Fax modem affecting ADSL?

(sorry, this started really short, honest...)

I have a fax modem on my ADSL line, and it seems that frequently incoming fax calls are causing the line to drop (the faxes come through OK). The line only appears to drop after the fax 'warbling' has started and voice calls don't appear to do this. This *seems* to happen less now that the fax modem and router have been moved further apart (they were right next to each other) though that may well just be luck.
Diagnostics are a bit tricky since the line is ADSL2+ and therefore has its own self-adjustment issues (normally only evening/night) even when the fax modem is disconnected (hard to tell if the line is more stable or not), and none of this is guaranteed to happen 'on demand', as it were.

I've tested with the original socket faceplate and with different filters and two different fax modems, but this seems to make no difference.

Both fax modems (hardware, external) seem to work properly, and the router (belkin F5D7632uk4A, wireless off) seems to work properly. Unfortunately the router/firmware combo is the one which does *not* have CLI access so diagnostics/tweaking availability is limited.
The line is shown as having
Noise Margin 8-9dB Up, 5-6dB Down, Attenuation 41dB
and generally syncs at 9600-10100, stable around 9900 or lower.

So...
1) are there known issues (in general) with faxes and/or fax modems on ADSL lines?
2) is it likely that *both* modems have developed a fault?
3) the ADSL line goes through the surge-protect on the UPS (everything powered by this), would it be better to put the fax modem through this instead?

I'm not clear if the fax modem is filtered, it needs to be and you might try two filters in series as an experiment.

In general modems, faxes and ADSL co-exist happily but as with some phones you can get the odd problem.

I wouldn't put an ADSL line through a surge protector personally, on a ver ylong line it was enough to stop it working - obviously you aren't in that situation but any unnecessary components are best avoided if you have a problem.

The fax modem is filtered, and I have even tried the double-filtering without any success.

The line doesn't lose sync with *every* fax, and on a test just now the line lost sync *before* the fax even picked up...

I had put the (ADSL post-filter) through the surge-protect as a precaution after a previous line problem - the engineer had mentioned a blown fuse at the exchange and I didn't fancy the idea of forking out for a new router. Though having checked prices it looks like a fax modem is now the valuable one that needs protecting!

I'll swap the surge protection and try again, with and without double-filtering.

Have you got the ring wire disconnected at the master socket? It may be extra noise affecting the ADSL, and the process of the fax ringing may just be enough to tip it over sometimes.

Some old Brother fax machines had issues in 2000, but it does not appear so often now.

In reply to a post by MrSaffron:

Some old Brother fax machines had issues in 2000, but it does not appear so often now.

At one site, I have two different Brother "all-in-ones" almost sat on top of the router. One is connected to a very noisy ADSL line whilst the other is connected directly to a dedicated FAX line. They have been swapped over several times due to ink running out etc and there's never been a problem.

Fortunately the ring wire isn't an issue - it ends at a box outside. Two pairs come in, one to the voice line, the other to the fax/DSL line.

- DSL straight into the 'main' filter (line now syncing around 11000 though stability TBC)
- fax now via the surge-protect then into extra filter then into 'main' filter
- fax now only protected by UPS, not powered by it (this one seemed to make a difference)

These seemed to work but then a quick couple of final checks (before posting news of my magnificent success) killed the line again so I see the proverbial square one looming again after what turns out to be only a temporary respite.

It *seems* to be the pickup 'click' (definitely not the ringing) that does it - the apparent death during 'warbling' was only because it was taking a few moments for the router to react.
Now that I've actually been going through testing and watching, it seems that roughly one in three faxes will kill the line though of course this might just be luck

And some extra mud for the water : while trying to pinpoint exactly when the line drops, I'm getting the impression that if there is DSL traffic on the line when a fax call comes in, it seems to stay up, with only one (sometimes two) 'truncated-ip' messages appearing from tcpdump (large numbers of these are a good warning that the line is in slow-death mode and that the router might be about to lock up or reset itself).

Just to add that now we are into evening time, the 11000-ish speeds have turned out to be unstable and on attempting to reconnect, unusable with "timeout in LCP negotiation" and so I plugged the line through the UPS surge-protect again. It means the line syncs at 10000-ish but at least I can get a connection, and I don't know any way on this router to set a maximum speed...

It seems that none of these various combinations is really making any difference, so is it possible that this is actually a problem with the router, either being faulty or just over-sensitive?

Possible router issue, on ASDL2+ the variations in what works for one person and doesn't work for another is much greater.

In reply to a post by yarwell:

I wouldn't put an ADSL line through a surge protector personally,

It can save a lot of expense.

I have two lines here and during a thunderstorm a few years ago Line One was fed through a surge protector and Line Two was not.

Does anybody here have any use for a fried Draytrek 2600VG router which was on Line Two? Many of my neighbours had their equipment fried, as well.

OK so I did the 'square one' thing, to start from a 'base case' as it were.

Original faceplate put back on
No power or line protection
Swapped router for a Netgear DG834v3

All set, line sync'd at 11776
Noise Margins initially 20 Up, 3 Down, Att. 38
(these now seem to have settled at 5-6 Up, 4-9 Down, Att. 38.)

Called the number and swore - same result, lost sync. Tried a couple of times, then went to change modem for a phone to see if there were any audible clues, swore again - I had forgotten to plug the fax back in.

So I left it with only the router plugged in, and after numerous calls established that I don't need to have anything else plugged in to kill the line. I even tried calling the number from a mobile in case it was one line interfering with the other - same result. Still not every time, line does not drop immediately, sometimes taking several rings.

Removed faceplate, added netgear-supplied filter to test socket, reconnected (DSL only, still no fax/phone), line OK, similar stats as above.

Same result. Call the number => line loses sync. Except at the moment (perhaps the rain is helping), when the Down NM has sunk to 0-1dB, occasionally showing a very large number which looks suspiciously like a mis-displayed negative value.

Unless I'm missing something really obvious, I think I've probably eliminated pretty much everything here as a possible cause. So, wiring somewhere?

Presumably the next step would be to contact BT and hope that it doesn't fail to fail on demand (as it is at the moment) if a demonstration is needed.