CRAAAAAZY update:
I've been pestering Grandstream and Flyingvoice for updates, and Grandstream
helpdesk came back to me with a most interesting comment:
"The development plan is to include the fax capabilities of the HT V1 into the
V2 series."
When I got this response, I was like whaaaaaaaat are you even talking about,
since (at least in my head) I had the same problem with the V1. Even if there
are differences in the DSP code between them, how will porting those forward
fix this specific issue?
So I broke the older revision of the HT802 hardware out again to re-test, so
that I could come back to them with captures in hand and say "told ya
so...don't waste your time".
Well. It works. The dadgum HT802 (retroactively: "HT802v1") actually works.
Looking back and reviewing the footage up to this point, I realized what had
happened that led me blend these two variants of this hardware in my mind: when
I first acquired the original HT802 in order to put it through its paces, I ran
into a bunch of T.38-related problems with it (shocker, I know). The biggest
one seemed to be that if the HT80Xv1 was the one that initiated the T.38
re-INVITE, it would always specify a "T38FaxMaxRate" of "9600" in the SDP
payload, and there was seemingly no way to change/adjust/influence this
behavior. And some of my orig & term providers did NOT like that, and would
send back SIP "488 Not Acceptable Here" in response, so the re-INVITE would
never succeed, and either the call would drop then and there, or it would
remain at/re-INVITE back to G.711 and eventually fail.
That issue was the first ticket I opened with Grandstream. Their response was
that HT80X was being sunsetted, and HT80Xv2 was replacing it, and also that
HT80Xv2 already had an option in the firmware to adjust the value it sent for
T38FaxMaxRate. (Fun aside: it did indeed, but that setting in the v2 firmware
didn't actually freaking work / was completely ignored, so I had to open a new
ticket about THAT, which they then eventually fixed. Sigh. Does anybody ever
actually test these things?...)
So I requisitioned meself an HT802v2, and that's when I ran into this
modem-always-fails-training-during-fax-reception problem. And that's the point
at which I started collecting a bunch of other ATA models from other vendors,
cobbling together a makeshift lab where I could run fax tests without involving
the PSTN, etc. All of these other ATAs were all failing in the very same or
very similar ways as the HT802v2 was, even in the lab. But because I never
even got to that point in testing the original HT802 because of a *different*
showstopper problem, I just lumped it in my head with all of the others and
assumed it also must have the same problem (because given that its successor
model does, and given how widespread the problem seems to be, why wouldn't it?).
Taking a look at detailed debug syslogs from both v1 and v2, it turns out that
the v1 series actually used a completely different DSP library under the
covers: while v2 has "SpanDSP" referenced all over the place, the log messages
from v1 look completely different and reference "LIBGSDSP" (where I have to
imagine "GS" stands for "Grandstream"). So apparently Grandstream had taken
the time and effort to develop a bespoke set of DSP algorithms in-house, but
when they came out with the newer hardware revision, they
didn't...bother...to...keep...it? And instead just took SpanDSP off the shelf
and slapped it into the v2 firmware??? (Perhaps they were offloading some of
the DSP functions in hardware on v1, and the SoC in the v2 doesn't have DSP
hardware-accel, or something? Heaven knows.)
I'm still flabbergasted that I have run across so many ATA models with T.38
gateway support that all have very similar modem interop problems. The Yeastar
T.38 implementation is also based on SpanDSP, so I suppose that explains that.
Flyingvoice's isn't, so perhaps that's why it works a little better (though
barely) than HT80Xv2 and Yeastar do. But I don't think the other models I've
played with have any SpanDSP code in them, so whatever faulty assumption
SpanDSP has been making seems to be shared at some level between multiple other
independent (?) implementations. At least, that's the best thesis I can
muster. Perhaps there is some ambiguous language in some standards document
somewhere that everybody is referencing but interpreting in different ways.
If Grandstream can actually manage to fix this in the v2, then perhaps that
will ensure there is at least *one* series of actively-produced ATA models out
there with a solid T.38 stack.
-- Nathan
-----Original Message-----
From: Nathan Anderson via VoiceOps [mailto:[email protected]]
Sent: Monday, February 16, 2026 19:56
To: 'Voiceops'
Subject: [VoiceOps] Re: Bizarre T.38 gateway/DSP modem interop problem
..switched from Windows Fax-and-Scan to HylaFAX on Linux, using the same
Conexant USB modem.
Behavior is identical.
-- Nathan
-----Original Message-----
From: Nathan Anderson via VoiceOps [mailto:[email protected]]
Sent: Monday, February 16, 2026 17:24
To: 'Voiceops'
Subject: [VoiceOps] Re: Bizarre T.38 gateway/DSP modem interop problem
I have tried both HT802 and HT802v2.
The Flyingvoice adapter actually does a better job with fax RX when paired with
my particular troublesome test modem than the Grandstreams do. At least the
Flyingvoice can successfully receive...it will only do so at 2400 or 4800bps,
and it takes like 2-3 minutes for it to finally train at that rate after trying
and failing at all of the others, but it will at least work. The Grandstream
just never manages to successfully train with the modem at any modulation rate,
period.
Grandstream and Yeastar both appear to use SpanDSP under-the-covers.
Flyingvoice's implementation appears to be different, though whether they wrote
theirs in-house or sourced it elsewhere, couldn't say.
Also, I should mention that after I first discovered this problem, I have
worked to try to isolate as many variables as I can in order to eliminate other
potential sources of failure from the equation. For example, I can still
reproduce the problem simply by having 2 ATAs register to the same IP-PBX and
have one fax machine call the sketch Conexant one without involving the PSTN at
all...all traffic remains local to the LAN, no origination carrier involved,
etc. Failure still occurs, and mode of failure is 100% identical.
Since the fax "machine" that I experience failures is a Class 1 PC fax modem, I
think my next test needs to be trying a different piece of fax software, just
to make 1,000% sure that it's the modem itself that is having difficulty with
the ATA, and not the software driving it that is the key differentiator (since
I've come to understand that "Class 1" is kinda-sorta like the "softmodem" of
fax standards, with not-inconsequential parts of it being processed on the host
CPU rather than internally within modem DSP/firmware itself).
What I still keep coming back to, though, is that the Motorola ATA's T.38
implementation *works fine* with this particular combo of fax modem + fax
software. It's just seemingly nearly every other ATA on the planet that
doesn't.
-- Nathan
-----Original Message-----
From: Enzo Damato via VoiceOps [mailto:[email protected]]
Sent: Monday, February 16, 2026 16:41
To: [email protected]
Subject: [VoiceOps] Re: Bizarre T.38 gateway/DSP modem interop problem
I'm not sure if this is much help, both because you said you tried
Grandstream and because I haven't tested fax reception (only
transmission), but the Grandstream ht802 has always been my go-to for
odd endpoints that will not work with anything else. So far, I've been
able to use it to hook up the following
- Payphones that need polarity reversal
- Fax machines (but I've only tried sending)
- Modems (results have been hit-and-miss, but reliable enough to do a
Nortel Millennium payphone config download after several tries)
- Rotary phones using pulse dialing
Regardless of what device you choose, the single most helpful thing that
I have found with faxes is to have the PBX/Switch/whatever capture the
whole fax and then attempt to send it on to the target number. This does
mean that you don't get confirmation that it was delivered, but it makes
the whole process much less awful since the t.38 is being terminated
much closer, and the ATA doesn't have to negotiate T.38 over long distances.
Thank you,
Enzo Damato
On 2/16/26 7:53 AM, Nathan Anderson via VoiceOps wrote:
> 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)...?
>
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