Common Supported Codec Set

Description

The Common Supported Codec Set (CSCS) is a fundamental concept in 3GPP networks for managing multimedia codec interoperability. It is defined as a standardized, operator-specific list of audio and video codecs that a network supports for services like Voice over LTE (VoLTE), Voice over NR (VoNR), and video telephony. The CSCS is not a single entity but a concept implemented within network elements and procedures to ensure that when a multimedia session is established, both the originating and terminating networks, along with the user equipment (UE), can agree on a common codec for the media stream.

Architecturally, the CSCS concept is embedded within the IP Multimedia Subsystem (IMS) core network. Key network elements involved include the Proxy-Call Session Control Function (P-CSCF), which acts as the first point of contact for the UE, and the Serving-Call Session Control Function (S-CSCF), which handles session control. The CSCS information is typically configured by the network operator within these IMS nodes. During session setup, such as with a SIP INVITE request, the UE includes its list of supported codecs in the Session Description Protocol (SDP) offer. The network (e.g., the P-CSCF) can then compare the UE's offered codecs against its configured CSCS. Based on local policy, the network may filter or reorder the codec list in the SDP before forwarding it towards the destination, ensuring that only mutually supported and operator-approved codecs are negotiated.

The primary role of CSCS is to act as a policy enforcement and interoperability mechanism. It prevents the negotiation of codecs that the network does not support, cannot transcode, or considers suboptimal from a quality or resource perspective. For example, an operator might configure its CSCS to prioritize the AMR-WB (Adaptive Multi-Rate Wideband) codec for high-definition voice, ensuring it is the preferred choice over narrowband alternatives when supported by both endpoints. The CSCS also plays a critical role in inter-operator scenarios. When a call traverses multiple networks, each network applies its own CSCS policy. The end-to-end codec selection becomes the intersection of the originating UE's capabilities, the originating network's CSCS, the terminating network's CSCS, and the terminating UE's capabilities.

Technically, the CSCS is defined in 3GPP specification TS 28.062, which falls under the umbrella of Management and Orchestration (MANO) specifications. This placement highlights that CSCS configuration is a management activity. The specification provides the framework and requirements for how this set is defined and managed, ensuring it can be consistently implemented across different vendor equipment. The actual codec negotiation happens dynamically during the SIP/SDP exchange, but the CSCS provides the static policy backdrop that governs this negotiation, making media sessions predictable, efficient, and aligned with the operator's service strategy.

Purpose & Motivation

CSCS was created to solve fundamental interoperability and quality management challenges in packet-switched multimedia services, which became critical with the introduction of IMS and LTE. Prior to standardized all-IP networks, circuit-switched voice used a limited, well-defined set of codecs (like AMR). The move to IP-based multimedia in IMS introduced a vast array of possible audio and video codecs from different standards bodies (3GPP, ITU, IETF). Without a managed approach, endpoints could negotiate codecs the network couldn't support, leading to call failures, or suboptimal codecs, degrading user experience. The lack of a common reference also complicated inter-operator roaming and interconnection agreements.

The core problem CSCS addresses is the disconnect between endpoint capability and network capability. A UE might support dozens of codecs, but the network operator needs to ensure service quality, manage transcoding resources efficiently, and maintain a consistent brand experience. CSCS provides the operator with a centralized policy control point for codec selection. It ensures that the network only agrees to media sessions using codecs it is prepared to handle, whether natively or via available media gateways. This prevents scenarios where a call is established with a codec the network cannot process, which would result in one-way audio or complete media failure.

Furthermore, CSCS enables strategic service differentiation. By defining a preferred codec order within the set, operators can steer sessions towards higher-quality codecs like EVS (Enhanced Voice Services) or AMR-WB, promoting high-definition voice as a market feature. It also future-proofs the network; as new, more efficient codecs like EVS are developed, operators can add them to their CSCS and gradually phase out older ones, managing the transition in a controlled manner. In essence, CSCS transforms codec negotiation from an uncontrolled endpoint-driven process into a network-managed policy, which is essential for reliable, high-quality commercial multimedia services.

Key Features

• Standardized operator-defined codec policy
• Enables network-controlled codec negotiation during IMS session setup
• Ensures interoperability between UEs and network media resources
• Supports quality-of-service management by prioritizing high-quality codecs
• Facilitates inter-operator roaming and interconnection agreements
• Provides a framework for managed evolution and introduction of new codecs

Evolution Across Releases

Defining Specifications