asterisk users - Feb 2008 - which codec over iax => pstn

using asterisk(A) over iax to another asterisk server(B) which connects 
to pstn over pri.

Doesn't B have translate to ulaw whatever goes out to the pstn, so 
therefore shouldn't A choose ulaw as the iax codec to B? That way 
there's no loss translating from {gsm, ildc, etc} to ulaw on the B server.

My partner thinks I'm nuts, and that gsm is much more "efficient"
as the
iax codec.

BTW, we have 512kbs over the iax connection.

Whom do you support?

sean

On Tue, 19 Feb 2008, sean darcy wrote:
> using asterisk(A) over iax to another asterisk server(B) which connects
> to pstn over pri.
>
> Doesn't B have translate to ulaw whatever goes out to the pstn,
Depends on the country, but ulaw or alaw...
> so
> therefore shouldn't A choose ulaw as the iax codec to B? That way
> there's no loss translating from {gsm, ildc, etc} to ulaw on the B
server.
It depends on how the call comes into A and how "bothered" A is about 
doing the transcoding, or letting B do it for them.

If the calls come into A via GSM, then you might as well send them out 
again in GSM as the call quality won't improve, but if they come into A as 
G711 (ulaw or ulaw) then keep them as G711 if you don't want to lose 
quality, else compress them to GSM if you don't have the bandwidth 
avalable.
> My partner thinks I'm nuts, and that gsm is much more
"efficient" as the
> iax codec.
GSM is more compressed than ulaw or alaw, so will use less bandwidth, but 
in doing so, it will sound worse. (mobile phone quality rather than 
landline quality)
> BTW, we have 512kbs over the iax connection.
G711 needs about 80Kb/sec each way to work. (It's 64Kb/sec plus IP 
overhead). GSM needs about 32Kb/sec (13Kb/sec plus IP overhead).

IAX is more efficient than SIP in packing multiple calls into the one data 
stream.
> Whom do you support?
My customers, and what they ask for...

Gordon