Enhanced Voice Services Adaptive Multi-Rate Wideband Interoperable Mode

Description

The EVS AMR-WB Interoperable (IO) mode is a component of the Enhanced Voice Services (EVS) codec, standardized in 3GPP Release 12 as part of the EVS specification (TS 26.445). EVS was developed to provide superior voice quality and efficiency for mobile networks, supporting narrowband, wideband, super-wideband, and fullband audio. However, to ensure backward compatibility with existing AMR-WB (Adaptive Multi-Rate Wideband) deployments, the EVS codec includes an AMR-WB IO mode. This mode allows EVS-capable devices to communicate with legacy AMR-WB devices using the same bitstream format, facilitating smooth migration and interworking.

The EVS AMR-WB IO mode comprises 9 codec modes and a Silence Insertion Descriptor (SID) frame. These modes correspond to the 9 active speech codec modes of AMR-WB, ranging from 6.6 kbps to 23.85 kbps, plus a SID frame for comfort noise generation during silence periods. All these bitstreams are fully interoperable with the AMR-WB codec as defined in 3GPP TS 26.171. This means that an EVS decoder can decode AMR-WB bitstreams, and an EVS encoder can produce bitstreams that an AMR-WB decoder can understand, when operating in this mode. The IO mode essentially embeds the AMR-WB codec within the EVS framework, allowing the EVS codec to switch between native EVS modes and AMR-WB IO mode depending on the capabilities of the far-end terminal and network conditions.

In operation, the EVS AMR-WB IO mode is used when the network or the remote party supports only AMR-WB. The EVS encoder then generates AMR-WB compatible bitstreams, and the EVS decoder can process incoming AMR-WB bitstreams. This mode does not provide the enhanced quality and efficiency of the native EVS Primary mode, but it ensures that EVS devices can interoperate with the vast installed base of AMR-WB devices. The mode is typically negotiated during call setup using Session Description Protocol (SDP) and the 3GPP codec negotiation procedures. The EVS codec can dynamically switch between EVS Primary and AMR-WB IO modes during a call, for example, during handover to a network that only supports AMR-WB, or when the far-end performs a codec change.

The EVS AMR-WB IO mode is defined in 3GPP TS 26.445, which specifies the EVS codec, and referenced in TS 26.114, which covers IP Multimedia Subsystem (IMS) multimedia telephony. The IO mode is also specified in TS 26.171 for AMR-WB, ensuring alignment. The mode includes the same frame structures, bit allocations, and error concealment mechanisms as AMR-WB, so that no transcoding is required when interworking with AMR-WB. This reduces latency and preserves voice quality. The SID frame is used for discontinuous transmission (DTX) and comfort noise generation, maintaining natural background noise during silence.

Overall, the EVS AMR-WB IO mode is a critical feature for the successful deployment of EVS, as it bridges the gap between the new EVS codec and the widely deployed AMR-WB codec. It enables operators to introduce EVS gradually while maintaining seamless voice services across their networks and with other operators. This interoperability is essential for global roaming and for ensuring that users experience consistent voice quality regardless of the codec capabilities of the endpoints.

Purpose & Motivation

The EVS AMR-WB IO mode was created to solve the interoperability challenge between the new EVS codec and the existing AMR-WB codec. When 3GPP developed EVS in Release 12, it aimed to significantly improve voice quality and capacity over AMR-WB, which was the prevailing wideband codec in LTE and UMTS networks. However, a wholesale replacement of AMR-WB was impractical due to the large installed base of AMR-WB-only devices and networks. Without interoperability, EVS devices would not be able to communicate with AMR-WB devices, leading to transcoding, degraded quality, and increased latency. The IO mode provides a seamless bridge by embedding AMR-WB compatibility within EVS.

The motivation was to enable a smooth migration path for operators. By including the AMR-WB IO mode, EVS-capable devices can fall back to AMR-WB when necessary, ensuring that calls between EVS and AMR-WB users are possible without transcoding. This is particularly important for roaming scenarios, where the visited network may only support AMR-WB. The IO mode also allows for dynamic codec switching during a call, such as during handover from an EVS-capable network to an AMR-WB-only network, maintaining call continuity. Furthermore, the IO mode simplifies network deployment because operators can introduce EVS without upgrading all devices simultaneously.

The EVS AMR-WB IO mode thus addresses the classic problem of technology evolution in telecommunications: how to introduce a superior technology while maintaining backward compatibility with legacy systems. It ensures that the benefits of EVS can be realized incrementally, without disrupting existing services. This approach has been used in other codec transitions, such as AMR to AMR-WB, and continues to be a best practice in 3GPP standardization.