Flow Failure Management Call Flows

The following call flow provides the details of the different flow failure scenarios during the EPS to 5GS handover. This call flow also describes how the SMF manages these failures and keeps the flows intact across 5GS network elements and the subscriber.

  • Flow failure from source in EPS to 5GS Handover

  • Flow failure from target in EPS to 5GS Handover

Flow Failure Management Call Flow
Flow Failure Management Call Flow Description

Step

Description

1

The EPS interworking capable UE initially camps on the EPS network.

2

This step involves taking the connected mode EPS to 5GS handover decision.

3

During the EPS to 5GS handover procedure, the PDN Connection in the Sm Context Create request from the AMF carries the EPS bearer contexts to be handed over to 5GS network.

4

The SMF identifies the bearer contexts that were established in EPS and missing in PDN connection as failed flows.

5

The SMF performs the resource allocation in 5GS network and sends it to the UPF excluding the failed flows.

6

The SMF sends the Sm Context Create Response with resource allocation request to the target RAN.

7

The AMF forwards the resource allocation request to the NG-RAN.

8

The NG-RAN sends the resource allocation response with the details of successful and failed flows to the AMF.

The target RAN node may not be able to allocate the resources for all the requested flows during EPS to 5GS handover procedure. The target RAN shares information about such failed flows in the resource allocation response.

9

The AMF sends the Sm Context Update Request with flows established and failed to establish in the target RAN.

10

The SMF identifies the failed flows at the target RAN.

11

The SMF performs the user plane resource update in 5G network excluding the failed flows.

12

The SMF sends the Sm Context Update Response with PDN connection including the bearer contexts handed over to the 5GS network.

13

The UE is moved to the 5GS network.

14

The SMF uses the EBI assignment procedure to release the EBIs of failed flows.

15

The SMF sends the N1N2 transfer request with the N1 NAS message to the UE to remove the resources of failed flows.

16

If the PCF has installed triggers to release the resources, then the SMF notifies the PCF about the failed flows.