Raising this thread from the dead to see if anybody else who might've missed it the first time has any bright ideas.
Shortly after I made the original post, a very kind gent with some actual DSP and fax-specific experience responded off-list, asking for some captures of working and non-working sessions. I sent those along, but unfortunately he seems to have dropped off the face of the earth. :-( Not that I really blame him...he was graciously volunteering his free time and expertise, and life is busy. But it just means I effectively lost one of the only leads I had. I'm desperate enough that now I'm willing to start naming names in public. At this point, I've run into nearly-identical T.38 receive-specific problems with products I've tested from all of these vendors: * Grandstream * Yeastar * Flyingvoice * (HP/)Poly(com) (f/k/a Obihai) * ...even Adtran It is mind-blowing to me that the only ATA I have ever found that works reliably with T.38 *reception* regardless of what modem I hook up to it is the freaking ancient Motorola model that I can't get anymore. The modes of failure across all of the newer ATAs that don't work are so strikingly similar that either I'm consistently doing something wrong without realizing it, or all of the engineers behind these products made the same wrong assumptions in their fax DSP code that do not hold true across all fax modems (or perhaps they share some [bad] code in common with each other! ...I do have reason to believe that at least 2 of the above vendors are using the open-source SpanDSP project/library to implement their T.38 gateway stack in their firmware!!) With the modem I've been testing against, the Grandstream just fails to receive entirely. The Flyingvoice adapter, on the other hand, will eventually succeed, but only after it trains all the way down to 2400-4800bps. I have had tickets open with both Grandstream and Flyingvoice for months now; they seem to be going nowhere, though to their credit they haven't given up (or at least the front-line support people updating the ticket continue to put on a brave face). Yeastar (which also just fails entirely) threw in the towel within days when I tried to ask them. I had forgotten that I ran into an extremely similar problem with Adtran a few years back that their support people also never solved. I have not yet tested Cisco ATA19x models. The only Poly/Obi one I've tried is a 300-series, which is now discontinued & replaced with the 400-series, so HP/Poly support won't touch it. I have considered acquiring a Poly 400 and a Cisco ATA192 and opening up tickets with both, but I just know I'm in for a bad time with both company's TACs if I do. Help me, Obi-Wan Kenobi... -- Nathan -----Original Message----- From: Nathan Anderson Sent: Thursday, November 13, 2025 23:21 To: Voiceops Subject: Bizarre T.38 gateway/DSP modem interop problem (...or, "Any currently-manufactured ATAs with a T.38 gateway implementation worth a damn?") Perhaps some will find this shocking, but for the longest time, we have been using Motorola VT1005 as our basic, low-port-count TA. We had lucked into a large source of overstock, still-new-in-box units for cheap some time ago, but that source is now gone. So we are shopping around for a new model to take its place. Part of the reason we stuck with the venerable Moto for so long was because our wish list looked like this: 1. Reasonable price point 2. Good performance for price 3. Solid T.38 implementation More to the point, we preferred a single TA that could fulfill all requirements, rather than having to stock multiple different models (e.g. one for voice-only, another for customers who actually cared about fax, etc.). And for the residential/SOHO crowd, it struck me as ridiculous that some 1-2 port count TAs out there often have MSRPs that are higher than the routers they're going to be sitting behind (I'm looking at you, Cisco...). The thing about the VT1005 is that not only did it have a solid T.38 gateway feature, but it was hands-down the MOST bullet-proof implementation I have EVER run across, period. It "just works". Even if I was okay with stocking a special model for our fax-using customers, and even if price was no object, I seemingly CANNOT buy another TA with as good an implementation for love nor money. It was the same story every time: every couple of years, I'd order another TA model and/or pull out some previous eval units we'd acquired before & update their firmwares, re-test them, and still run into tons of issues. So as long as the Moto was still available, I just kept kicking the can down the road. I'm going through that same hell again now, and I have realized over the last few weeks of opening tickets with hardware vendors & tearing my hair out that there is a common thread to my failing fax tests. 1. Fax TRANSMISSION always works fine (T.38 gatewaying kicks in, the modems train with each other at 14400bps, pages are sent successfully). 2. Fax RECEPTION is what breaks down (T.38 gatewaying kicks in, but the receiving modem -- the one plugged into the TA on our side -- always Fails To Train at virtually any speed) 3. ...though #2 is only true with CERTAIN fax modems, while others can receive faxes with non-Moto ATAs JUST FINE, at speeds up to 14400bps The fax modem I usually run my tests through is a cheap little USB-based hardware modem, but one with only Class 1.0 fax support. It's based on what seems to be a fairly ubiquitous Conexant chipset, the CX93010. When paired with Windows Fax & Scan and connected to a Motorola VT1005, receiving faxes via T.38 works just *fine*. But when paired with literally any other ATA with T.38 support that I've tried, it will either attempt but fail to train at 14400bps all the way down to 2400bps, or (with one ATA in particular) it will finally successfully train and send CFR after training all the way down to 4800bps, or 2400bps at the worst. As far as I can tell, this is not strictly speaking a T.38 problem per-se. This is an issue seemingly with the DSP on the ATA that's emulating the remote modem, and there is something about most of these DSP implementations that at least this particular Conexant-based modem does NOT like. It can send faxes through these ATAs all day long, but whatever tones these TAs are generating, the Conexant just isn't having it. If I sub in a different fax machine in its place (e.g. big HP multifunction Laserjet), fax reception (mostly) works great through a lot of these same ATAs. And similarly, if I just put the Moto back in service with the Conexant modem, that also works just fine. Now, sure, blaming the modem is fair game. And I don't discount the possibility that there is something that it's doing wrong. The thing is...the Moto VT just freaking works with it. And the fact that there is at least one modem model out there -- one with a common enough chipset -- that virtually all currently-manufactured TA models out there spouting T.38 support cannot interop with makes me concerned that I'm likely going to run into more such interop problems in the field with customers' own fax equipment, after we start deploying & the TA we choose to go with is suddenly exposed to a much more, erm, diverse crowd of fax machine models. What on earth could this modem could be so sensitive to that it doesn't work with any of the TAs I've tested with it (other than the Moto)...? -- Nathan Anderson First Step Internet, LLC [email protected] _______________________________________________ VoiceOps mailing list -- [email protected] https://lists.voiceops.org/postorius/lists/voiceops.voiceops.org/ To unsubscribe send an email to [email protected]
