Step by Step VoNR Call Flow in 5G Voice Over New Radio

telcomatraining.com – Voice over New Radio (VoNR) is a next-generation voice service that leverages the full capabilities of 5G networks. Unlike legacy voice services such as VoLTE (Voice over LTE) or CSFB (Circuit Switched Fallback), VoNR is designed to operate natively on a 5G standalone (SA) network, offering ultra-low latency, higher call quality, and faster call setup times. In this article, we’ll explore the step-by-step VoNR call flow and explain how it works within the 5G architecture.

What is VoNR?

VoNR, or Voice over New Radio, enables voice communication purely over the 5G standalone core network. It eliminates the dependency on 4G LTE for voice services, thereby utilizing the 5G NR (New Radio) and 5G Core (5GC) for both data and voice transmission.

VoNR Call Flow: Step-by-Step Breakdown

Here is a detailed walkthrough of the typical VoNR call setup process in a 5G standalone environment:

1. UE Registration to 5G Network

The User Equipment (UE) first performs registration to the 5G standalone core network. This involves:

• Initial Access via 5G NR
• Authentication through the Access and Mobility Management Function (AMF)
• Session setup with the Session Management Function (SMF)
• Policy control via PCF and user data interaction via UDM

The registration also includes capabilities exchange where the UE declares VoNR support.

2. IMS Registration

After successful 5G registration, the UE initiates IMS (IP Multimedia Subsystem) registration using the SIP (Session Initiation Protocol) over the established PDU session. This IMS registration is essential for voice services.

• The UE sends a SIP REGISTER request to the IMS server
• The P-CSCF (Proxy Call Session Control Function) handles the request and forwards it to the I-CSCF (Interrogating CSCF)
• The request is then passed to the S-CSCF (Serving CSCF) after HSS/UDM authentication

3. VoNR Call Initiation (SIP INVITE)

Once the UE is registered with IMS, it can initiate a voice call:

• The UE sends a SIP INVITE message to the IMS core via the P-CSCF
• This INVITE contains the caller ID, called number, codec information, and media session parameters (SDP)
• The SIP message traverses through the IMS network and reaches the recipient’s S-CSCF

4. IMS Session Establishment and Bearer Setup

Upon receiving the SIP INVITE, the network performs:

• SIP signaling negotiation between both UEs
• Codec matching and session description agreement
• Quality of Service (QoS) flow establishment through the 5GC

The SMF and UPF (User Plane Function) coordinate to set up the bearer paths with QoS Class Identifier (QCI) for voice, ensuring low latency and high reliability.

5. Voice Path Establishment (RTP Setup)

Once SIP signaling is completed:

• RTP (Real-Time Protocol) streams are established between the UEs
• Media path flows through UPFs ensuring packet forwarding
• Voice packets use dedicated 5QI (typically 1 or 5) for guaranteed quality

6. In-Call Mobility and QoS Maintenance

During the call, the network manages:

• Seamless handovers between gNBs (5G base stations)
• QoS re-negotiation if needed
• Measurement reports from UE for maintaining connection stability

7. Call Termination

When either party ends the call:

• A SIP BYE message is sent and acknowledged
• RTP stream is torn down
• Network releases QoS bearers and updates session states accordingly

Benefits of VoNR

• Low Latency: Near real-time communication with call setup times under one second
• Enhanced Voice Quality: HD voice with support for EVS (Enhanced Voice Services) codec
• Efficient Resource Usage: Native 5G voice eliminates reliance on legacy LTE or CS fallback
• Faster Call Setup: Direct call establishment without switching between networks

Conclusion

VoNR represents the future of voice communication in a 5G world. With its efficient call flow and native 5G integration, it offers a seamless voice experience for users and enables operators to fully capitalize on their 5G SA deployments. As 5G adoption continues to grow, understanding the VoNR call flow becomes critical for network engineers, telecom professionals, and stakeholders in the mobile ecosystem.