Codec

A codec (coder-decoder) is a software algorithm that encodes and decodes audio streams for transmission over IP networks. In VICIdial, codecs determine the audio quality, bandwidth consumption, and CPU usage for every call. The codec negotiation happens during SIP call setup, where the calling and receiving sides agree on a common audio format.

How It Works in VICIdial

Asterisk in VICIdial supports numerous codecs, but the most commonly used are G.711 (ulaw/alaw), G.729, and Opus. G.711 ulaw is the default for North American deployments — it provides toll-quality audio at 64 kbps per call with minimal CPU overhead. G.729 compresses audio to 8 kbps, reducing bandwidth by 87% but requiring more CPU processing and a commercial license. Opus is a modern, open-source codec used primarily with WebRTC connections.

Codec configuration is set per SIP or PJSIP endpoint and trunk. When two endpoints support different codecs, Asterisk can transcode between them, but this increases CPU usage significantly. For optimal performance, configure all carrier trunks and agent endpoints to use the same codec, eliminating the need for transcoding.

The codec also affects recording file sizes — G.711 recordings consume approximately 480 KB per minute, while G.729 recordings are roughly 60 KB per minute.

Why It Matters

Codec selection directly impacts three critical areas: call quality, bandwidth costs, and server capacity. A VICIdial server handling 200 concurrent G.711 calls needs approximately 100 Mbps of bandwidth for audio alone, while the same call volume using G.729 needs only about 13 Mbps. However, G.729 transcoding can limit a server to fewer concurrent calls due to CPU constraints.

For most VICIdial deployments, G.711 ulaw is the best default — it provides the highest quality with the lowest CPU overhead. Reserve G.729 for bandwidth-constrained environments like remote agents on limited connections. See the VICIdial setup guide for codec configuration recommendations.