5.17.2.1 General

Interworking with EPC in this clause refers to mobility procedures between 5GC and EPC/E-UTRAN, except for clause 5.17.2.4. Network slicing aspects for EPS Interworking are specified in clause 5.15.7

In order to interwork with EPC, the UE that supports both 5GC and EPC NAS can operate in single-registration mode or dual-registration mode:

• In single-registration mode, UE has only one active MM state (either RM state in 5GC or EMM state in EPC) and it is either in 5GC NAS mode or in EPC NAS mode (when connected to 5GC or EPC, respectively). UE maintains a single coordinated registration for 5GC and EPC. Accordingly, the UE maps the EPS-GUTI to 5G GUTI during mobility between EPC and 5GC and vice versa following the mapping rules in Annex B. To enable re-use of a previously established 5G security context when returning to 5GC, the UE also keeps the native 5G-GUTI and the native 5G security context when moving from 5GC to EPC.
• In dual-registration mode, UE handles independent registrations for 5GC and EPC using separate RRC connections. In this mode, UE maintains 5G-GUTI and EPS-GUTI independently. In this mode, UE provides native 5G-GUTI, if previously allocated by 5GC, for registrations towards 5GC and it provides native EPS-GUTI, if previously allocated by EPC, for Attach/TAU towards EPC. In this mode, the UE may be registered to 5GC only, EPC only, or to both 5GC and EPC.

Dual-registration mode is intended for interworking between EPS/E-UTRAN and 5GS/NR. A dual-registered UE should not send its E-UTRA connected to 5GC and E-UTRAN radio capabilities to NR access when connected to 5GS/NR to avoid being handed over to 5GC-connected E-UTRA or to E-UTRAN.

NOTE 1: This is to prevent the dual registered UE from being connected to the same E-UTRA cell either connected to EPC or 5GC simultaneously using separate RRC connections via single RAN node as a result of handover. If a dual- registered UE implementation chooses to send its E-UTRA capability when connected to 5GS/NR, the UE and the network behaviour when UE enters a 5GC-connected E-UTRA is not further specified. If however the UE is registered with 5GS/NR only, the UE can send its E-UTRA capability in order to allow inter-RAT handover to E-UTRA/5GC and Dual Connectivity with multiple RATs.

If a dual-registered UE had not sent its E-UTRA connected to 5GC and E-UTRAN radio capabilities to 5GS and the UE needs to initiate emergency services, it shall locally re-enable its E-UTRA connected to 5GC and E-UTRAN radio capabilities in order to perform domain selection for emergency services as defined in TS 23.167 [18].

NOTE 2: However even in this case, the UE is still not expected to connect to E-UTRAN/EPC and E-UTRA/5GC simultaneously using separate RRC connection via single RAN node as a result of the domain selection for emergency services.

The support of single registration mode is mandatory for UEs that support both 5GC and EPC NAS.

During E-UTRAN Initial Attach, UE supporting both 5GC and EPC NAS shall indicate its support of 5G NAS in UE Network Capability described in clause 4.11.1.5.2 of TS 23.502 [3].

During registration to 5GC, UE supporting both 5GC and EPC NAS shall indicate its support of EPC NAS.

NOTE 3: This indication may be used to give the priority towards selection of SMF+PGW-C for UEs that support both EPC and 5GC NAS.

If the UE supports 5GC NAS, at PDN connection establishment in EPC, the UE may allocate a PDU Session ID and sends it via PCO, regardless of N1 mode status (i.e. enabled or disabled) in the UE.

NOTE 4: UE providing a PDU Session ID at PDN connection establishment even when N1 mode is disabled allows for IP address preservation during EPC to 5GC mobility once the UE re-enables N1 mode.

NOTE 5: For the case the MME has selected a standalone PGW for a PDN connection, if the UE re-enables N1 mode and reports the change in UE Network Capability, the MME can initiate PDN disconnection with reactivation required as described in clause 4.11.0a.8 of TS 23.502 [3] to allow selection of SMF+PGW-C thus session continuity at EPC to 5GC mobility.

If the EPC supports "Ethernet" PDU Session Type and the 5GSM Capabilities indicate that the UE supports Ethernet PDN type in EPC, then PDU Session type "Ethernet" is transferred to EPC as "Ethernet". Otherwise, PDU Session types "Ethernet" and "Unstructured" are transferred to EPC as "non-IP" PDN type (when supported by UE and network). If the UE or EPC does not support Ethernet PDN type in EPC, the UE sets the PDN type to non-IP when it moves from 5GS to EPS and after the transfer to EPS and the UE and the SMF shall maintain information about the PDU Session type used in 5GS, i.e. information indicating that the PDN Connection with "non-IP" PDN type corresponds to PDU Session type Ethernet or Unstructured respectively. This is done to ensure that the appropriate PDU Session type will be used if the UE transfers to 5GS.

