EVS (Enhanced Voice Services) is the primary voice codec for VoNR, defined in 3GPP TS 26.441. It supports narrowband, wideband, super-wideband (up to 14.4 kHz), and fullband (up to 20 kHz) audio at bit rates from 5.9 kbps to 128 kbps. It also provides significantly better performance than AMR-WB in packet loss conditions.
| Mode | Frequency range | Bit rate | Quality descriptor |
|---|---|---|---|
| Narrowband (NB) | 50 Hz – 8 kHz | 5.9, 7.2, 8.0, 9.6 kbps | Better than AMR-WB NB at lower bit rate |
| Wideband (WB) | 50 Hz – 8 kHz | 5.9 – 128 kbps | Equivalent to or better than AMR-WB |
| Super-wideband (SWB) | 50 Hz – 14.4 kHz | 9.6 – 128 kbps | Noticeably richer than HD Voice |
| Fullband (FB) | 20 Hz – 20 kHz | 9.6 – 128 kbps | Full-range audio – CD quality equivalent |
One of EVS’s less-discussed advantages is its packet loss concealment capability. AMR-WB degrades audibly when packet loss exceeds 1-2% on a wireless link. EVS includes enhanced forward error correction and a more sophisticated packet loss concealment algorithm that maintains acceptable audio quality at higher loss rates. This matters for edge-of-coverage deployments and in congested environments.
EVS is backward compatible with AMR-WB. When a VoNR device calls a VoLTE device that does not support EVS, the call negotiates down to AMR-WB via SDP. The user on the VoNR device hears AMR-WB quality; the VoLTE user hears standard HD Voice. The codec negotiation happens transparently in the SIP INVITE/200 OK exchange.
EVS is not exclusive to VoNR. Some VoLTE implementations support EVS as a higher-quality option when both endpoints support it. In practice, EVS on VoLTE is limited because it requires both the handsets and the operator’s IMS to support EVS, which is not universal. VoNR standardises EVS as the primary codec, making fullband quality the expected baseline rather than an optional enhancement.