5G to EPS Handover Without Using N26 Interface
This section describes the call flow for the 5G to EPS handover procedure without using the N26 interface.

Step | Description |
---|---|
1 - 2 | The UE initiates the TAU procedure by sending, to the eNodeB, a TAU Request. |
3 | The eNodeB forwards the TAU Request message to the new MME. |
4 | If the MME determines 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 | The UE initiates an Attach Request to the eNodeB. |
6 |
The eNodeB relays the Attach Request to the new MME. |
7 | Refer to the E-UTRAN Initial Attach procedure in TS 23.401. |
8 |
If the MME has changed since the last detach, or if there is no valid subscription context for the UE in the MME, or if the UE provides an IMSI or the UE provides an old GUTI which doesn't refer to a valid context in the MME, or for some network sharing scenario if the PLMN-ID of the TAI supplied by the eNodeB is different from that of the GUTI in the UE's context, the MME sends an Update Location Request message to the HSS. |
9 | The HSS acknowledges the Update Location message by sending an Update Location Ack (IMSI, Subscription data) message to the new MME. |
10 | Refer to the E-UTRAN Initial Attach procedure in TS 23.401. |
11 | After the MME receives the Modify Bearer Response (EPS Bearer Identity) message, it sends a Notify Request including the APN and PDN GW identity to the HSS for mobility with non-3GPP accesses. The message includes information that identifies the PLMN in which the PDN GW is located. |
12 |
In the case of non-emergency services, the HSS stores the APN and PDN GW identity pair. In the case of emergency services, the HSS stores the "PDN GW currently in use for emergency services". The HSS then sends a Notify Response to the MME. |
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 and sets the Request Type to "handover". UE provides an APN and the PDU Session ID corresponding to the PDU Session it wants to transfer to EPS. |
14 | The PGW-C+SMF initiates release of the PDU Session(s) in 5GS transferred to EPS. |