Section: 4.11.2.2 5GS to EPS Mobility
Content: The following procedure is used by UEs in single-registration or dual registration mode on mobility from 5GS to EPS. In the case of network sharing the UE selects the target PLMN ID according to clause 5.18.3 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. Figure 4.11.2.2-1: Mobility procedure from 5GS to EPS without N26 interface The UE operating in single-registration mode can start the procedure from Step 1 or Step 5. The UE operating in dual-registration mode starts the procedure from Step 5. NOTE 1: The network has indicated the " Interworking without N26" to the UE. To support IP address preservation, the UE in single-registration mode starts the procedure from Step 5. If the UE in single-registration mode starts the procedure from Step 1, the IP address preservation is not provided. 0. UE is registered in 5GS and established PDU sessions. The FQDN for the S5/S8 interface of the SMF+PGW-C is also stored in the UDM by the SMF+PGW-C during PDU Session setup in addition to what is specified in clause 4.3.2.2.1 and clause 4.3.2.2.2. NOTE 2: At 5GS to EPS mobility, the MME use the FQDN for the S5/S8 interface of the SMF+PGW-C to find the SMF+PGW-C and when UE moves back from EPS to 5GS, the AMF uses FQDN for the S5/S8 interface of the SMF+PGW-C to find the SMF+PGW-C. 1. Step 1 as in clause 5.3.3.1 (Tracking Area Update) in TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [13]. 2. Step 2 as in clause 5.3.3.1 (Tracking Area Update) in TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [13] with the following modifications: The UE shall provide a EPS-GUTI that is mapped from the 5G-GUTI following the mapping rules specified in TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. The UE indicates that it is moving from 5GC. 3. Step 3 as in clause 5.3.3.1 (Tracking Area Update) in TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [13]. 4. If the MME determined that the old node is an AMF based on UE's GUTI mapped from 5G-GUTI and the MME is configured to support 5GS-EPS interworking without N26 procedure, the MME sends a TAU Reject to the UE. 5. Step 1 as in clause 5.3.2.1 (E-UTRAN Initial Attach) in TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [13] with the modifications captured in clause 4.11.2.4.1. 6. Step 2 as in clause 5.3.2.1 (E-UTRAN Initial Attach) in TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [13]. 7. Steps 4-7 as in clause 5.3.2.1 (E-UTRAN Initial Attach) in TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [13], with the modifications captured in clause 4.11.2.4.1. 8. Step 8 as in clause 5.3.2.1 (E-UTRAN Initial Attach) in TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [13], with the modifications captured in clause 4.11.2.4.1. 9. Step 11 as in clause 5.3.2.1 (E-UTRAN Initial Attach) in TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [13], with the following modifications: The subscription profile the MME receives from HSS+UDM includes per DNN/APN at most one SMF+PGW-C FQDN as described in in clause 5.17.2.1 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. 10. Steps 12-24 as in clause 5.3.2.1 (E-UTRAN Initial Attach) in TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [13], with the modifications as described in clause 4.11.2.4.1. 11. Step 25 as in clause 5.3.2.1 (E-UTRAN Initial Attach) in TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [13]. 12. Step 26 as in clause 5.3.2.1 (E-UTRAN Initial Attach) in TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [13]. 13. If the UE has remaining PDU Sessions in 5GS which it wants to transfer to EPS and maintain the same IP address/prefix, the UE performs the UE requested PDN Connectivity Procedure as specified in clause 5.10.2 of TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [13] and sets the Request Type to "handover" or "handover of emergency bearer services" in Step 1 of the procedure with modification captured in clause 4.11.2.4.2. UE provides an APN and the PDU Session ID corresponding to the PDU Session it wants to transfer to EPS. The UE provides the PDU Session ID in PCO as described in clause 4.11.1.1. UEs in single-registration mode performs this step for each PDU Session immediately after completing the E-UTRAN Initial Attach procedure. UEs in dual-registration mode may perform this step any time after the completing of E-UTRAN Initial Attach procedure. Also, UEs in dual-registration mode may perform this step only for a subset of PDU Sessions. The MME determines the SMF+PGW-C address for the Create Session Request based on the APN if received from the UE, local Emergency Configuration Data (as in clause 4.11.0a.4) and the subscription profile which may include the Emergency Information received from the HSS+UDM in Step 9 or when the HSS+UDM notifies the MME for the new SMF+PGW-C ID in the updated subscription profile. The SMF+PGW-C uses the PDU Session ID to correlate the transferred PDN connection with the PDU Session in 5GC. As a result of the procedure the PGW-U+UPF starts routing DL data packets to the Serving GW for the default and any dedicated EPS bearers established for this PDN connection. 14. For Non-Roaming case and Roaming with Local Breakout, the SMF+PGW-C initiates release of the PDU Session(s) in 5GS transferred to EPS as specified in clause 4.3.4.2 with the following clarification: - In step 2, the SMF+PGW-C shall not release IP address/prefix(es) allocated for the PDU Session; - If UP connection of the PDU Session is not active, step 3b is not executed, thus the steps triggered by step 3b are not executed; If UP connection of the PDU Session is active, the SMF invokes the Namf_Communication_N1N2MessageTransfer service operation in step 3b without including N1 SM container (PDU Session Release Command); - In step 11, Nsmf_PDUSession_SMContexStatusNotify service operation invoked by the SMF to notify AMF that the SM context for this PDU Session is released due to handover to EPS. For Home Routed roaming, the SMF+PGW-C initiates release of the PDU Session(s) in 5GS transferred to EPS as specified in clause 4.3.4.3 with the following clarification: - In step 3a, the H-SMF invokes the Nsmf_PDUSession_Update service operation without including N1 SM container (PDU Session Release Command); - In step 16a, Nsmf_PDUSession_StatusNotify operation invoked by H-SMF to notify the V-SMF that the PDU session context is released due to handover to EPS; - In step 16b, Nsmf_PDUSession_SMContexStatusNotify service operation invoked by the V-SMF to notify AMF that the SM context for this PDU Session is released due to handover to EPS.