Terminology » 5G » 5G VoNR: Architecture, Call Flow, and Setup

5G VoNR: Architecture, Call Flow, and Setup

This page describes 5G VoNR (Voice over NR) architecture, VoNR call flow, and VoNR call setup registration and termination messages.

Introduction

5G NR (New Radio) supports both NSA (Non-Standalone) and SA (Standalone) modes. 5G NR NSA supports voice over IP (VoIP) services by falling back to the 4G LTE network. 5G NR SA mode supports voice services with the help of IMS (IP Multimedia Subsystem). Voice over IP calls in 5G NR are referred to as VoNR or Vo5G.

5G VoNR (Voice over NR) Architecture

The figure depicts the 5G NR VoNR architecture and its elements: RAN, Core, and IMS. The following are the basic requirements of the VoNR architecture in order to offer voice services through it. This architecture supports a service-based interface using the HTTP protocol. DDN (Data Network Name) in 5G NR is similar to APN (Access Point Name) in 4G LTE.

VoNR architecture

VoNR key features are as follows:

VoNR relies on IMS for voice call setup, its management, and release.
UE PDCP needs to have features such as RTP/RTCP protocols, ROHC compression, etc. The MAC layer should support DRX functionality.
The SIP protocol is used between the UE and IMS for signaling messages.
It uses 5QI=5 for SIP signaling messages and 5QI=1 for speech packets after the connection is established.
gNB should support RLC-AM mode for SIP signaling DRBs and RLC-UM mode for voice traffic DRBs.
EVS (Enhanced Voice Services) codec is recommended by 3GPP.
The AMF entity supports a “UE capability request” to get support for IMS voice services. gNB takes care of UE capability enquiry and its response to the UE.
UE uses the following IEs to declare its support for IMS voice services, which are “ims-VoiceOverNR-FR1-r15” and “ims-VoiceOverNR-FR2-r15”.

5G VoNR Call Setup Steps

The VoNR call flow is similar to the VoLTE call flow. It includes the following 5 stages:

Step 1: Detection of robust 5G NR cell by UE and registration with 5G gNB/5GC.
Step 2: Default PDU session establishment with data DDN using non-GBR QoS flow having 5QI equals 6 to 9.
Step 3: UE establishes a default internet PDU session with IMS DDN using non-GBR QoS flow having 5QI equals 5.
Step 4: UE completes registration with IMS using SIP protocol messages.
Step 5: UE establishes VoNR MO call or MT call over a dedicated PDU session using 5QI equals 1 (called as GBR).

VoNR Call Setup

The RTP protocol is used for the flow of voice traffic over a dedicated PDU session. After the conversation is completed, the VoNR call is terminated and PDU session resources are released.

5G VoNR Call Flow

Let’s understand the VoNR call flow using the following figure.

Voice Over NR 5G VoNR Call Flow

When UE initiates a VoNR MO call, an RRC connection is established between the calling UE and the serving gNB.
5G core (5GC) establishes QoS flow with calling UE using 5QI equals 5 in order to carry SIP signaling. The serving gNB sets up corresponding DRBs (data radio bearers) for SIP signaling.
An RRC connection is also established between the called UE and its serving gNB.
5GC establishes QoS flow (using 5QI=5) with called UE in order to carry SIP signaling. The serving gNB sets up corresponding DRBs for called UE.
The calling UE, called UE, and IMS perform SIP negotiation on codec type, IP addresses, port numbers, and other information of calling/called UEs and other voice service information.
After SIP negotiation is successful, the 5GC establishes QoS flow using 5QI=1 for both calling UE and called UE. This is required to carry RTP (Real-Time Protocol) and RTCP (Real-Time Control Protocol) data flows. Serving gNB and target gNB initializes corresponding DRBs for these data flows.
Now the call is being answered and voice conversation is carried between calling and called UEs.
After the call ends, calling UE and called UE release QoS flow having 5QI equals 1. At the same time, both gNBs release corresponding DRBs.