4.11.0a Impacts to EPS Procedures

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

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

4.11.0a.1 General

This clause captures changes to procedures in TS 23.401 [13] that are common to interworking based on N26 and interworking without N26.

4.11.0a.2 Interaction with PCC

When interworking with 5GS is supported and a "SMF+PGW-C" is selected for a PDN connection, policy interactions between PDN GW and PCRF specified in TS 23.401 [13] are replaced by equivalent interactions between SMF+PGW-C and PCF as follows:

– IP-CAN Session Establishment procedure defined in TS 23.203 [24] is replaced by SM Policy Association Establishment Procedure as described in clause 4.16.4.The SMF+PGW-C includes the information elements received in Create Session Request message into the Npcf_SMPolicyControl_Create Service as follows: the SUPI contains the IMSI, the DNN contains the APN, the PEI contains either the IMEISV or IMEI, the Session AMBR contains the APN-AMBR and the default QoS information that contains the default EPS bearer QoS, note that QCI values are mapped into 5QI values. The SMF+PGW-C may receive PCC Rules and PDU Session Policy Information, 5G QoS information in the PCC Rule and in PDU Session Policy Information are mapped into EPS QoS information as defined in clause 4.11.1.1 and Annex C.

– (PCEF-initiated) IP-CAN Session Modification procedure defined in TS 23.203 [24] is replaced by SM Policy Association Modification procedure as described in clause 4.16.5.1.

– The SMF+PGW-C includes the information elements received in Modify Bearer Request or Modify Bearer Command message into the Npcf_SMPolicyControl_Update Service with the following modifications, the subscribed Session AMBR includes the subscribed APN-AMBR and subscribed default QoS information includes the default EPS bearer QoS, note that QCI values are mapped into 5QI values. The SMF+PGW-C includes the stored SUPI. The SMF+PGW-C may receive PCC Rules and PDU Session Policy Information, 5G QoS information in the PCC Rule and in PDU Session Policy Information are mapped into EPS QoS information as defined in clause 4.11.1.1 and Annex C.

– The SMF+PGW-C includes the information elements received in Delete Bearer Command message into the Npcf_SMPolicyControl_Update Service with the following modifications, The SMF+PGW-C includes the stored SUPI.

– (PCRF-initiated) IP-CAN Session Modification procedure defined in TS 23.203 [24] is replaced by SM Policy Association Modification procedure as described in clause 4.16.5.2. The SMF+PGW-C may receive PCC Rules and PDU Session Policy Information, 5G QoS information in the PCC Rule and in PDU Session Policy Information are mapped into EPS QoS information as defined in clause 4.11.1.1 and Annex C.

– IP-CAN Session Termination procedure defined in TS 23.203 [24] is replaced by SM Policy Association Termination procedure as described in clause 4.16.6. The SMF+PGW-C includes the information elements received in Delete Session Request message by the SMF+PGW-C into the Npcf_SMPolicyControl_Delete Service. The SMF+PGW-C includes the stored SUPI.

4.11.0a.3 Mobility Restrictions

The UE’s subscription may include access restriction for NR in 5GS and restriction for Core Network Type (5GC). If so, the HSS provides these restrictions to the MME. The MME includes these restrictions in the Handover Restriction List to the E-UTRAN. The MME and E-UTRAN use these restrictions to determine if mobility of the UE to 5GS or NR connected to 5GS should be permitted.

4.11.0a.4 PGW Selection

When the UE requests to establish a non-emergency PDN connection to an APN, the MME may use the UE’s support for 5GC NAS indication included in the UE Network Capability and/or UE’s subscription from HSS that includes UE’s mobility restriction parameters related to 5GS and/or indication of support for interworking with 5GS for this APN to determine if SMF+PGW-C or a standalone PGW-C should be selected. If both PGW-C and SMF+PGW-C is available, then MME may select SMF+PGW-C when UE’s subscription from HSS indicate support for interworking with 5GS for the APN.

NOTE: If restriction for Core Network Type indicates that the UE can access to 5GC, it implies that the UE has 5G subscription data.

When the UE performs emergency attach or requests to establish an emergency PDN connection, the MME may use the UE’s support for 5GC NAS indication included in the UE Network Capability and/or local configuration to determine if an emergency SMF+PGW-C or a standalone emergency PGW-C should be selected. An emergency SMF+PGW-C needs to be configured in Emergency Configuration Data in the MME if it is to be selected.

To enable a differentiation of the emergency gateway based on UE´s support of 5GC NAS, the Emergency Configuration Data in the MME defined in TS 23.401 [26] is extended with the IE listed in Table 4.11.0a.4-1.

Table 4.11.0a.4-1: MME Emergency Configuration Data Extensions

Field	Description
Emergency SMF+PGW-C identity	The statically configured identity of a combined SMF+PGW-C used for emergency APN. The SMF+PGW-C identity may be either an FQDN or an IP address. (NOTE)
NOTE: The FQDN always resolves to one SMF+PGW-C.

