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

[sean darcy]

Gordon Henderson wrote:
> 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...

ulaw
> 
>> 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.
> 
the call comes into A over an analog port on a TDM400P. I assume it's ulaw.
> 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).
> 
So with DSL 512kbs up and 3mbs down, plenty of room for G711.

Of course, I could figure out how to configure QOS in iptables for 
asterisk, it'd be a lot better.

> 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...
> 

Great. Thanks for the help.
> Gordon
> 
sean