[VoiceOps] VoIP T38 Fax reliability

Justin Randall jrandall at comwave.net
Thu Jan 7 12:39:49 EST 2010

Faxing over T.38 from experience has certainly been a better experience
than over G711u bypass.  Of course there are some implementations of
T.38 that interoperate with each other better than others.  My
experience with AudioCodes, Linksys/Sipura, and Grandstream has been
very positive.


One of the common potential problems with either approach is the
behaviour of the signaling in response to which end (caller, callee, or
both) of the session is responsible for initiating the re-INVITE for
T.38.  When supporting wholesale customers it is not always possible to
configure the behaviour of the ATAs/IADs and the only way to ensure fax
will at least always be "detected" is to have one of endpoint
gateways/ATAs/IADs detect fax as the caller and the callee.  The next
fun piece to tackle once that's configured is to ensure that all
endpoints properly support handling SIP 491 Request Pending response
codes to avoid a race condition which can cause INVITE "glare" if one
endpoint is configured to detect faxes and re-INVITE for both caller and
callee and the other is set to detect faxes and re-INVITE on either.







From: voiceops-bounces at voiceops.org
[mailto:voiceops-bounces at voiceops.org] On Behalf Of Ujjval Karihaloo
Sent: Wednesday, January 06, 2010 7:01 PM
To: voiceops at voiceops.org
Subject: [VoiceOps] VoIP T38 Fax reliability


I know this a thorn in everyones flesh, I would like to know of
experiences with T38 Voip Faxing. We have had good and bad. Those that
don't work just don't and they don't even want to troubleshoot and just
switch over to an Analog line for Fax. We mostly use AudioCodes and
Sipura T38 ATAs.


Any of you folks do T38 and whats the experience like.


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