[asterisk-dev] how does a channel driver know the result of codec negotiation ?

[Kevin P. Fleming]

Luigi Rizzo wrote:

> because typically you cannot afford to do video transcoding in
> the middle box.
> So if the caller tells "i can do H261 | H263 | MPEG4" and the callee
> can only decode say H263, the caller still has no idea of what
> format to use. For incoming streams it is not much of a problem - if
> you get one of your supported formats you switch to that on the
> first incoming frame. But for outgoing data you have no clue...

This situation is in no way unique to video; users with non-transcodable
(or expensively transcodable) audio formats have the same problem.

Until this issue is solved in Asterisk in some architecturally-proper
way, then it will be a problem for video formats as well. The way
Asterisk works now is that a format is chosen for the callee's channel
before any attempt to make an outbound channel has been made (and there
may not even be an attempt to make an outbound channel if the callee
ends up running an Asterisk application instead) and that format is
never changed, unless we receive a different format from the callee for
some reason during the call.

-- 
Kevin P. Fleming
Director of Software Technologies
Digium, Inc. - "The Genuine Asterisk Experience" (TM)