[Asterisk-Users] G.729? Worth it?

[Stewart Nelson]

The MOS (Mean Opinion Score) scale is:
5=Excellent; 4=Good; 3=Fair; 2=Poor; 1=Bad.

Some values, taken from "Carrier Grade Voice over IP" by
Daniel Collins:

G.711          4.3
G.729          4.0
G.729AB        3.9
GSM(full rate) 3.7

The above scores assume no packet loss, minimal delay, no echo.

However, IMO such scores are generally only useful for choosing
among compression codecs.

If you have plenty of bandwidth and minimal packet loss, you
should use G.711, not only for better quality, but because it
avoids issues with conferencing, DTMF relay, etc.  Also, if your
ITSP has upstream routes that use a different compression scheme,
G.711 avoids cascaded codecs, which sound really awful, MOS < 3
for sure.

If you don't have enough bandwidth to handle the desired number
of simultaneous calls with G.711, you obviously need to use
compression; IMHO G.729 is a good choice.

If you have >1% packet loss (or packets effectively lost due to
excessive jitter), then G.729 may actually sound better.  Lost
G.711 samples are replaced with silence, sometimes with pops
at the transitions.  OTOH, most G.729 implementations have
"packet loss concealment", which continues the previous sound,
gradually fading out.  With 5% loss, a good G.729 system sounds
like a mediocre cellular call, but G.711 sounds terrible.

There are systems that use G.711 when traffic is light, but
switch to compression codecs under heavy traffic to conserve
bandwidth.  I don't know how/if this can be done in Asterisk.

--Stewart