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Voice-over-NR(VoNR) Explained

  • Writer: Venkateshu
    Venkateshu
  • 1 day ago
  • 4 min read

Introduction

Voice over NR (VoNR) delivers real-time voice and video over 5G Standalone’s ultra-low latency, high capacity architecture, using native NR air interface, a fully cloud-native 5GC, and IMS for service control. It supports next-level user experience—crystal-clear call quality, fast setup, and uninterrupted 5G data during calls.


VoNR Architecture


Component

Role in VoNR Call Establishment

UE (User Equip.)

Initiates NR registration, SIP signaling, and RTP media

gNB

5G NR base station for RRC/NAS and user-plane

AMF

Access and Mobility mgmt on 5GC

SMF

Session Mgmt, QoS enforcement, slice assignment

UPF

User-plane anchoring, RTP/IMS traffic routing

IMS Core

SIP signaling, voice service authentication, call routing

PCF

Policy and charging flows for QoS/Slice

VoNR Callflow: RAN, 5GC & IMS

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Below is a step-by-step walkthrough with Industry-standard Information Elements, UE/Network logs, and what to look for in protocol traces.

1. NR Registration & Capability Exchange

  • UE sends RRC Setup Request with IMS/VoNR support, UE-CapabilityRAT-ContainerList, SIB1 indicating IMS emergency voice over NR support (ims-EmergencySupport).

  • AMF triggers authentication via UDM, retrieves subscriber features: voice centric/data centric, IMS voice support, EPS fallback (if enabled).

  • Log: UE: “RRC ConnectionSetupComplete, IMS-VoiceOverNR-FR1-r15: Supported”

2. IMS Slice PDU Session Setup

  • UE sends NAS PDU Session Establishment Request with DNN = “ims”, SST = 1 (service slice for voice/data).

  • AMF/SMF negotiate PDU session, set up N6 route for IMS SIP/RTP traffic, allocate default QoS Flow (5QI=5).

  • Log: “IMS PDU Session Established, 5QI=5, SUPI:xxxx”

3. SIP Registration on IMS

  • UE IMS stack starts SIP REGISTER, traverses all P/I/S-CSCF entities.

  • Authentication challenge/response exchanges (401, 407, 200 OK).

  • Log: “SIP REGISTER sent – 200 OK, Public User ID: sip:IMSI@ims”

4. VoNR Mobile-Originated Call (MO)

  • SIP INVITE from UE triggers end-to-end call setup.

  • Codec/SDP negotiation (EVS, AMR-WB), call routing at IMS core, alerting (180 Ringing).

  • PDU session modification request for a dedicated GBR QoS flow (5QI=1, RTP stream for voice).

  • SMF, PCF, UPF configure paths for real-time media; gNB sets DRX for voice.

  • RTP stream initiated upon SIP ACK exchange.

Log Snippets:

  • “SIP INVITE (codec=EVS/AMR, ptime=20ms) → 180/200 OK / ACK”

  • “QoS Flow Modification: Adding GBR, 5QI=1 for RTP”

  • “RTP started (GBR): payload=EVS, jitter=<xx>ms, MOS=4.5”

5. Call Release

  • SIP BYE → 200 OK triggers DRB/GBR release in NR user plane, and IMS session closure.

  • Log: “SIP BYE sent, GBR released, call teardown success”

6. KPIs, Error Analysis, Realtime Log Deltas

  • Track call setup time, RLF counters (n310/n311, t310 for failures).

  • Handover logs: "Xn/N2 handover start, IRAT fallback triggered" if VoNR not supported.


VoNR vs VoLTE: Deep Protocol Differences

Feature

VoNR (5G SA)

VoLTE (LTE)

RAN

gNB New Radio (5G)

eNB LTE

Core

5GC (completely cloud-native)

EPC

IMS Connection

PDU session with N6 routing

EPS bearer to IMS APN

QoS Flow

5QI=5 (SIP) / 5QI=1 (RTP, GBR)

QCI=5 (SIP) / QCI=1 (RTP)

Codec

EVS, AMR-WB

AMR-WB

Call Setup

Lower latency; native 5G

Slightly higher

Handover/Fallback

EPS fallback to LTE if no VoNR coverage

SRVCC to 2G/3G/WiFi

Troubleshooting VoNR issues

1. VoNR Call Setup Failure

  • Issue: UE cannot establish VoNR call; call attempts fall back to VoLTE or PS data only.

