VoIP glossary

What is Codec?

A codec is the algorithm that converts speech into digital packets and back again, trading audio quality against the bandwidth each call uses.

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Codec explained

Every VoIP call uses a codec on each leg. G.711 sends uncompressed telephone-quality audio at about 87 kbit/s per direction including overhead. G.729 compresses the same speech to roughly 31 kbit/s at a small cost in quality. Wideband codecs such as G.722 and Opus carry a wider range of frequencies for clearer voices, but only stay wideband if every hop supports them.

Both ends must agree on a codec during call setup. If they cannot, the call fails with a 488 error or a device in the middle has to transcode, which adds delay and can reduce quality. Listing codecs in the same order on your PBX and your trunk avoids most problems.

Example

A branch office with a slow uplink sets G.729 first and G.711u second, so most calls use less bandwidth but still connect if the far end does not support G.729.

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