Section: 5.17.2.2 Interworking Procedures with N26 interface 5.17.2.2.1 General
Content: NOTE 1: Additional network slicing and Interworking with EPS with N26 aspects are specified in clause 5.15.7. Interworking procedures using the N26 interface, enables the exchange of MM and SM states between the source and target network. The N26 interface may be either intra-PLMN or inter-PLMN (e.g. to enable inter-PLMN mobility). When interworking procedures with N26 is used, the UE operates in single-registration mode. For the 3GPP access, the network keeps only one valid MM state for the UE, either in the AMF or MME. For the 3GPP access, either the AMF or the MME is registered in the HSS+UDM. The support for N26 interface between AMF in 5GC and MME in EPC is required to enable seamless session continuity (e.g. for voice services) for inter-system change. The UE's subscription may include restriction for Core Network Type (EPC) and RAT restriction for E-UTRA. If so, the UDM provides these restrictions to the AMF. The AMF includes RAT and Core Network type restrictions in the Handover Restriction List to the NR. The AMF and NR use these restrictions to determine if mobility of the UE to EPS or E-UTRA connected to EPS should be permitted. When the UE moves from 5GS to EPS, the SMF determines which PDU Sessions can be relocated to the target EPS, e.g. based on capability of the deployed EPS, operator policies for which PDU Session, seamless session continuity should be supported etc. The SMF can release the PDU Sessions that cannot be transferred as part of the handover or Idle mode mobility. However, whether the PDU Session is successfully moved to the target network is determined by target EPS. Similarly, the UE's subscription may include restriction for Core Network Type (5GC) and RAT restriction for NR. If so, the HSS provides these restrictions to the MME. The MME includes RAT and Core Network type 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. If the SMF+PGW-C receives the PDU session ID from UE via PCO and know 5GC is not restricted for the PDN connection by user subscription, the SMF+PGW-C sends the mapped QoS parameters to UE. When the UE moves from EPS to 5GS, for the case when the MME has selected SMF+PGW-C even for PDN connections that cannot be relocated to the target 5GS, the SMF+PGW-C determines which PDN Connections can be relocated to the target 5GS, e.g. based on capability of the deployed 5GS, subscription and operator policies for which PDN Connection, seamless session continuity should be supported etc. The SMF+PGW-C and NG-RAN can reject the PDN Connections that cannot be transferred as part of the handover or Idle mode mobility. For the case when the MME has selected standalone P-GW for a PDN connection for which session continuity is not supported and the AMF cannot retrieve the address of the corresponding SMF during EPS to 5GS mobility, the AMF does not move the PDN connection to 5GS. NOTE 2: When applying the AMF planned removal procedure or the procedure to handle AMF failures (see clause 5.21.2) implementations are expected to update the DNS configuration to enable MMEs to discover alternative AMFs if the MME tries to retrieve a UE context from an AMF that has been taken out of service or has failed. This addresses the scenario of UEs performing 5GS to EPS Idle mode mobility and presenting a mapped GUTI pointing to an AMF that has been taken out of service or has failed. In the case of mobility from 5GS to EPS, if the MME lacks certain capability, e.g. MME not supporting 15 EPS bearers, the 5GC shall not transfer the UE EPS bearers and/or EPS PDN connections that are not supported by the EPC network. If the MME does not support 15 EPS bearers, the AMF determines which EBIs cannot be transferred to EPS, and retrieves the EPS bearer contexts from the SMF+PGW-C for the EBIs that can be transferred to EPS. NOTE 3: How the AMF determines which EBIs can be transferred to EPS is according to local configuration, e.g. according to DNN, S-NSSAI, ARP associated with an EBI. In the case of mobility from 5GS to EPS, if the mobility is a result of the PCF modifying the RFSP Index value for the UE to indicate that EPC/E-UTRAN access is prioritized over the 5GS access, the AMF may be sent with a RFSP in Use Validity Time by the PCF as specified in TS 23.503[ Policy and charging control framework for the 5G System (5GS); Stage 2 ]  [45]. If the AMF receives RFSP in Use Validity Time and selects the RFSP Index in use identical to the authorized RFSP Index as specified in clause 5.3.4.3, then the AMF provides the MME with the RFSP Index in use and the RFSP in Use Validity Time, which indicates the time by which the RFSP Index in use will be used in the MME as specified in clause 4.11.1.5.8 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3]. NOTE 4: The RFSP in Use Validity Time is to allow the UE to stay in EPS for a period of time to avoid the potential ping-pong issue (i.e. 5GS keeps sending the UE to EPS based on authorized RFSP Index from PCF, and the EPS keeps sending the UE back to 5GS immediately based on the subscribed RFSP Index. NOTE 5: The RFSP in Use Validity Time applies only to EPS but not to 5GS, therefore in the case of mobility from EPS to 5GS, the RFSP in Use Validity Time if received from MME is ignored by the AMF.