  • Resolution Steps:

    1. Verify UE supports 5G SA capable of VoNR.

    2. Check network 5G Standalone (SA) coverage is available.

    3. Confirm IMS PDU session is correctly established (valid DNN=ims).

    4. Validate 5GC configuration allows IMS voice slice with correct QoS (5QI values).

    5. Inspect SIP signaling logs for failures (e.g., missing SIP 200 OK).

    6. Synchronize UE and network firmware versions supporting VoNR.

    7. Test with different SIMs/carriers to isolate provisioning errors.

2. Voice Quality Issues (Jitter, Packet Loss)

  • Issue: Calls experience audio glitches, jitter, or echo.

  • Resolution Steps:

    1. Monitor real-time RTP packet loss and jitter using network probes.

    2. Check QoS flow settings (5QI=1 for GBR RTP traffic).

    3. Validate proper configuration of UPF and QoS enforcement.

    4. Adjust jitter buffer settings in UE and network.

    5. Identify network congestion points and optimize routing.

    6. Ensure codec settings (EVS/AMR-WB) are compatible and prioritized.

3. Frequent Call Drops or RLFs

  • Issue: Calls drop unexpectedly or radio link failures occur during voice sessions.

  • Resolution Steps:

    1. Analyze radio-level logs (RRC Reconfig, handover failures).

    2. Improve coverage, especially in indoor or low signal areas.

    3. Check handover configurations and parameters (especially Inter-RAT handovers).

    4. Review SMF/AMF logs for session continuity issues.

4. IMS Registration Failures

  • Issue: UE unable to register with IMS core, blocking voice calls.

  • Resolution Steps:

    1. Validate SIP REGISTER messages flow and successful 200 OK response.

    2. Check UE SIP credentials and authentication.

    3. Monitor PCSCF, ICSCF, and SCSCF node health and logs.

    4. Ensure correct DNS routing and firewall port openings for SIP and RTP.

    5. Review UE IMS configuration and update if needed.

5. PDU Session Issues for IMS

  • Issue: IMS PDU session establishment or modification failure impacting VoNR voice bearers.

  • Resolution Steps:

    1. Validate NAS signaling for PDU Session Establishment (Message IE checks).

    2. Check SMF logs for session creation/modification errors.

    3. Inspect UPF N4 interface for data plane setup failures.

    4. Verify slice and QoS configuration matches IMS services.

6. Incorrect QoS Flow Setup

  • Issue: Voice packets assigned incorrect QoS, causing delays or packet loss.

  • Resolution Steps:

    1. Confirm QoS flow (5QI=1) is mapped for RTP during call setup.

    2. Check PCF policies and ensure GBR parameters are enforced.

    3. Validate DRB (Data Radio Bearer) setups via NGAP messages.

    4. Update network policy control modules if mismatches found.

7. EPS Fallback (VoNR to VoLTE) Triggering Unexpectedly

  • Issue: Calls drop back to LTE during 5G call even in good coverage.

  • Resolution Steps:

    1. Review UE configuration to ensure VoNR preferred over fallback.

    2. Check network handover/coverage parameters for 5G SA.

    3. Update AMF and SMF with appropriate policy and session configurations.

    4. Monitor SIP signaling for forced fallback reasons, e.g., SIP 503.

8. UE Firmware/Software Bugs

  • Issue: Device firmware glitches affecting VoNR registration or calls.

  • Resolution Steps:

    1. Recommend full device update to latest OS and vendor VoNR patches.

    2. Test with other 5G VoNR-compatible devices to isolate issue.

    3. Report vendor with detailed logs if bugs found.

9. Call Setup Time is Too High

  • Issue: Longer call setup time affects user experience.

  • Resolution Steps:

    1. Optimize IMS signaling path and SIP proxy processing.

    2. Minimize PDU session establishment time by pre-establishing IMS sessions.

    3. Streamline NGAP and NAS signaling procedures.

    4. Use fast RRC resume mechanisms.

References

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