Immediate Active Codec Set

Description

The Immediate Active Codec Set (IACS) is a concept defined in 3GPP specifications, particularly within the context of Circuit-Switched FallBack (CSFB) and voice service continuity. It refers to a pre-defined, limited set of speech codecs that a network and a User Equipment (UE) are guaranteed to support without the need for explicit capability negotiation during call establishment. When a voice call is initiated—especially in scenarios like CSFB where a UE falls back from LTE/5G to a 2G/3G circuit-switched network—time is critical. The IACS provides a known common denominator of codecs to avoid the delay of a full codec negotiation procedure.

Operationally, the IACS is typically a very small set, often consisting of a single, mandatory codec like the Adaptive Multi-Rate (AMR) codec at a specific rate (e.g., AMR 12.2 kbps). The network, when triggering a fallback or setting up an emergency call, can directly assign a codec from the IACS. The UE, by virtue of complying with 3GPP standards, is required to support the codecs in the IACS. This mutual guarantee allows the call setup signaling to bypass the typical 'codec list' exchange. The network includes the chosen IACS codec in the call setup message (e.g., in a GSM or UMTS Assignment Command or in certain SIP messages for IMS), and the UE immediately activates that codec for the voice bearer.

The IACS plays a crucial role in the architecture of inter-RAT mobility for voice. In CSFB procedures specified in 23.272 and referenced in 28.062, the MSC (Mobile Switching Centre) uses the IACS to instruct the target 2G/3G radio access network to allocate resources for a voice call using a known codec. This ensures that the fallback process is streamlined and the call setup time is minimized, which is vital for user experience and critical for emergency calls. The IACS is also relevant in the context of Single Radio Voice Call Continuity (SRVCC), where a voice call is handed over from a packet-switched domain (like VoLTE) to a circuit-switched domain, again requiring rapid and guaranteed codec alignment to maintain call continuity without interruption.

Purpose & Motivation

The Immediate Active Codec Set was created to solve the problem of call setup delay, particularly in scenarios involving fallback from packet-switched to circuit-switched networks for voice services. With the introduction of LTE, which initially lacked a native circuit-switched voice domain, CSFB was developed as a transitional voice solution. A key challenge was minimizing the additional delay incurred when redirecting a UE from LTE to a 2G/3G network to answer a voice call. A significant portion of call setup time in legacy systems was spent on codec negotiation—exchanging lists of supported codecs and selecting a common one.

The IACS eliminates this negotiation step for fallback and emergency calls by pre-defining a minimal, universally supported codec set. This purpose is driven by the need for service reliability and speed. For emergency calls, every millisecond counts. For commercial CSFB, reducing call setup time improves user perception and aligns with regulatory requirements for call establishment times. Historically, before such mechanisms, inter-system handovers or fallbacks could suffer from call setup failures or prolonged delays if codec negotiation failed or was slow. The IACS provides a deterministic, fast-path solution. It addresses the limitation of full-flexibility codec negotiation by sacrificing some codec choice optimization for the sake of speed and guaranteed success in time-critical scenarios, ensuring robust voice service continuity across different generations of mobile networks.