EPS Fallback and 5G VoNR

5G Voice Solution

There are two primary solutions in the 5G voice evolution path: EPS Fallback and Voice over New Radio (VoNR).

EPS Fallback Procedure

EPS Fallback chooses to switch or redirect to EPS based on the presence or absence of the N26 interface.

1. The UE resides on the NG-RAN of the 5G system and initiates the establishment of an MO/MT IMS voice session.
2. The NG-RAN receives a network-initiated PDU session modification request to establish a QoS flow for voice services.
3. NG-RAN evaluates whether to allow VoNR, considering factors such as:
   • UE capabilities
   • N26 interface deployment status
   • LTE radio conditions
   • Whether VoNR is enabled/supported in the network
4. If NG-RAN decides on EPS Fallback, it rejects the PDU session modification request with a specific cause code. The 5GC then waits for the UE to fall back to 4G.
5. NG-RAN triggers fallback by either switching or redirecting the UE to EPS, depending on terminal capability and network configuration.
6. Once the UE connects to the EPS:
   1. With N26 interface: The UE performs a Tracking Area Update (TAU).
   2. Without N26 interface: The UE performs an EPS attach and sets the request type to “Handover” in the PDN connection request.
7. After successful fallback to EPS, the SMF/PGW re-initiates the establishment of a dedicated bearer for IMS voice services.
8. The voice call proceeds over the 4G network using VoLTE.

5G VoNR Procedure

5G VoNR is similar to the VoLTE process, except that a PDU session must first be modified to establish QoS flows for voice services.

1. In a calling or called scenario, the IMS system triggers QoS establishment based on SIP signaling.
2. The 5GC sends a PDU session modification request to the NG-RAN to set up a dedicated QoS flow for voice.
3. NG-RAN reconfigures the user plane resources accordingly.
4. NG-RAN confirms successful PDU session modification and notifies the 5GC, PCF, and IMS.
5. The call continues end-to-end over the 5G network via the IMS domain—functionally identical to VoLTE but carried natively on 5G NR.

Relevant Resources:
IMS-PRODUCT-INFORMATION.pdf
Products: Voice over NR (VoNR)