Voice over 5G NR Call Flow | VoNR call setup architecture

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 fallback to 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 as VoNR or Vo5G.

5G VoNR (Voice over NR) architecture

The figure depicts 5G NR VoNR architecture and its elements viz. RAN, Core and IMS. Following are the basic requirements of VoNR architecture in order to offer voice services through it. This architecture supports service based interface using 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 the voice call setup, its management and release.
• UE PDCP needs to have features such as RTP/RTCP protocols, ROHC compression etc. MAC layer should support DRX functionality.
• SIP protocol is used between UE and IMS for signaling messages.
• It uses 5QI=5 for SIP signaling messages and 5QI=1 for speech packets after 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.
• AMF entity supports "UE capability request" to get support for IMS voice services. gNB takes care of UE capability enquiry and its response to UE.
• UE uses 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

VoNR call flow is similar to VoLTE call flow. It includes 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 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 dedicated PDU session using 5QI equals 1 (called as GBR).

VoNR Call Setup

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

5G VoNR call flow

Let us understand VoNR call flow using following figure.

Voice Over NR 5G VoNR Call Flow

• When UE initiates VoNR MO call, RRC connnection is established between calling UE and serving gNB.
• 5G core (5GC) establishes QoS flow with calling UE using 5QI equals 5 in order to carry SIP signaling. Serving gNB sets up corresponding DRBs (data radio bearers) for SIP signaling.
• RRC connection is also established between 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.
• Calling UE, Called UE and IMS performs SIP negotiation on codec type, IP addresses, port numbers and other information of calling/called UEs and other voice service informations.
• 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.


• 3GPP TS 23.228 Rel-15 IP IMS
• 3GPP TS 23.501 Rel-15 5G system
• 3GPP TS 23.502 Rel-15
• https://www.techplayon.com/

Refer advantages and disadvantages of Voice over LTE and Voice over 5G NR to understand difference between VoLTE and VoNR.

5G NR Numerology | 5G NR Terminology

5G NR Control channels | 5G NR Traffic Channels | 5G NR Reference Signals and sequences


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5G NR Physical layer
5G NR MAC layer
5G NR RLC layer
5G NR PDCP layer

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