[asterisk-users] Inquiry:Asterisk different codec schemes?

[Olle E. Johansson]

5 jan 2010 kl. 10.08 skrev hadi motamedi:

> 
> 
> On Tue, Jan 5, 2010 at 8:59 AM, Olle E. Johansson <oej at edvina.net> wrote:
> 
> 4 jan 2010 kl. 14.46 skrev Kevin P. Fleming:
> 
> > hadi motamedi wrote:
> >
> >> Sorry . I didn't get the point clearly . In the SIP Invite message , it
> >> says "my audio endpoint is IP x.x.x.x port x, and I can use codecs
> >> A,B,C". The remote endpoint responds with a 200 OK, saying "my audio
> >> stream is at IP y.y.y.y port y, and I choose codec B". Can you please do
> >> me favor and let me know if my understanding is right or not ?
> >> Thank you
> >
> > No, you are not understanding the SDP offer/answer model properly. If
> > one endpoint offers codecs A, B and C in its SDP, it is willing to
> > *receive* media in those formats. The receiver of that offer can choose
> > to send media to the offerer in any of those formats, at any time. If
> > the answering endpoint includes only codec B in its SDP, then it is
> > willing to *receive* only codec B. In that scenario, it is possible for
> > media to flow from endpoint 1 to endpoint 2 using codec B, and from
> > endpoint 2 to endpoint 1 using codec A (or C), but this will not happen
> > if Asterisk is an endpoint in this scenario.
> >
> > When Asterisk receives a media frame, if the format of that frame is not
> > the format that it is currently sending to the other endpoint, it will
> > switch to that format automatically. If it cannot do so because the
> > other endpoint did not offer to receive that format, then the call's
> > audio will probably fail. This is the reason why I responded before that
> > Asterisk does not support asymmetric formats in a media session.
> >
> > In reality, it is extremely uncommon for a SIP endpoint to want to send
> > media in a format that it is not also willing to receive; in fact, I
> > can't say I've ever seen this situation arise in any testing I've done
> > or in any issues reported in our issue tracker.
> 
> But it's fairly common to have asymmetric media in the call. If the caller offers A, B and C and the callee responds with B, the caller sends B but the callee might send A.
> 
> /O
> 
>  
>  
> Sorry . You mean we can have asymmetric codecs in Asterisk ?
>  
As Kevin stated, for Asterisk, the server switches to the format we receive, so no. I just pointed out that it happens quite often that a call is asymmetric, and you will see Asterisk trying to follow the other side.

/O