[VoiceOps] VoIP T38 Fax reliability

Antoine Reversat a.reversat at gmail.com
Thu Jan 7 15:39:45 EST 2010

We have a very good success rate here using Linksys spa2102s and
asterisk 1.6. We ran into several problems mainly in t.38 negotiation.

The things I would be most careful about :

- Make sure the t.38 is correctly negotiated. That has been an issue
in the past. A good thing that asterisk comes with enough debugging to
be able to troubleshoot that.
- Get rid of any SIP ALGs. We have had a strange issue where both ends
couldn't negotiate t.38 because the SIP ALG implementation (namely
D-LINKs implementation) would just not understand the re-invite part.

In my experience once your fax voip call uses t.38 you're good. Or as
somebody else said if you have 0 packet loss / 0 jitter then you're
good too (as long as you use g711).

On Thu, Jan 7, 2010 at 12:55 PM, Alex Balashov
<abalashov at evaristesys.com> wrote:
> Been there, done that, got the t-shirt.  Main problem I ran into is that (1)
> Super G3 detection doesn't work on many switches, and (2) even the ones on
> which it does work there is not a vendor-neutral way to pass that knowledge
> through to the terminating gateway, and (3) T.38 support at Super G3 speeds
> is lacking in most implementations at this time.
> Between this and the problems you mention, it's just not worth it.
> Seriously, it's one of the least Pareto-optimal propositions in the entire
> business.  Just sell them analog lines.
> I've actually managed to convert a few customers - even steadfastly
> traditional, paper-intensive ones - to using fast doc scanners + e-mail.
>  :-)
> On 01/07/2010 12:47 PM, Ujjval Karihaloo wrote:
>> 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.
>> Regards,
>> Justin
>> ------------------------------------------------------------------------
>> *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.
>> _______________________________________________
>> VoiceOps mailing list
>> VoiceOps at voiceops.org
>> https://puck.nether.net/mailman/listinfo/voiceops
> --
> Alex Balashov - Principal
> Evariste Systems
> Web     : http://www.evaristesys.com/
> Tel     : (+1) (678) 954-0670
> Direct  : (+1) (678) 954-0671
> _______________________________________________
> VoiceOps mailing list
> VoiceOps at voiceops.org
> https://puck.nether.net/mailman/listinfo/voiceops

More information about the VoiceOps mailing list