[asterisk-users] which codec over iax => pstn

[Gordon Henderson]

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