Voice Over NR (VoNR Call Flow)

5G Call Flow, 5G Core, 5G Network Architectures, Call Flow, NAS Signalling, New Radio, NR, Signaling, Tech Fundas, Voice Services

VONR Introduction

We are seeing 5G Stand Alone (SA) data services are rolling out throughout the world, it is very much essential for MNOs to provide services such as Voice/Video calls. To achieve this, 3GPP standards have defined that the 5G network must provide the Voice services using 5G RAN, 5GC and IMS. 5G radio technology is known as New Radio (NR), and the Voice services using 5G RAN, 5G Core and IMS is referred to as Voice over New Radio-VoNR .

Voice Over NR Network Architecture

Voice Over NR network Architecture is consist of 5G RAN, 5G Core and IMS network. A high level architecture is shown below. (Only major network functions are included). This network architecture supports Service based interface using HTPP protocol.


VoNR Key Pointers

  • VoNR rely upon IP Multimedia Subsystem (IMS) to manage the setup, maintenance and release or voice call connections.
  • UE PDCP should support RTP and RTCP, RoHC compression and MAC layer should support DRX
  • SIP is used for signaling procedures between the UE and IMS.
  • VoNR uses a QoS Flow with 5QI= 5  for SIP signaling messages and QoS Flow with 5QI= 1
    • QoS Flows with 5QI= 5 is non-GBR but should be treated with high priority to ensure that SIP signaling procedures are completed with minimal latency and high reliability.
    •  QoS Flow with 5QI= 1 is GBR. This QoS Flow is used to transfer the speech packets after connection establishment
  • gNB uses RLC-AM mode DRB for SIP signaling and RLC-UM mode for Voice Traffic (RTP) DRBs
  • 3GPP has recommended  ‘Enhanced Voice Services’ (EVS) codecs  for 5G
    • EVS codec supports a range of sampling frequencies to capture a range of audio bandwidths.
    • These sampling frequencies are categorized as Narrowband, Wideband, Super Wideband and Full band.
  • VoNR UE provides capability information during the NAS: Registration procedure with IE ‘ UE’s Usage Setting’ indicates that the higher layers of the UE support the IMS Voice service.
  • The AMF can use the UE Capability Request to get UE’s support for IMS Voice services. gNB can get UE Capability with RRC: UE Capability Enquiry and UE Capability response to the UE.  The UE indicates its support for IMS voice service with following IEs
      • ims-VoiceOverNR-FR1-r15: This field indicates whether the UE supports IMS voice over NR FR1
      • ims-VoiceOverNR-FR2-r15: This field indicates whether the UE supports IMS voice over NR FR2
      • within feature set support IE ims-Parameters: ims-ParametersFRX-Diff voiceOverNR : supported

VoNR Call Flow

VoNR Call flow is very much similar with VoLTE call. The overall VoNR call flow includes five stages as shown in following picture. It starts from detecting a 5G NR Cell and performing Registration to 5G gNB and 5GC and establish the default PDU session with non-GBR QoS Flow with 5QI=6-9 with Data DNN (DDN is equal to APN in 4G LTE).


After this, UE  establish the Default Internet PDU session with IMS DNN with non-GBR QoS Flow and 5QI=5. Then UE does Registration process with IMS using SIP messaging over Default IMS PDU and establish VoNR MO/MT call over dedicated PDU session with GBR QoS Flow and 5QI=1. The Voice Traffic flow with RTP protocol over dedicated PDU session and once conversion is over the VoNR call can be terminated with releasing dedicated PDU session. You can read details 5G QoS in this blog post.

High Level Call Flow is shown in below:

Note: Below Call flow is drawn by referring multiple online resources and available information from 3GPP and just for reference purpose, the call flow messaging sequence may differ based on the implementation from different UEs, RAN, Core and IMS vendors. 

5G Registration: Detailed Registration Call Flow here


Default Internet and IMS PDU session Establishment


IMS Registration and VoNR Call 


VoNR Call Termination



  • 3GPP TS 23.228, Release 15.3.0 –  IP Multimedia Subsystem (IMS); Stage 2
  • 3GPP TS 23.501, Release 15.4.0 – 5G; System Architecture for the 5G System
  • 3GPP TS 23.502, Release 15.4.0 – 5G; Procedures for the 5G System
  • Blogs: http://volteromania.blogspot.com/p/5gsa.html

Related Posts