Operational mode

Description

The Operational mode is a concept defined in 3GPP specifications for the Enhanced Voice Services (EVS) codec, primarily in TS 26.114, TS 26.445, and TS 26.980. The EVS codec is a high-quality speech codec designed for Voice over LTE (VoLTE) and 5G voice services. It operates in two main modes: EVS Primary mode and EVS AMR-WB IO mode. The Operational mode parameter is used to distinguish between these two modes, ensuring that the codec operates correctly and interoperates with legacy AMR-WB codecs.

EVS Primary mode is the native mode of the EVS codec, offering enhanced quality and features such as super-wideband and full-band audio, improved compression efficiency, and better error resilience. EVS AMR-WB IO mode is an interoperability mode that allows the EVS codec to function as an AMR-WB codec, enabling seamless interworking with networks and devices that only support AMR-WB. The Operational mode parameter is signaled during session setup (e.g., via SDP) to indicate which mode is being used.

The Operational mode is crucial for ensuring backward compatibility and smooth migration from AMR-WB to EVS. When a call involves a legacy AMR-WB device, the EVS codec can switch to AMR-WB IO mode to maintain compatibility. The parameter is also used in the codec's internal configuration to select the appropriate coding algorithms and bit rates. In 3GPP specifications, the Operational mode is defined as part of the codec's configuration and is used by both the encoder and decoder.

Architecturally, the Operational mode is part of the media negotiation process. It is included in the Session Description Protocol (SDP) offer/answer exchange, allowing endpoints to agree on the mode. The EVS codec's flexibility in supporting multiple modes makes it a versatile solution for mobile operators, who can deploy EVS while ensuring interoperability with existing AMR-WB infrastructure. The Operational mode parameter is also used in the context of IMS (IP Multimedia Subsystem) and VoLTE, where it helps manage codec selection and transcoding.

Since its introduction in Release 12, the definition of Operational mode has remained stable, reflecting its fundamental role in EVS codec operation. The term is essential for understanding how EVS achieves both high quality and backward compatibility. As voice services continue to evolve towards 5G, the Operational mode remains a key concept for ensuring seamless voice communication across different network generations.

Purpose & Motivation

The Operational mode was introduced to address the need for a high-quality voice codec that could also interoperate with existing AMR-WB deployments. The EVS codec was designed to provide superior voice quality, but operators needed a way to ensure that EVS-enabled devices could communicate with legacy AMR-WB devices. The Operational mode solves this by allowing the EVS codec to operate in two distinct modes: EVS Primary mode for enhanced quality and EVS AMR-WB IO mode for interoperability. This dual-mode capability enables a smooth transition from AMR-WB to EVS without disrupting existing services. The parameter also simplifies codec negotiation by providing a clear indication of the mode being used. Its creation was motivated by the widespread deployment of AMR-WB and the need to protect investments in legacy infrastructure while advancing voice quality.