The SMF+PGW-C invokes this procedure to perform network slice availability check on the number of PDU Sessions and update for S-NSSAI associated with the PDN connection during PDN connection establishment. In this case the SMF in figure 4.2.11.4a-1 is replaced with SMF+PGW-C.
• Step 8: The NSACF determines whether or not to accept the request as described in clause 5.15.11.5 of TS 23.501. When the local maximum number is reached, the NSACF may interact with the Primary NSACF before it returns the response back to the SMF+PGW-C as defined in clause 4.2.11.4a.

The SMF+PGW-C performs NSACF discovery that supports controlling the maximum number of UEs per network slice as described in clause 6.3.22 of TS 23.501 and in clause 5.2.7.3.2. If non-Hierarchical NSAC or Centralized NSAC architecture is deployed at the network, the following impacts are applicable to clause 4.2.11.2 (Number of UEs per network slice availability check and update procedure):

• The SMF+PGW-C invokes this procedure to perform network slice availability check on the number of UEs and update for S-NSSAI associated with the PDN connection during PDN connection establishment. In this case the AMF in figure 4.2.11.2-1 is replaced with SMF+PGW-C.
• Step 2: The SMF+PGW-C includes in the message the S-NSSAI, identity of SMF+PGW-C, UE ID and update flag. In a centralized NSAC architecture, the SMF+PGW-C additionally includes the NSAC service area if available.
• Step 3: The AMF ID is replaced by the SMF + PGW-C ID.
• Step 3: The NSACF determines whether or not to accept the request as described in clause 5.15.11.5 of TS 23.501.

If Hierarchical NSAC architecture is deployed at the network, the following impacts are applicable to clause 4.2.11.2a:

• The SMF+PGW-C invokes perform network slice availability check on the number of UEs and update for S-NSSAI associated with the PDN connection during PDN connection establishment. In this case the AMF in figure 4.2.11.2a-1 is replaced with SMF+PGW-C.
• Step 2: The SMF+PGW-C includes in the message the S-NSSAI, identity of SMF+PGW-C, UE ID and update flag.
• Step 3: The AMF ID is replaced by the SMF + PGW-C ID.
• Step 8: The NSACF determines whether or not to accept the request as described in clause 5.15.11.5 of TS 23.501. When the local maximum number is reached the NSACF may interact with the Primary NSACF before it returns the response back to the SMF+PGW-C as defined in clause 4.2.11.2a.

The SMF+PGW-C shall continue the PDN Connection Establishment procedure only when the Network Slice subject to NSAC is available based on both numbers of PDU Sessions and number of UEs.

Support of NSAC in conjunction with interworking with EPC for maximum number of UE with at least one PDU session/PDN connection is described in clause 5.15.11.5a of TS 23.501. For option 1, the same mechanisms in clause 4.2.11.2 (Number of UEs per network slice availability check and update procedure) is used with the following additions:

• The SMF+PGW-C invokes this procedure when the UE establishes the first PDU Session/PDN Connection and when the last PDU Session/PDN Connection associated with the network slice is released. In this case, the AMF in figure 4.2.11.2-1 is replaced with SMF+PGW-C.
• Step 2: The SMF+PGW-C includes in the message the S-NSSAI, identity of SMF+PGW-C, UE ID and update flag. The update flag may include either 'increase', 'decrease' or 'update' values.
• Step 2: The AMF ID is replaced by the SMF+PGW-C ID.
• Step 3: The 'update' value indicates that for existing UE registration the Access Type is to be replaced with a new Access Type during inter access mobility. If the update flag parameter from the SMF+PGW-C indicates update value, the NSACF locates the existing entry with UE ID and NF ID and replaces the Access Type in the existing entry. The NSACF may take the Access Type into account for UE counting based on local policy. If there was one UE entry in the new Access Type with same NF ID then NSACF may not increase the count again. If there was one UE entry in the new Access Type with different NF ID then NSACF may add this new NF ID in the UE entry and not increase the count. If there was one entry with same NF ID in the old Access Type then NSACF may remove the UE entry and decrease the count.
• The NSACF determines whether or not to accept the request as described in clause 5.15.11.5a of TS 23.501

For option 2, the same mechanisms in clause 4.2.11.4 (Number of PDU Sessions per network slice availability check and update procedure) are used with the following additions:

• The NSACF also counts the maximum number UE with at least one PDU session/PDN connection based on the update value received from SMF+PGW-C.
• If the update flag parameter from the SMF/SMF+PGW-C anchoring the PDU session/PDN connections indicates update, the NSACF locates the existing entry with UE ID and NF ID and replaces the Access Type in the existing entry. The NSACF may take the Access Type into account for UE counting based on local policy. If there was one UE entry in the new Access Type with same NF ID then NSACF may not increase the count again. If there was one UE entry in the new Access Type with different NF ID then NSACF may add this new NF ID and not increase the count. If there was one entry with same NF ID in the old Access Type then NSACF may remove the UE entry and decrease the count.
• The NSACF determines whether to accept the request as described in clause 5.15.11.5a of TS 23.501.
• The NSACF performs other actions in addition to the above to keep track of the last PDU Session/Connection released so as to decrease the count for the number of Registered UEs with at least one PDU Session/one PDN Connection.

If Hierarchical NSAC architecture is deployed in the network, there are the following additional enhancements:

• For option 2, the NSAC for the number of UEs with at least one PDU session/PDN connections and the NSAC for the number of PDU sessions shall be handled within the same NSACF.
• For both options, when the local maximum number of UEs with at least one PDU Session/PDN Connection or local threshold is reached, the NSACF may interact with the Primary NSACF before it returns the response back to the SMF+PGW-C. For more details on handling between the NSACF and Primary NSACF see clause 4.2.11.2a.
• For NSAC for number of PDU Sessions, when the local maximum number is reached, the NSACF may interact with the Primary NSACF before it returns the response back to the SMF+PGW-C. For more details on handling between the NSACF and Primary NSACF see clause 4.2.11.4a.

Figure 4.11.6.1-1 represent the information flow subscribing and notifying the availability or expected level of support of a service API. For the subscription to Nnef_APISupportCapability service, the subscription request may include the Duration of Reporting. If the Duration of Reporting is expired, the SCEF+NEF deletes the subscription without any explicit signalling interaction. For the CN type Change Event subscription to the HSS+UDM, the subscription request may include the Duration of Reporting. If the Duration of Reporting is expired, the HSS+UDM locally unsubscribe the CN Type Change Event without any explicit signalling interaction. The SCEF+NEF informs the AF of the API Indication which indicates list of available north-bound API(s).

Figure 4.11.6.2-1 represent the information flow unsubscribing to Continuous report type subscription of the availability or expected level of support of a service API. If the AF invokes Nnef_APISupportCapability_Subscribe service to SCEF+NEF node with the Duration of Reporting parameter for Continuous report type, the subscription on the SCEF+NEF and HSS+UDM are implicitly unsubscribed if the Duration of Reporting timer expires, i.e. the explicit unsubscribe service operation is not needed. If the explicit unsubscribe operation is needed, the information flow is as follows.

Figure 4.11.6.3-1 represent the information flow to configure monitoring events applicable to both EPC and 5GC using 5GC procedures towards UDM in scenarios where interworking between 5GS and EPC is possible.