5GS to EPS Handover for Single Registration Mode with N26 Interface

Call flow for the 5GS to EPS Handover for Single Registration Mode with N26 Interface
Important

The IP address preservation cannot be supported if PGW-C+SMF in the HPLMN doesn't provide the mapped QoS parameters.

Call Flow Description for the 5GS to EPS Handover for Single Registration Mode with N26 Interface
Step Description
1 The NG-RAN sends a Handover Required (Target eNB ID, Source to Target Transparent Container, inter-system handover indication) message to the AMF.
2a AMF sends Nsmf_PDUSession_ContextRequest requests the V-SMF to provide SM Context that also includes the mapped EPS Bearer Contexts.
Note

The AMF knows the MME capability to support non-IP PDN type or not through local configuration.

Note

In a home routed roaming scenario, the UE's SM EPS Contexts is obtained from the V-SMF.

2b The PGW-C+SMF send N4 Session modification to PGW-U+UPF to establish the CN tunnel for each EPS bearer and provide EPS Bearer Contexts to AMF.
2c PGC+SMF sends Nsmf_PDUSession_ContextResponse requests the V-SMF to provide SM Context that also includes the mapped EPS Bearer Contexts.
3

The AMF sends a Forward Relocation Request with modifications and clarifications.

4 MME sends Create_Session_Request to S-GW.
5

S-GW sends Create_Session_Response to MME.

6 MME sends Handover Request(may contain information Handover Restriction List with information about PLMN IDs) to E-UTRAN.
7 E-UTRAN sends Handover Request Acknowledgment to MME.
8 Creates indirect data forwarding tunnel request or response between MME and S-GW.
9

MME sends Relocation Response to AMF.

10a AMF sends the Nsmf_PDUSession_UpdateSMContext Request (Serving GW Addresses and Serving GW DL TEIDs for data forwarding) to the PGW-C+SMF, for creating indirect data forwarding tunnel.
10b N4 Session modification request is sent between PGW-C+SMF and PGW-U+UPF.

10c

The PGW-C+SMF returns an Nsmf_PDUSession_UpdateSMContext Response (Cause, CN tunnel Info for Data Forwarding, QoS flows for Data Forwarding) to AMF for creating indirect data forwarding. Based on the correlation between QFIs and Serving GW Addresses and TEIDs for data forwarding, the PGW-U+UPF maps the QoS flows into the data forwarding tunnels in EPC.

11a

The AMF sends the Handover Command to the source NG-RAN.

11b

The NG-RAN sends the Handover Command to the source UE.

12a

The UE sends the Handover Complete Command to the source UE NG-RAN.

12b

The E-UTRAN sends the Handover Notify to the source UE MME.

12C

The MME sends Relocation Complete Notification to AMF.

12d

The AMF acknowledges MME with Relocation Complete Acknowledge message. A timer in AMF is started to supervise when the resources in the NG-RAN and PGW-C+SMF are released.

13

MME sends Modify Bearer Request to S-GW.

14

S-GW sends Modify Bearer to PGW-C+SMF.

15

The PGW-C+SMF initiates a N4 Session Modification procedure in the direction of UPF+PGW-U to update the User Plane path downlink User Plane, for the indicated PDU Session is switched to E-UTRAN. The PGW-C+SMF releases the resource of the CN tunnel for PDU Session in UPF+PGW-U.

16

PGW-C+SMF sends Modify Bearer Response to S-GW.

17

S-GW sends Modify Bearer Response to MME.

18

The UE initiates a Tracking Area Update procedure(TAU).

19

The PGW-C+SMF initiates dedicated bearer activation procedure for non-GBR QoS Flows.

20

Indirect Data Forwarding Tunnel Request and Response deleted in MME and S-GW

21a

Indirect Data Forwarding Tunnel Request and Response deleted in AMF and PGW-C+SMF.

21b

N4 Session modification request is sent between PGW-C+SMF and PGW-U+UPF.