5.17.2.2.2 Mobility for UEs in single-registration mode

When the UE supports single-registration mode and network supports interworking procedure with the N26 interface:

• For idle mode mobility from 5GS to EPS, the UE performs either TAU or Attach procedure with EPS GUTI mapped from 5G-GUTI sent as old Native GUTI, as described in clause 4.11.1.3.2.1 of TS 23.502 [3] and indicates that it is moving from 5GC. The UE includes in the RRC message a GUMMEI mapped from the 5G-GUTI and indicates it as a native GUMMEI and should in addition indicate it as "Mapped from 5G-GUTI". The MME retrieves the UE's MM and SM context from 5GC. For connected mode mobility from 5GS to EPS, either inter-system handover or RRC Connection Release with Redirection to E-UTRAN is performed. At inter-system handover, the AMF selects target MME based on 2 octet TAC format used in the Target ID as specified in TS 38.413 [34]. During the TAU or Attach procedure the HSS+UDM cancels any AMF registration associated with the 3GPP access (but not AMF registration associated with the non-3GPP access): an AMF that was serving the UE over both 3GPP and non-3GPP accesses does not consider the UE as deregistered over non 3GPP access.
• For the first TAU after 5GC initial Registration, the UE and MME for the handling of UE Radio Capabilities follow the procedures as defined in clause 5.11.2 of TS 23.401 [26] for first TAU after GERAN/UTRAN Attach.

NOTE 1: MMEs supporting interworking with N26 interface are not required to process the indication from the UE that it is moving from 5GC and will assume that the UE is moving from another MME.

• For idle mode mobility from EPC to 5GC, the UE performs mobility Registration procedure with the 5G GUTI mapped from EPS GUTI and indicates that it is moving from EPC. The UE derives GUAMI from the native 5G-GUTI and includes GUAMI in the RRC message to enable RAN to route to the corresponding AMF (if available). If the UE holds no native 5G-GUTI, then the UE provides in the RRC message a GUAMI mapped from the EPS GUTI and indicates it as "Mapped from EPS". The AMF and SMF retrieve the UE's MM and SM context from EPC. For connected mode mobility from EPC to 5GC, either inter-system handover or RRC Connection Release with Redirection to NG-RAN is performed. At inter-system handover, the MME selects target AMF based on TAC used in the Target ID as specified in TS 38.413 [34]. During the Registration procedure, the HSS+UDM cancels any MME registration.

NOTE 2: During a transition period, the source eNB may be configured via O&M to know that the MME is not upgraded and thus supports only 2 octet TAC. The Target ID for the NG-RAN node is set as "Target eNB ID" in the existing IEs as defined in TS 38.413 [34].

For both idle mode and connected mode mobility from EPC to 5GC:

• The UE includes the native 5G-GUTI as an additional GUTI in the Registration request; the AMF uses the native 5G-GUTI to retrieve MM context identified by the 5G-GUTI from old AMF or from UDSF (if UDSF is deployed and the old AMF is within the same AMF set).
• If this is the first mobility event for a PDU Session that was established while being connected to EPC, the UE shall trigger the PDU Session Modification procedure and:
• should indicate the support of Reflective QoS to the network (i.e. SMF) if the UE supports Reflective QoS functionality. If the UE indicated support of Reflective QoS, the network may provide a Reflective QoS Timer (RQ Timer) value to the UE;
• shall indicate the number of supported packet filters for signalled QoS rules, if the UE supports more than 16 packet filters for the PDU Session. The network shall store this information so that subsequent mobility events do not require another signalling of it.
• should indicate the support of Multi-homed IPv6 PDU session to the network (i.e. SMF) if the UE supports Multi-homed IPv6 PDU session. If the UE indicated support of Multi-homed IPv6 PDU session, the network shall consider that this PDU session is supported to use multiple IPv6 prefixes.
• should provide the UE Integrity Protection Maximum Data Rate to the network (i.e. SMF). The network shall consider that the maximum data rate per UE for user-plane integrity protection supported by the UE is valid for the lifetime of the PDU session.