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

[Olivier]

2009/3/20 Klaus Darilion <klaus.mailinglists at pernau.at>

>
>
> Steve Underwood schrieb:
> > Hi Olivier,
> >
> > Olivier wrote:
> >>     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).
> > That is what typically what happens when the calling end doesn't obey
> > the spec. It comes down to a race for who initiates the re-invite first.
> > If you are lucky the two ends sort themselves out. If you are unlucky
> > you end up with both ends re-inviting, and you may get a call failure.
>
> This is what I see very often - both sides send reINVITE (overlapping),
> both sides reject with 491 (request pending) with the result that the
> switch to T.38 failed. I have only once seen handling overlapping
> reINVITEs smart (ComISDN client)
>
> Theoretically it would be perfect if the reINVITE is always triggered by
> the callee (as the spec says) - but there are ATAs which need up to 15
> seconds to detect a fax and send reINVITE - and in this cases you
> sometimes have to work around by allowing the caller to send reINVITE too.


Linksys 3102 have an option specifying if reINVITE should be inbound,
outbound or both.
Unfortunately, I couldn't find this option in Patton SmartNodes.


>
> regards
> Klaus
>
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