VoNR and VoLTE share the same IMS signalling architecture. The differences are in the radio (5G NR vs LTE), the core network (5GC vs EPC), the codec (EVS vs AMR-WB), and new capabilities like network slicing. For most users the experience improvement is primarily audio quality. For network engineers the change is in the core architecture and QoS model.
| Feature | VoLTE (4G) | VoNR (5G SA) | Change |
|---|---|---|---|
| Radio access | LTE (E-UTRAN, eNodeB) | 5G NR (NG-RAN, gNB) | New radio layer |
| Core network | EPC (MME/SGW/PGW) | 5GC (AMF/SMF/UPF) | New core – SA required |
| Signalling | SIP via IMS | SIP via IMS | No change |
| Media protocol | RTP/UDP | RTP/UDP | No change |
| IMS elements | P-CSCF, S-CSCF, TAS, HSS | Same elements | No change |
| Voice codec (mandatory) | AMR-WB (IR.92) | EVS (GSMA 5G voice profile) | Major quality upgrade |
| Audio frequency | 50 Hz – 7 kHz | 50 Hz – 20 kHz (fullband EVS) | Near 3x wider |
| QoS model | EPS bearers, QCI | QoS flows, 5QI | Architecture change |
| Voice QoS value | QCI 1 | 5QI 1 | Same parameters |
| Packet delay budget | 100ms | 100ms | No change |
| Network slicing | No | Yes – dedicated voice slice | New capability |
| UPF placement | PGW (centralised) | UPF (can be distributed/edge) | Lower latency potential |
| SIM requirement | ISIM on UICC | ISIM on UICC | No change |
| SRVCC equivalent | SRVCC to 2G/3G | EPS fallback to VoLTE | Falls to VoLTE not 2G/3G |
| UK availability | Live – all 4 operators | Limited – 5G SA rollout in progress | Deploying now |
The most perceptible improvement for end users is audio quality. AMR-WB (VoLTE HD Voice) was already a significant step over 3G narrowband. EVS fullband is another step again – comparable to a high-quality digital audio recording rather than a telephone call.
VoLTE uses SRVCC to hand off to 2G/3G if LTE coverage is lost mid-call. VoNR uses EPS Fallback to hand off to VoLTE on the LTE bearer if 5G SA coverage is lost. From the user’s perspective the call continues; the network transfers the IMS session from the 5G NR bearer to an LTE VoLTE bearer transparently. This means VoNR deployments are resilient even where 5G SA coverage is incomplete, provided VoLTE coverage exists in the same area.
For in-depth coverage of the IMS architecture, SIP call setup, QCI 1 bearer setup, SRVCC, and SIM provisioning for VoLTE – all of which carries over to VoNR – see voltesim.co.uk.