[asterisk-users] T.38 - Which endpoint shall reINVITE ? caller or callee ?

[Olivier]

2009/3/17 Steve Underwood <steveu at coppice.org>

> Vlasis Hatzistavrou (KTI) wrote:
> > Olivier wrote:
> >
> >> Hi,
> >>
> >> I've been playing with T.38.
> >>
> >> I observed that mostly but not always, it's the "calling endpoint" that
> >> reINVITE the other party to drop current SIP/G711 session and start a
> >> new T.38.
> >> But sometimes, it's also the callee party that reINVITE the calling
> party.
> >>
> >> Which is the "standardized" or most common, way to start a T.38 session
> ?
> >> Shall it come from callee or from caller ?
> >>
> >> Regards
> >>
> >
> > Fax transmission can be initiated from any one of the parties. AFAIK
> > T.38 as well as the PSTN fax standards do not show any preference
> > whether fax transmission is requested from a or b party.
> >
> > In practice, the caller usually initiates a fax transmission, but this
> > doesn't mean that the called party cannot initiate it, too.
> >
> > Best regards,
> > Vlasis Hatzistavrou.
> >
> Hey, why bother looking at a spec when its so much more fun to make it
> up as we go along?
>
> T.38 says that if the call starts in audio mode it is the called end
> which should initiate a re-invite to change from audio to T.38. This
> makes sense, as that is the end which has the best chance of figuring
> out if a FAX machine answers the call. In practice many T.38
> implementations will send out a re-invite when they are the calling
> side, so any practical implementation has to allow for this. Clashes are
> possible, if both ends send re-invite, and this is not always handled
> properly


Yesterday, with 2 consecutive sendings on the same setup (same fax file,
same ATAs, same servers), on the first try, I've seen the reINVITE coming
from callee on from the caller on the second try.
I don't remember I changed anything between both tries (though I may have
done without noticing this).



>  Also many implementations will only listen for a FAX machine
> at the beginning of a call, so if a human answers and later presses the
> start button on their FAX machine the T.38 gateway might miss this.
>
> Regards,
> Steve
>
>
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