Local Active Codec Set

Description

The Local Active Codec Set (LACS) is a management concept and information element used in 3GPP circuit-switched (CS) voice services, specified in TS 28.062 for management purposes. It represents the set of speech codecs that a specific User Equipment (UE) is currently capable of using and that are authorized by the network for a given call or context. The LACS is not a static property of the UE but a dynamic list that can be modified by network signaling. The core network, typically the Mobile Switching Center (MSC), determines and communicates the LACS to the UE during call establishment or modification procedures.

From a procedural standpoint, the LACS works as part of the codec negotiation process. When a mobile-originated or mobile-terminated CS voice call is initiated, the network assesses various factors such as the subscriber's profile, network policies, and current radio conditions. Based on this assessment, the network constructs the LACS—a subset of the UE's total supported codecs—and includes it in the call setup message (e.g., a SETUP or CALL CONFIRMED message). The UE must then restrict its codec selection for this call to only those codecs present in the received LACS. This allows the network to enforce policies, prioritize certain codecs for bandwidth efficiency or quality, and ensure interoperability with the called party's network.

The role of the LACS is critical for efficient radio resource management and service quality in CS domain. By controlling the active set, the network can prevent the use of codecs that are not suitable for the current radio bearer configuration or that would consume excessive bandwidth. It also enables features like Transcoder Free Operation (TrFO) and Tandem Free Operation (TFO), where the LACS can be coordinated between networks to allow an end-to-end compatible codec without intermediate transcoding. The LACS is a key parameter in the Operation and Maintenance (O&M) framework for monitoring and controlling voice service performance across the network.

Purpose & Motivation

The LACS was created to solve the problem of uncontrolled and suboptimal codec selection in circuit-switched mobile voice calls. Early systems had limited codec support, but as networks evolved (e.g., from GSM FR/HR to AMR and AMR-WB), UEs began supporting multiple codecs with different bitrates and quality levels. Without network control, a UE might select a high-bitrate codec unsuitable for a congested cell, or a codec incompatible with the remote end, forcing wasteful transcoding. The purpose of the LACS is to give the network centralized, policy-driven control over the codec negotiation process.

Its introduction was motivated by the need for more intelligent radio resource management and enhanced voice quality features. It addressed the limitations of static, terminal-centric codec lists by allowing the network to dynamically adapt the available codec set based on real-time conditions and service policies. This enables operators to optimize spectrum usage, ensure consistent quality of service, and implement advanced voice services like wideband audio (AMR-WB) selectively. The LACS provides the mechanism for the network to 'guide' the UE's behavior, ensuring network-wide objectives for efficiency and interoperability are met during the call setup phase.

Key Features

• Dynamic, network-controlled list of codecs available for a specific CS voice call
• Signaled from the network (MSC) to the UE during call establishment
• Restricts the UE's codec selection to the authorized subset
• Enables network policy enforcement for codec usage and radio resource management
• Facilitates advanced voice features like Transcoder Free Operation (TrFO)
• Allows adaptation of codec support based on subscriber profile and current network conditions

Evolution Across Releases

Defining Specifications