[Asterisk-Dev] IAX codec negociation (again...)

Kevin P. Fleming kpfleming at digium.com
Mon Jun 20 07:15:01 MST 2005

Goldenear wrote:

> When I see how codec négociation works with sip/ser, I just guess it 
> could be as easy with * :)

Asterisk is not a proxy.

> I'm wondering, couldn't * ask both sides of a call their codecs 
> capabilities before "bridging" them and so force them to use  the same 
> codec if there is a common one ?

Asterisk is not a proxy. The codec decision for the peer originating the 
call is made long before we have any idea if the call is going to be 
terminated to another peer (or an application, or anything else).

> In this way, it works :) A switch from gsm to ulaw during the call.
> So, I wonder why having asterisk in the path would break codecs 
> nogociation.

Asterisk is not a proxy :-) Putting it in between two peers will _not_ 
be transparent, under any circumstances. In this scenario, A does not 
'switch from gsm to ulaw", the call is set up that way and never 
changes. What happens if B transfers A to peer C, which only supports 
GSM? How would you expect Asterisk to handle that?

With all that said, it _is_ possible for this to be done, but it will 
require Asterisk to be able to know when it needs to renegotiate a codec 
with a peer, and to initiate that operation, and peers/protocols that 
support it.

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