[VoiceOps] VoIP T38 Fax reliability

Justin Randall jrandall at comwave.net
Fri Jan 8 12:28:51 EST 2010

I never actually got the reply from Ujjval so I'm going to reply to this

My experience with the FCS-BAD problems has typically been related to
TDM problems not relating to VoIP or T.38 in any way.  While it is
possible that a buggy T.38 stack might cause all sorts of unexpected
issues, FCS is related to framing and I've seen this issue resolved in
certain media gateways by ensuring that the TDM clocking on the media
gateway cards is tuned properly.

One thing people often forget is that faxing doesn't even work 100% in
the PSTN.  Line noice, interference, etc. all contribute to failures.
These types of failures are simply propagated through the T.38 stack
into the UDPTL signaling.

For your issues with not receiving DIS I would try and perform a G711u
bypass fax call and export the audio from the RTP through Wireshark and
listen to the analog signaling handshake exchange to see if the fax
machines are even properly communicating at a basic level.  If you have
PSTN bearer tracing capabilities you could also perform a T.38 test call
and listen to see if the initial fax signaling tones are sent and
acknowledged at the analog side.

Hope this is helpful,


-----Original Message-----
From: voiceops-bounces at voiceops.org
[mailto:voiceops-bounces at voiceops.org] On Behalf Of Alex Balashov
Sent: Thursday, January 07, 2010 12:55 PM
To: voiceops at voiceops.org
Subject: Re: [VoiceOps] VoIP T38 Fax reliability

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
> 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
> than over G711u bypass. Of course there are some implementations of
> that interoperate with each other better than others. My experience
> 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,
> 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
> 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
> 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
> configured to detect faxes and re-INVITE for both caller and callee
> 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
> 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

More information about the VoiceOps mailing list