PDN type "non-IP" is transferred to 5GS as "Unstructured" PDU Session type if it is successfully transferred.

It is assumed that if a UE supports Ethernet PDU Session type and/or Unstructured PDU Session type in 5GS it will also support non-IP PDN type in EPS. If this is not the case, the UE shall locally delete any EBI(s) corresponding to the Ethernet/Unstructured PDU Session(s) to avoid that the Ethernet/Unstructured PDU Session(s) are transferred to EPS.

MTU size consideration for PDU Sessions and PDN Connections towards a SMF+PGW-C follows the requirements in clause 5.6.10.4.

Networks that support interworking with EPC, may support interworking procedures that use the N26 interface or interworking procedures that do not use the N26 interface. Interworking procedures with N26 support provides IP address continuity on inter-system mobility to UEs that support 5GC NAS and EPC NAS and that operate in single registration mode. Networks that support interworking procedures without N26 shall support procedures to provide IP address continuity on inter-system mobility to UEs operating in both single-registration mode and dual-registration mode. In such networks, AMF shall provide the indication that interworking without N26 is supported to UEs during initial Registration in 5GC or MME may optionally provide the indication that interworking without N26 is supported in the Attach procedure in EPC as defined in TS 23.401 [26].

If the network does not support interworking with EPC, network shall not indicate support for "interworking without N26" to the UE.

When the HSS+UDM is required to provide the subscription data to the MME, for each APN, only one SMF+PGW-C FQDN and associated APN is provided to the MME according to TS 23.401 [26].

For interworking without N26 interface:

• if the PDU session supports interworking, the SMF+PGW-C stores the SMF+PGW-C FQDN to SMF context in HSS+UDM when the SMF is registered to HSS+UDM.
• For an APN, the HSS+UDM selects one of the stored SMF+PGW-C FQDN based on operator's policy.

For interworking with N26 interface:

• For a DNN, AMF determines PDU session(s) associated with 3GPP access in only one SMF+PGW-C supporting EPS interworking via EBI allocation procedure as described in clause 4.11.1.4.1 of TS 23.502 [3].
• If the network supports EPS interworking of non-3GPP access connected to 5GC, the AMF serving 3GPP access notifies the UDM to store the association between DNN and SMF+PGW-C FQDN which supports EPS interworking as Intersystem continuity context, to avoid MME receiving inconsistent SMF+PGW-C FQDN from AMF and HSS+UDM.
• The AMF updates Intersystem continuity context if the SMF+PGW-C and DNN association is changed due to the AMF selecting another SMF+PGW-C for EPS interworking for the same DNN.
• If the SMF+PGW-C FQDN and associated DNN exists in Intersystem continuity context, the HSS+UDM provides MME with SMF+PGW-C FQDN and associated APN.

It does not assume that the HSS+UDM is aware of whether N26 is deployed in the serving network. The HSS+UDM check the Intersystem continuity context first. If no SMF+PGW-C FQDN associated with an DNN exists in Intersystem continuity context, the HSS+UDM selects one of the SMF+PGW-C FQDN for the APN from SMF context based on operator's policy.

In entire clause 5.17.2 the terms "initial attach", "handover attach" and "TAU" for the UE procedures in EPC can alternatively be combined EPS/IMSI Attach and combined TA/LA depending on the UE configuration defined in TS 23.221 [23].

If a UE in MICO mode moves to E-UTRAN connected to EPC and any of the triggers defined in clause 5.4.1.3 occur, then the UE shall locally disable MICO mode and perform the TAU or Attach procedure as defined in clause 5.17.2. The UE can renegotiate MICO when it returns to 5GS during (re-)registration procedure.

IP address preservation for IP PDU sessions cannot be ensured on subsequent mobility from EPC/E-UTRAN to GERAN/UTRAN to a UE that had initially registered in 5GS and moved to EPC/E-UTRAN.

NOTE 6: The SMF+PGW-C might not include the GERAN/UTRAN PDP Context anchor functionality. Also, 5GC does not provide GERAN/UTRAN PDP Context parameters to the UE when QoS Flows of PDU Session are setup or modified in 5GS. Hence, the UE might not be able to activate the PDP contexts when it transitions to GERAN/UTRAN.

IP address preservation for IP PDU sessions cannot be ensured on subsequent mobility from EPC/E-UTRAN to 5GS for a 5GS NAS capable UE that had initially established a PDP context via GERAN/UTRAN and moved to EPC/E-UTRAN. IP address preservation for IP PDU session mobility between EPC/E-UTRAN and 5GS may be re-ensured as specified in clause 5.17.2.4 when UE moves from GERAN/UTRAN into EPC/E-UTRAN.

NOTE 7: The PGW acting as a GGSN might not support SMF+PGW-C functionality. GPRS does not support 5GS parameters transfer between UE and SMF+PGW-C (e.g. providing of PDU session ID and 5GS QoS information).

