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Collaboration, Voice & AV · Foundational

G.729

An ITU-T 8 kbps CS-ACELP voice codec widely used to save WAN bandwidth, with annexes A (low complexity) and B (VAD).

Definition

G.729 compresses narrowband speech to 8 kbps using conjugate-structure algebraic code-excited linear prediction, with 10 ms frames and a MOS of about 3.9. G.729a reduces DSP complexity with slightly lower quality, and G.729b adds VAD and comfort noise. It does not reliably carry DTMF tones, fax or modem signals in-band, so out-of-band DTMF and fax relay are required.

Example

A G.729 call with 20 ms packetization uses about 24 kbps at Layer 3 and 31.2 kbps on Ethernet.

Exam tip

Because G.729 distorts tones, pair it with out-of-band DTMF (RFC 2833/4733, KPML or SIP Notify) and T.38 for fax.

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