asterisk users - Jul 2005 - Zap channel(s), meetme and codecs/licences

Hi all,

Some simple questions about codecs:

What codec does the Zap channel use by default?
Can this default be changed, and to what? (g729 too?)

What codec does meetme use? (I think this is ulaw, but asking to be sure)
Can you use another codec, or does everything have to be transcoded to ulaw?

Finally ... if I have a 3way call going, between 1 g729 caller and two
other callers, do I need one or two available licences? (I'm guessing that
zap doesn't do g729, and am wondering if I have an FXO caller and a local
FXS person talking to a VoIP caller using g729, how it would work)

Trying to nail down how all of the codecs and figure out how the licence
costs might work.

Thanks,
J.

> Some simple questions about codecs:
> 
> What codec does the Zap channel use by default?
None/all. Look in /usr/src/asterisk/configs/zapata.conf.sample and
you won't find any reference to codecs. Think of the zap channels
sort of as a layer-two protocol interacting with external devices
such as T1's, pstn lines, etc, to support "signaling". Layer-two
protocols do not care about the _content_ of the packet.

The devices attached to the zap channel _do_ care about content.
In the US, a T1 zap channel will always be ulaw as an example.
If you attach a channel bank to a zap channel, the channel bank
will implement a codec (ulaw in most/all US products).
> Can this default be changed, and to what? (g729 too?)
Nope, but you might be able to specify g729 in whatever device you
have attached to the zap channel. Right now, I can't think of a
single device where that choice is valid though. (There might be
some, but I simply don't know.)
 
> What codec does meetme use? (I think this is ulaw, but asking to be sure)
> Can you use another codec, or does everything have to be transcoded to
ulaw?
Again, meetme facilitates the handling of voice packets, and if one
of those participants happens to use a g729-capable phone, a g729
license will be used to do the conversion. You really don't care
what meetme happens to use internally; its all about the end-points
and what _they_ need to access the meetme channel.
 
> Finally ... if I have a 3way call going, between 1 g729 caller and two
> other callers, do I need one or two available licences? (I'm guessing
that
> zap doesn't do g729, and am wondering if I have an FXO caller and a
local
> FXS person talking to a VoIP caller using g729, how it would work)
Someone else might want to chime in here, but it seems to me (as a non-
programmer) that internal handling of voice packets (within *) were
primarily slinear or something like that. If two end points can communicate
with the same codec, the voice data is simply passed through (no
conversion). If one user is g729 only communicating with other users,
that g729 user consumes one license instance to convert to whatever
the other users might be using. Two g729 users and one g711 user, likely
uses two g729 licenses.

Without a better understanding of what devices you are truly trying to
use, I don't believe anyone is going to be able to answer your questions
relative to zap channels and codec selection/conversion.
> Trying to nail down how all of the codecs and figure out how the licence
> costs might work.
Since the g729 license is fairly inexpensive, just by four/five and
play around with whatever devices you've got access to. Depending on
exactly what you're trying to do, you may just decide that you don't
like the call quality and might try to minimize the g729 use instead
of minimizing bandwidth.

Also keep in mind that a g729 user that happens to receive a pre-canned
audio message ("sorry, this number is invalid") will consume a license
since the messages are usually in gsm format.