4.11.5 Impacts to 5GC Procedures

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

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

4.11.5.1 General

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 [2].

4.11.5.2 Registration procedure

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 [13].

NOTE 1: The steps above apply to interworking with N26.

– 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 [2]), 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 [2]), 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.

NOTE 2: Upon mobility from GERAN/UTRAN to 5GS as specified in clause 5.17.2.4 of TS 23.501 [2], the previously registered SGSN needs to be cancelled from HSS/HLR, otherwise the incoming session can fail.

– Step 15: As described in clause 6.3.7.1 , clause TS 23.501 [2], 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 TS 23.501 [2].

– 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.

NOTE 3: The details how synchronization can be done is left to stage 3.

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):

In clause 4.3.2.2.1 Non-roaming and Roaming with Local Breakout:

– Step 17: Additional trigger for step 17 Nsmf_PDUSession_UpdateSMContext are:

– If status of interworking with EPS for a PDU session changes, e.g. due to change of 5GMM capability (e.g. "S1 mode supported"), the UE subscription data change (e.g. Core Network Type Restriction to EPC), the AMF invokes Nsmf_PDUSession_UpdateSMContext (EPS Interworking Indication with N26 or without N26) to SMF. The SMF determines whether the PDU session supports interworking with EPS need be changed. If it needs to 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 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:

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

4.11.5.3 UE Requested PDU Session Establishment procedure

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:

In clause 4.3.2.2.1 Non-roaming and Roaming with local breakout:

– Step 1: In PDU Session Establishment Request message, the UE includes also the UE capability of Ethernet PDN type support in EPS to the SMF (or H-SMF in home routing roaming);

– Step 3: The AMF determines that a PDU Session supports EPS interworking with N26 or without N26, based on e.g. 5GMM capability (e.g. "S1 mode supported"), UE subscription data (e.g. Core Network Type Restriction to EPS, EPC interworking support per (S-NSSAI, subscribed DNN)) and network configuration if EPS interworking with N26 or without N26 is supported. The AMF then includes in the Nsmf_PDUSession_CreateSMContext an indication whether the PDU Session supports EPS Interworking with N26 or without N26.

For PDU Session with Request Type "initial emergency request", the AMF decides the EPS interworking with N26 or without N26 based on 5GMM capability and local configuration.

For PDU Session with Request Type "Existing Emergency PDU Session", the AMF shall use Emergency Information received from HSS+UDM and the S-NSSAI locally configured in Emergency Configuration Data.

If the Request Type indicates "Existing PDU Session" the AMF selects the SMF based on SMF-ID or SMF+PGW-C FQDN received from UDM during the Registration or Subscription Profile Update Notification procedure. The case where the AMF does not recognize the PDU Session ID or the subscription context that the AMF received from UDM neither contains an SMF ID nor a SMF+PGW-C FQDN corresponding to the PDU Session ID constitutes as an error case.

NOTE 1: If the AMF receives from the UDM, for a PDU Session, both a SMF ID and a SMF+PGW-C FQDN, the SMF ID takes precedence.

If the AMF has stored APN Rate Control Status and the PDU Session is considered a new first PDU Session to a DNN that is the same as the APN in stored APN Rate Control Status and interworking with EPC is enabled for this PDU Session, then the AMF sends the APN Rate Control Status to the SMF.

The AMF indicates to the SMF whether the UE support User Plane Integrity Protection with EPS and whether the AMF has associated functionality.

– Step 4: If the EPS Interworking indication received from AMF indicates that the UE supports EPS interworking and the SMF determines, based on the EPS interworking support indication from the AMF and additional UE subscription data (e.g. whether UP integrity protection of UP Security Enforcement Information is not set to required, EPS interworking is allowed for this DNN and S-NSSAI), that the PDU Session supports EPS interworking, the SMF+PGW-C FQDN for S5/S8 interface is included in the Nudm_UECM_Registration Request.

– Step 10a: If APN Rate Control Status is received from the AMF then the SMF provides the configured APN Rate Control Status to the PGW-U+UPF.

– Step 13 In PDU Session Establishment Accept message, the SMF also includes indication of Ethernet PDN type supported if the Ethernet PDN type is supported by both the UE and the SMF+PGW-C. The SMF and the UE stores the information if Ethernet PDN type is supported for later use when UE moves from 5GS to EPS.

– Step 16c: For PDU Session establishment with Request Type "initial PDU Session", if the SMF+PGW-C selects the same PCF as the PCF ID received from AMF as specified in clause 4.3.2.2.1 and if the PDU Session supports EPC interworking, the SMF provide the selected PCF ID in the UDM using the Nudm_UECM_Registration service operation.

NOTE 2: The subscription data "EPS interworking support indication" is used by AMF when determining the EPS interworking support for the PDU Session. Therefore, when the UE establishes the PDU Session via the 3GPP access, the SMF does not need to consider the same subscription data "EPS interworking support indication" again.

In clause 4.3.2.2.2 Home-routed Roaming:

– Step 3a: Same impact as for step 3 for the non-roaming and roaming with local breakout case above.

– Step 5: Same impact as for step 10a for the Non-roaming and Roaming with Local Breakout case above.

– Step 6 The V-SMF pass the EPS interworking support indication received from the AMF to the H-SMF in Nsmf_PDUSession_Create.

– Step 7: If the EPS interworking indication received from V-SMF indicates that the PDU Session supports EPS interworking and the H-SMF determines, based on the EPS interworking support indication from the AMF and additional information such as UP integrity protection of UP Security Enforcement Information as described in clause 4.11.1.1, that the PDU Session supports EPS interworking, the SMF+PGW-C FQDN for S5/S8 interface is included in the Nudm_UECM_Registration Request.

– Step 15: Same impact as in step 13 for the non-roaming and roaming with local breakout case above with the difference that it’s the home SMF+PGW-C that includes the indication of Ethernet PDN type supported.

For interworking with the N26 interface, if the PDU Session supports interworking with EPS, the SMF+PGW-C invokes EBI allocation as described in clause 4.11.1.4.1.

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.

4.11.5.4 UE or Network Requested PDU Session Modification procedure

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 clause 3a of 4.3.3.2 above.

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

4.11.5.5 Xn based inter NG-RAN handover

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.

4.11.5.6 Inter NG-RAN node N2 based 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).

4.11.5.7 UE or Network Requested PDU Session Release procedure

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.

4.11.5.8 Network Configuration

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.

4.11.5.9 Network Slice Admission Control

Support of NSAC in conjunction with interworking with EPC is described in clause 5.15.11.5 of TS 23.501 [2].

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 [2] 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 [2].

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 [2] and in clause 5.2.7.3.2.

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.

– Step 3: The AMF 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 [2].

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.