4.11.0a.5 PDN Connection Establishment

During establishment of non-emergency PDN connection in the EPC, the UE and the SMF+PGW-C exchange information via PCO as described in clause 5.15.7 of TS 23.501 [2]. For UE with 5GC NAS capability disabled (i.e. N1 mode is disabled), the UE may also allocate a PDU Session ID and send it to the SMF+PGW-C via PCO. If the SMF+PGW-C supports more than one S-NSSAI and the APN is valid for more than one S-NSSAI, before the SMF+PGW-C provides an S-NSSAI to the UE, the SMF+PGW-C should check such that the selected S-NSSAI is among the UE’s subscribed S-NSSAIs and that the S-NSSAI is not subject to Network Slice-Specific Authentication and Authorization, by retrieving the Subscribed S-NSSAI from UDM using the Nudm_SDM_Get service operation (the SMF+PGW-C discovers and selects a UDM as described in clause 6.3.8 of TS 23.501 [2]). If the SMF+PGW-C is in a VPLMN, the SMF+PGW-C uses the Nnssf_NSSelection_Get service operation to retrieve a mapping of the Subscribed S-NSSAIs to Serving PLMN S-NSSAI values. If the S-NSSAIs supported by the SMF+PGW-C are all subject to NSSAA, then the SMF+PGW-C should reject the PDN connection establishment. If the selected S-NSSAI is subject to NSAC and EPS counting is required for the S-NSSAI, the SMF+PGW-C uses the Nnsacf_NSAC_NumberOfUEsUpdate services operation and/or the Nnsacf_NSAC_NumberOfPDUsUpdate services operation to check if the selected S-NSSAI is available as described in clause 4.11.5.9. The SMF+PGW-C uses the Nudm_SDM_Subscribe service operation to subscribe the change of the Session Management Subscription data. If the SMF+PGW-C is notified from UDM with subscription data change, the SMF+ PGW-C takes actions for the PDN connection as described in clause 5.17.2.1 of TS 23.501 [2].

As described in TS 23.548 [74], during establishment of a PDN connection, a UE that hosts EEC(s) may indicate to the SMF+PGW-C, in the PCO, that it supports the ability to receive ECS address(es) via NAS and to transfer the ECS Address(es) to the EEC(s). If the UE indicated in the PCO that it supports the ability to receive ECS address(es) via NAS, the SMF+PGW-C may provide the ECS Address Configuration Information (as described in clause 6.5.2 of TS 23.548 [74]) to the UE in the PCO. The SMF+PGW-C may derive the Edge Configuration Server Information based on local configuration, the UE’s location, and/or UE subscription information.

The SMF+PGW-C may use the bearer modification procedure without bearer QoS update to send the UE a PCO with updated ECS Address Configuration Information as defined in clause 6.5.2 of TS 23.548 [74] to the UE.

During establishment of non-emergency PDN connection in the EPC, if PGW-C+SMF is selected for a UE that has 5GS subscription, the SMF may be configured to obtain the subscribed IP index from UDM as part of subscription data using the Nudm_SDM_Get service operation (the PGW-C+SMF discovers and selects a UDM as described in clause 6.3.8 of TS 23.501 [2]).

During establishment of non-emergency PDN connection in the EPC, if SMF+PGW-C is selected for a UE that has 5GS subscription but does not support 5GC NAS and is accessing via EPC/E-UTRAN and if the SMF+PGW-C supports more than one S-NSSAI and the APN is valid for more than one S-NSSAI, the SMF+PGW-C+PGW-C may proceed as specified in first paragraph of this clause or select any S-NSSAI associated with the APN of the PDN connection. The SMF+PGW-C shall not provide any 5GS related parameters to the UE.

NOTE 1: The SMF+PGW-C knows that the UE does not support 5GS NAS if the UE does not provide PDU Session ID in PCO (see clause 5.15.7 of TS 23.501 [2]).

During establishment of emergency PDN connection:

– The SMF+PGW-C is to be derived from the emergency APN or to be statically configured in the Emergency Configuration Data in MME.

– 5GC interworking support with N26 or without N26 is determined based on UE’s 5G NAS capability and local configuration (in the Emergency Configuration Data in MME).

– The S-NSSAI configured for the emergency APN in SMF+PGW-C is not sent to the UE by the SMF+PGW-C. One S-NSSAI is configured for the emergency APN.

During establishment of non-emergency PDN connection and emergency PDN connection, if SMF+PGW-C is selected for a UE that does not support 5GC NAS, the SMF+PGW-C creates unique PDU Session ID for each PDN connection of the UE.

– The unique PDU Session ID can be created based on the EPS Bearer IDs assigned by the MME for the PDN Connections associated with the UE and not be in the range of PDU Session ID values that can be created by a 5GC NAS capable UE.

– If handover between EPS and EPC/ePDG (as specified in clause 8.6 of TS 23.402 [26]) is required, the SMF+PGW-C, based on operator configuration, may perform the following to ensure the uniqueness of PDU Session ID:

– the SMF+PGW-C queries the UDM for any PDU Session ID(s) that are already registered in the UDM for the UE by invoking Nudm_UECM_Get service operation.

