G.726 in the WAN

[keschb]

I am fighting a lot of voice quality issues on VoIP calls over an MPLS WAN. It does not appear to be related to any network QoS as loss, delay and jitter are within tolerance. I think the issue might be delay due to the G.729AB Codec. We are doing some transcoding, sometimes multiple times in certain call scenarios. Some calls go from IP-TDM while others are all IP, but go from G.711 to G.729AB and maybe to TDM or to G.711 again. We have a signicant ammount of Inter PBX traffic and remote IP phones / Gateways in our Voice Network. When we switch to G.711 all of the issues seem to clear up. I was thinking about using G.726 as a compromise between G.729 and G.711. I don't know much about this Codec or anyone who is using it.

[ssaini]

G.726 is a codec that evolved from the older G.721 from 1984 and G.723 from 1988 (now obsolete). It is an ADPCM codec with several possible bitrates (16, 24, 32, 40 kB/s). This codec was finalized in 1990.
G.726 was not only a merge of the older 721 and 723, it also saw the new 16 kB/s rate introduced. The most popular mode of the codec is the 32 kB/s version since this offers exactly twice the amount of capacity over the G.711.
G.726 is frequently used for international lines in the phone network, it has also become standard in modern DECT phones.
The codec comes in these possible different sample rates:

• <LI class=NTXT>16 kB/S <LI class=NTXT>24 kB/s <LI class=NTXT>32 kB/s
• 40 kB/s

As the codec generates a bitstream, it neads packetization in order to establish the frame length. It is a waveform speech encoder based on ADPCM (Adaptive Differential PCM).
The standard description can be found here: www.itu.int/rec/T-REC-G.726/e