Supported Codec List

Description

The Supported Codec List (SCL) is a critical information element used within 3GPP networks, specifically defined in the context of Out-of-Band Transcoder Control (OoBTC). It is a structured list, typically carried within session control protocols like SIP or within specific 3GPP NAS/RRC messages, that enumerates all the speech and multimedia codecs a User Equipment (UE) is capable of encoding and decoding. This list includes codec identifiers and their associated configuration parameters, such as bit rates, frame sizes, and other operational modes. The primary technical function of the SCL is to facilitate end-to-end codec negotiation before a call or session is established.

During session initiation, the originating UE includes its SCL in the session setup request. This request traverses the core network, potentially involving the MSC Server, MGW, or IMS entities. The network examines the SCL and compares it with the capabilities of the terminating UE, which also provides its own SCL. Based on operator policy, network capabilities, and the intersection of the two SCLs, the network selects the optimal codec for the session. This selection is then communicated back to both UEs via the answer message, ensuring both parties use the same codec from the start of the media flow.

The architecture for SCL utilization is integrated into the call control and session management layers. In Circuit-Switched (CS) domains, it interacts with Transcoder Free Operation (TrFO) procedures. In the IP Multimedia Subsystem (IMS), it is carried within SIP/SDP offer-answer exchanges. The SCL enables the network to bypass intermediate transcoding units (Traffic Bearer Transcoder, TTB) if a common codec is supported end-to-end, establishing a TrFO or IMS-based High-Quality Voice call. This direct media path is crucial for preserving voice quality, as each transcoding step can introduce distortion and delay. The management and processing of the SCL are therefore key to implementing efficient OoBTC, a fundamental feature for modern high-definition voice services like VoLTE and VoNR.

Purpose & Motivation

The SCL was created to solve the classic telephony problem of codec mismatch and mandatory in-path transcoding. In early mobile networks, voice calls often involved multiple codec conversions as the call traversed network nodes, because the originating phone, network equipment, and terminating phone might all support different codecs. A fixed, default codec (like AMR-NB) was often used in the radio access, but the core network would transcode this to another format (like G.711) for interconnection. Each transcoding operation degrades audio quality, adds latency, and consumes additional processing resources in network nodes like the Media Gateway (MGW).

The motivation for SCL and OoBTC was to move this codec negotiation and selection process 'out-of-band'—that is, to perform it in the control plane before the media path is set up. This allows the network to determine if the two endpoints support a common codec. If they do, the network can configure a direct, transcoder-free bearer between them, dramatically improving perceived voice quality and reducing mouth-to-ear delay. This was a significant step towards high-definition voice services. The introduction of SCL in Rel-8 was aligned with the broader push for all-IP networks and IMS, where end-to-end IP connectivity made transcoder-free operation a more attainable goal, directly addressing the quality limitations of legacy tandem-free operation (TFO) which was an in-band solution.