When a PDU session is moved from 5GS to EPS, the SMF+PGW-C keeps the registration and subscription in HSS+UDM until the corresponding PDN connection is released.

The SMF+PGW-C receives notification of subscription update regarding the 5G parameters (e.g. DNNs, S-NSSAIs) which are associated with the established PDN connection(s) connecting via EPS. If the subscription regarding DNN, QoS profile or PDN connection type associated with the PDN connection is updated, then the MME receives the subscription update and triggers corresponding actions according to TS 23.401 [26]. If the SMF+PGW-C receives subscription updates (e.g. change of 5GS Subscribed NSSAI) from the UDM the SMF+PGW-C triggers corresponding actions for the PDN connection. This may include (depending on the modified parameter) to:

• update the UPF (e.g. for a change of Framed Route information); or
• release the PDN connection with an appropriate cause (e.g. for change of EPS/5GS interworking support, for subscription removal of S-NSSAI associated with the PDN connection);
• do nothing about changes to DNN and/or PDU Session type that are handled by the MME.

If header compression is used for Control Plane CIoT EPS/5GS Optimisations and when the UE moves from EPS to 5GS or from 5GS to EPS, the UE may initiate the PDU Session Modification Procedure or UE requested bearer resource modification procedure to renegotiate the header compression configuration and to establish the compression context between the UE and MME/SMF, see TS 23.401 [26] and TS 24.501 [47].

If the UE is moving from 5GS to EPS and the RAT type is also moving from a "broadband" RAT (e.g. NR or WB-E-UTRA) to NB-IoT in EMM-IDLE state, the UE should set the mapped EPS bearer context for which the EPS bearer is a dedicated EPS bearer to state BEARER CONTEXT INACTIVE as for NB-IoT UEs in EPS only support the default bearers. In addition, UE shall locally deactivate the related bearers according to the maximum number of supported UP resources and send the latest bearer context status in the TAU Request.

If APN Rate Control is used when the UE moves from EPC to 5GC then the P-GW/SCEF and UE store the current APN Rate Control Status for an APN. If while connected to 5GC the last PDU Session to a DNN that is the same as the APN identified in the APN Rate Control Status is released then the APN Rate Control Status may be stored in the AMF in addition to the Small Data Rate Control Status and the UE discards the APN Rate Control Status. The APN Rate Control Status is stored in the AMF so it can be provided to the MME during mobility to EPC and subsequently applied at establishment of a new first PDN Connection to the same APN, if valid. The APN Rate Control Status is provided to the UPF+PGW-U if a first new PDU Session is established towards the DNN that is the same as the APN identified in the APN Rate Control Status if the UE moves back to EPC, taking into account its validity period.

The UE may be provided with initial APN Rate Control parameters by the SMF when a first new PDU Session is established for a DNN and S-NSSAI that supports interworking with EPS and the DNN matches an APN. The SMF provides the APN Rate Control Status for the APN that matches the DNN, if available at the SMF, otherwise the configured APN Rate Control parameters for the APN that matches the DNN are provided as the initially applied parameters. If the initially applied parameters differ from the configured APN Rate Control parameters and the first APN Rate Control validity period expires, the UE is updated with the configured APN Rate Control parameters once the UE has moved to EPC.

NOTE 8: If the APN Rate Control Status is provided to a UPF+PGW-U it is not used for Small Data Rate Control while the UE is connected to 5GC, it is only used as the APN Rate Control Status if the UE moves to EPC.

NOTE 9: Encoding of APN and DNN specified in TS 23.003 [19] allows the comparison of EPS APN and 5GS DNN.

If a Service Gap timer is running in the AMF when the UE moves from 5GC to EPC, the AMF stops the running Service Gap timer. If the UE returns to 5GC from EPC the AMF provides the Service Gap Time to the UE as described in clause 5.31.16.

If a Service Gap timer is running in the MME when the UE moves from EPC to 5GC, the MME stops the running Service Gap timer. If the UE returns to E-UTRAN connected to EPC from 5GC the MME provides the Service Gap Time to the UE as described in TS 23.401 [26].

If a Service Gap timer is running in the UE when the UE moves to from 5GC to EPC and if Service Gap Time is received from the MME, the UE stores the received Service Gap Time for later use when the timer needs to be started next time and the Service Gap timer that was started before the system change is kept running in the UE and applied for EPC. If a Service Gap timer is running in the UE when the UE moves to 5GC and if Service Gap Time is received from the AMF, the UE stores the received Service Gap Time for later use when the timer needs to be started next time and the Service Gap timer that was started before the system change is kept running in the UE and applied in 5GS.

For UE currently served by EPC, a SMF+PGW-C may support L2TP tunnelling on N6, as described in clause 5.8.2.16.