Hi Temi
> I am having some issues ensuring t38 faxes are being received reliably. I am
> currently using kamailio as an inbound sbc with freepbx as the endpoint. I
> understand that potentially stripping other codecs except for g711a/u makes
> inbound faxes reliable, but beyond that I am having trouble determining how
> to ensure reliable faxing. Is this something kamailio can handle (such as an
> error correction mode or pass through setting or module?) I would appreciate
> any insight as I have hit a wall when it comes to this issue.
Fax over VoIP is a very complicated issue, there are various very
different implementations.
audio works fine, if you have basically no jitter end to end.
t.38 tends to work better if there is jitter.
In my experience, it is best to basically try to leave the codec
negotiation to the involved endpoints and not to try to build some kind
of T.38 to audio gateway.
If the involved endpoints fail in negotiate a codec, they probably
misbehave.
What you can do to try to improve the situation if there is an issue
with T.38, is to force the endpoints to use audio and strip image from
the codec list. But this is not as easy as in the usual situation
where the connections starts with audio, then one party detects
fax-tone and re-invites with only image (maybe several times when the
fax-tone is re-detected on every page boundary), you can not just strip
image and forward the empty codec list to the other party, this is sure
to end up with an error.
You would need to reply 488 to the party initiating T.38 and hope it
correctly falls back to audio. This can either happen with a re-invite
back to audio, or just by doing nothing. 488 according to RFC should
result in the previously negotiated codec to stay valid!
So when you using rtpengine, make sure you have something like this in
your reply route:
if ($rs != 488) {
# do NOT manage reply on 488 to allow t.38 fallback to
previously negotiated codec.
rtpengine_manage("some options");
}
Mit freundlichen Grüssen
-Benoît Panizzon-
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