– the SMF+PGW-C creates a PDU session ID that does not collide with the received PDU session ID(s).

– When the SMF+PGW-C establishes the PDN connection successfully, the SMF+PGW-C performs UDM registration using the Nudm_UECM_Registration service operation.

NOTE 2: If the scenario that a UE handovers a PDN connection with APN1 established via MME to ePDG and then establishes a second PDN connection with APN2 via MME needs to be supported in a given deployment, then, based on operator configuration, the SMF+PGW-C can query the UDM to avoid that the same PDU Session ID value of first PDN connection would be assigned by the SMF+PGW-C for the second PDN connection.

When the SMF+PGW-C establishes the PDN connection successfully, the SMF+PGW-C provides the ID of the PCF ID selected for the PDN connection in the UDM using the Nudm_UECM_Registration service operation.

A SMF+PGW-C may support L2TP as described in clause 4.3.2.4. In this case step 1 and step 7 of Figure 4.3.2.4‑1 correspond to a PDN Connection establishment and a SMF+PGW-C replaces the SMF in that Figure.

To support User Plane Integrity Protection with EPS and policies that Require User Plane integrity protection to be used, at PDN connection establishment, the MME shall indicate to the SMF+PGW-C (via the Serving GW) whether the UE, the current eNB and the MME support User Plane Integrity Protection in EPS. If the MME and the UE support User Plane Integrity Protection, then the SMF+PGW-C informs the MME of the User Plane integrity protection policy (Required, Preferred, Not Needed) applicable to the PDN connection on a per-EPS bearer basis. In turn, the MME informs the eNB.

4.11.0a.6 Network Configuration

To avoid the need for identifier coordination between MMEs, AMFs and SGSNs, the MME provides to the eNB its served GUMMEIs by separating the values between native MME GUMMEI values, values mapped from AMF and values mapped from SGSN.

4.11.0a.7 Interactions with DN-AAA Server

EAP-based secondary authentication and authorization at PDU session establishment is supported as defined in clause 4.3.2.3. EAP-based secondary authentication and authorization at PDN connection establishment is supported as defined in Annex H.

This clause 4.11.0a.7 defines the support of secondary authorization over EPS without EAP-based authentication when one of the UE, the 5GC, and the DN does not support (or is configured not to use) the mechanisms defined in Annex H.

NOTE 1: This implies that when data connectivity to the DN is initiated as a PDU session over 5GC it can be subject to EAP based secondary authentication mechanism (see TS 33.501 [15]) whereas, when data connectivity to the same DN is initiated as a PDN connection over EPC it cannot be subject to EAP based secondary authentication mechanism. This discrepancy occurs regardless of whether the data connection is later on moved between EPC and 5GC.

NOTE 2: Secondary authorization without authentication means no signalling exchange with the UE.

If secondary authentication and authorization has been performed for the PDU Session while the UE was in 5GS and the UE has moved to EPS, the following applies:

– DN-AAA re-authorization (without re-authentication signalling) can be performed even when the UE is in EPS, e.g. to provide new parameters from the DN-AAA Server to SMF+PGW-C.

– Re-authentication cannot be performed while the UE is in EPS because there is no support of the related signalling in EPS. In case the SMF+PGW-C receives a re-authentication request from the DN-AAA, the SMF+PGW-C informs the DN-AAA Server that the UE is not available for re-authentication at the moment. The SMF+PGW-C should not initiate PDN connection release at this point: the DN-AAA decides the actions to take, based on the reply from SMF+PGW-C and local policy which may also trigger a DN-AAA Server request to release the PDU Session/PDN connection of the UE.

4.11.0a.8 5GC NAS capability (re-)enabled and disabled

When 5G NAS (i.e. N1 mode) capability is (re-)enabled, the UE triggers Tracking Area Update procedure as specified in clause 5.3.3.0 of TS 23.401 [13].

If the MME determines that interworking with 5GS is changed to Supported and if the MME has selected a standalone PGW, the MME may initiate PDN disconnection with reactivation required as specified in clause 5.10.3 of TS 23.401 [13] at the end of the tracking area update procedure.

NOTE 1: MME requesting PDN disconnection with reactivation required is to allow MME to select a SMF+PGW-C so that session continuity can be achieved when UE moves to 5GS.

If a UE disables its support of N1 mode and moves from 5GS to EPS, the UE and the SMF+PGW-C maintain the mapped 5GS parameters for PDU session(s) that are transferred to EPS.

NOTE 2: The mapped 5GS parameters are used when the UE re-enables N1 mode and moves back to 5GS.

4.11.0a.9 PDN Connection Release

If the SMF+PGW-C has registered with the UDM, the SMF+PGW-C shall deregister with UDM (using Nudm_UECM_Deregistration) during the PDN Connection Release procedure.

NOTE: SMF+PGW-C registration in the UDM is specified in clause 4.11.0a.5 or clause 4.3.2.