Call Flows

The following call flow depicts the EPS Fallback procedure.

EPS Fallback Call Flow
EPS Fallback Call Flow Description
StepDescription
1The UE initiates Mobile-Originated (MO) or a Mobile-Terminated (MT) IMS voice establishment procedure with NG-RAN in 5GS.
2The Network-initiated PDU Session modification request to set up QoS flow for voice reaches the NG-RAN.
3

The NG-RAN is configured to support the EPS fallback for IMS voice. Based on the UE functionalities, indication from the AMF to redirect EPS fallback for voice, network configuration, and radio conditions, the NG-RAN triggers fallback to EPS. If the NG-RAN determines not to trigger the fallback to EPS, then the procedure stops, and the following steps are not performed.

The NG-RAN initiates measurement report solicitation from the UE including E-UTRAN as target.

4

The NG-RAN rejects the PDU Session modification request received in Step 2 with an indication that mobility due to fallback for IMS voice is ongoing.

The NG-RAN indicates the rejection of the PDU session modification to configure QoS flow for IMS voice that is received in Step 2 as PDU Session Response message toward the SMF through the AMF. This message includes the details on the ongoing mobility due to fallback for IMS voice. The SMF maintains the PCC rules that are associated with the QoS flows.

For a roaming scenario, the PDU Session Response message is sent toward H-SMF through V-SMF.

Or

NG-RAN responds with configured cause which it might trigger fallback to EPS. If the cause sent by gNB matches with EPS Fallback causes configured at SMF, it continues to behave as mentioned in steps from step. 5 onwards.

Note

If configuration at SMF always checks for the configured cause even if gNB rejects N2 message with cause “IMS voice EPS fallback or RAT fallback triggered”.

5

Based on the UE functionalities, the NG-RAN initiates handover to the EPS. The SMF reports change of the RAT type, if the PCF is subscribed for it.

A timer starts to track failure in the EPS fallback. After the timer expires, the SMF notifies the PCF about the dedicated bearer creation failure and new statistics, with the “smf_eps_fb” and “timeout” labels, is incremented.

6aFor 5GS to EPS handover, the UE initiates TAU procedure.
6bThe UE attaches the PDN connectivity request with the “handover” request type.
7After the completion of the 5GS to EPS handover procedure, the SMF or PGW-C reinitiates the configuration of the dedicated bearer for IMS voice and mapping the 5G QoS to EPC QoS parameters. The SMF notifies about the Successful Resource Allocation and Access Network Information, if the PCF is subscribed for it.
8The IMS voice session establishment continues.