Based on the presence or absence of the N26 interface, Evolved Packet System (EPS) Fallback opts to switch or redirect to EPS.

The AMF performs and involves in the following activities:

• It supports the IMS voice over PS (VoPS) session and indicates towards the UE during the Registration procedure.

• It sends the value of redirection-eps-fallback information elements towards the gNodeB.

• The redirection-eps-fallback IE is based on the UE 5GMM capability to support Request Type flag handover and CLI configuration for redirection-eps-fallback .

This feature supports the following functionalities:

• 5GS interworking without N26 interface indicator in Registration Accept.

• Redirection for EPS fallback for voice as part of the ICSR

• Handover Request and Path Switch Request ACK to fill Redirection IE.

• N26, Xn, and N2 handovers

To configure this feature, use the following configuration:

config 
    amf-global 
        call-control-policy ccp_name
             feature-support-ie 
                [no] iwk-n26-supported 
                [no] redirection-eps-fallback { not-supported | supported } 
                end 

Note	As a default action, the AMF doesn't send the redirection information element (Redirection IE). It's an action sent only to RAN, which is based on the value of CLI, and the capability of UE.

NOTES:

• call-control-policy ccp_name —Specify and configure the Call Control Policy or Profile, as applicable.

• feature-support-ie —Configure and specify about supported or unsupported AMF or 5GC features.

• iwk-n26-supported —Specify the "Interworking without N26" indicator supported within the 5GS network functionality support. It gets applied only when the "Interworking without N26" indicator in the 5GS network functionality is in a supported state. When not supported, the "unsupported status" doesn’t have a reference to the status.

• redirection-eps-fallback —Configure the UE support and redirection for the EPS Fallback for voice, as a part of ICSR.

• not-supported | supported —Specify if the support is available or not. The nonsupported option indicates the disabled 5G VoPS 3GPP support.

• 5G IMS Voice over Packet-Switched (VoPS) 3GPP Sessions—Specify if the UE capability support gets enabled or not. Also, to specify, if the enabled UE configuration is with the UE Radio capability or not. The default value is true, indicating it’s a supported value.