Codecs
G.729Also known as G.729a, G.729ab, CS-ACELP
Definition
G.729 is an ITU-T narrowband speech codec that compresses telephone audio to 8 kbit/s using CS-ACELP (conjugate-structure algebraic code-excited linear prediction). It works on 10 ms frames and delivers near-toll quality at one-eighth the bitrate of G.711, which made it the standard choice for bandwidth-constrained VoIP and wholesale links.
Variants and features
- G.729 Annex A (G.729a): reduced-complexity version, interoperable with base G.729; by far the most common.
- Annex B (G.729b / “ab”): adds voice activity detection and comfort noise, so silence sends almost nothing.
- RTP payload type: static type 18; SDP “annexb=no” disables Annex B.
Its patents have expired, so G.729 is now royalty-free, which removed the per-channel licence costs that once limited its use in open-source PBXs.
Bandwidth vs quality
At 20 ms packetisation each packet carries 20 bytes of audio plus 40 bytes of RTP/UDP/IPv4 header: about 24 kbit/s at the IP layer and about 31 kbit/s on Ethernet per direction, against about 87 kbit/s for G.711. The trade-offs:
- Slightly lower quality than G.711, and noticeably worse after multiple transcodes (tandeming).
- In-band DTMF and fax tones are distorted; use RFC 4733 events and T.38 instead.
- More CPU per channel, which matters on large transcoding boxes.
Where G.729 is used
As bandwidth has become cheaper, many networks have moved back to G.711 or to wideband codecs, keeping G.729 for satellite, cellular backhaul and congested international links where every kilobit still counts. When a call already passes through a G.729 hop, avoid transcoding it again, because each lossy step removes more quality.
G.729 remains popular in wholesale VoIP and on links where many channels share limited bandwidth. GSMCalls supports G.729 and G.711 on the trunk side, so you can pick per trunk based on bandwidth and your carrier’s requirements; see the features page.
Frequently asked questions
How much bandwidth does G.729 use?
8 kbit/s of payload, which is about 24 kbit/s per direction at the IP layer and about 31 kbit/s on Ethernet with 20 ms packets.
Is G.729 still licensed?
No. The key patents have expired and G.729 can now be used royalty-free.
Does DTMF work with G.729?
Use RFC 4733 telephone-events. In-band tones are distorted by the compression and are unreliable.