[VoiceOps] VoIP T38 Fax reliability

Ujjval Karihaloo ujjval at simplesignal.com
Thu Jan 7 12:47:41 EST 2010

Correct, I have been able to get around INVITE glare etc...by tweaking ATA configs, however the problem mainly lies in UDPTL T3/T38 signaling in the RTP, E.g. TAs keep sending no signal msgs to the Provider and they don't sends a DIS back and it then just re-invites for G711 Pass-though...and that never works...

I have also seen FCS-BAD (HDLC frame signal)  coming from the Provider when UDPTL data is sent.

From: Justin Randall [mailto:jrandall at comwave.net]
Sent: Thursday, January 07, 2010 10:40 AM
To: Ujjval Karihaloo; voiceops at voiceops.org
Subject: RE: [VoiceOps] VoIP T38 Fax reliability

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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