VoNR Call Flow

Description 

Each operator has a unique set of the allocated spectrum and its own deployment strategy. VoNR features can be different to meet the operator’s call quality, voice coverage, service continuity, mobility, call setup delay, call success rates, etc. VoLTE and VoNR use the same IMS network for voice calls. Technological improvements in New Radio, 5GC, and 5G devices are expected to provide a superior user experience in VoNR compared to VoLTE. EPS Fallback (E-UTRAN connected to EPC) and RAT(E-UTRAN connected to 5GC) are two deployment scenarios that support redirection or handover to E-UTRAN during the call set-up phase. The EPS FB mechanism will use the existing IMS Core and EPS for delivering voice or video services. 3GPP also supports several interworking options with the Evolved Packet Core. Network Operators should consider the impact to voice services while finalizing the interworking mode with EPC.  

Call flow

• When a UE initiates a call, an RRC connection is set up between the UE and its serving gNB. 
• The 5G Core Network (5GC) sets up a QoS (Quality of service) flow with a 5G QoS Identifier and gNB sets up the corresponding data radio bearer (DRB). 
• An RRC connection is set up between the UE and its serving gNB. 
• The 5GC sets up a QoS flow to carry SIP signaling and gNB sets up the corresponding DRB. 
• The calling and called UE’s and the IMS perform SIP negotiation on the codec scheme, IP address, port number, information of both calling and called UE’s and other voice service information. 
• After SIP negotiation is successful, the 5GC sets up QoS flow with a 5QI of 1 for both the calling and called UEs to carry Real-time transport protocol (RTP) and Real-time transport control protocol (RTCP) data flows, and the gNBs set up the corresponding DRBs. 
• After the call ends, the calling and called UEs to release their QoS flows.

5G IMS Support

The IMS (IP Multimedia Subsystem) represents connection management for voice services in 5G. A network offering voice services in 5GS needs to support IMS with the following functionality

• Indication towards the UE if an IMS voice-over PS session is supported. 
• Capability to transport the proxy call session control function (P-CSCF) addresses to a UE. 
• Paging policy differentiation for IMS
• IMS emergency service
• Domain selection for UE originating sessions
• Terminating domain selection for IMS voice 
• Support for the P-CSCF restoration procedure 
• Network repository function based P-CSCF discovery 
• NRF or SCP based HSS discovery 

Process

• SIP Registration 
• Authentication 
• Addressing 
• Call establishment and termination 
• SIP precondition considerations 
• Early media and announcements 
• Forking 
• Signaling compression 
• SIP timer operation 
• RCS feature-capability discovery 

EPS fallback call flow 

UE resides on the NG-RAN of the 5G system and initiates the establishment of the MO/MT IMS voice session. The NG-RAN receives a network-initiated PDU session modification request that establishes a QoS stream for voice services. 

For the inter-RAT handover case, it is easy to detect from signaling information that an EPS fallback was triggered. In the source-eNodeB-to-target-eNodeB-transparent-container sent by the gNB to the eNB. If a re-direction from voice EPS Fallback is possible or not is indicated in the NGAP Initial Context Setup Request, handover request, and path switch request acknowledge messages, all sent by the AMF to the gNB.