Local Supported Codec Set

Description

The Local Supported Codec Set (LSCS) is a fundamental concept in multimedia session establishment within IMS and CS networks. It represents the complete set of audio, video, and other media codecs that a terminal (UE) or a network node (like a Media Gateway) is capable of encoding and decoding. This set is defined locally by the device's hardware and software capabilities. During session initiation, such as placing a Voice over LTE (VoLTE) call or a video call, the LSCS is not transmitted directly. Instead, it forms the basis for constructing the offer/answer in the Session Description Protocol (SDP).

The process works as follows: When a UE initiates a session, it consults its internal LSCS to create an SDP offer. This SDP offer contains a prioritized list of codecs from the LSCS that it prefers to use for the session. This list includes details for each codec, such as the RTP payload type, clock rate, and any codec-specific parameters. The offer is sent to the remote party via SIP signaling. The receiving UE compares the offered codec list with its own LSCS. It then generates an SDP answer, selecting the highest-priority codec that is also present in its local supported set.

This mutual negotiation ensures that both ends of the session use a codec they both support, which is critical for interoperability. The LSCS is dynamic; it can change based on factors like current radio conditions (e.g., a UE might exclude high-bitrate codecs in poor coverage) or service restrictions from the network. Network entities like the P-CSCF or MRFC can also influence the final codec selection based on network policy, transcoding capabilities, or bandwidth management, potentially modifying the effective codec set used from the terminals' original LSCS.

Purpose & Motivation

The LSCS concept exists to solve the fundamental problem of media interoperability in heterogeneous telecommunications networks. As networks evolved from circuit-switched voice (supporting a single, fixed codec like AMR) to packet-switched multimedia (IMS), the number of possible audio and video codecs proliferated. Different manufacturers and service providers adopted different codecs, leading to potential call failures if endpoints had no common supported format.

The LSCS, used within the framework of SDP negotiation, provides a standardized mechanism for devices to advertise their capabilities and mutually agree on a working set of media encodings. This addresses the limitations of static, pre-configured codec assignments. It enables rich media services, allows for the introduction of new, more efficient codecs (like EVS or VP9) without breaking backward compatibility, and supports quality adaptation by allowing the selection of a codec appropriate for the current network conditions and device capabilities.

Key Features

• Defines the inherent media encoding/decoding capabilities of a UE or network node
• Serves as the source for building SDP offers and answers during session negotiation
• Typically includes multiple codecs with configurable modes and bitrates (e.g., AMR, AMR-WB, EVS, H.264, H.265)
• Can be updated dynamically based on UE configuration, network policy, or radio conditions
• Essential for ensuring end-to-end media interoperability in IMS and VoLTE
• Used in conjunction with network policies for bandwidth management and transcoding control

Evolution Across Releases

Defining Specifications