BT DLM nightmare

After over 6 days online with no disconnections i reboot the router and BTs DLM decides its going to raise my snr from 9 to 12 for no apparent reason, I have been told by my ISP i now have to wait 72 hours for it to be reset
This is a joke

Edited by deleted (Tue 18-Jan-11 10:58:01)

Single resync should not do that, possibly you were not seeing lots of other resyncs, or the number of errors that the error correction was masking was very high.

In reply to a post by MrSaffron:

or the number of errors that the error correction was masking was very high.

That's one of my beefs with the DLM- if an "error" is correctable and corrected, it's not really an error is it?

Certainly hard errors should be taken into account in determining the target SNRM, but I don't see why corrected ones should.

ISp said my line had been up for a good 6 and a half days before my resync
10697 crc in that time

In reply to a post by iwish:

10697 crc in that time

I can't remember what my CRC error rate was when I was on ADSL, but that sounds rather high... though I'm quite prepared to be contradicted!

The masking can often mean, not a corrected packet, but a retransmit of a data cell, thus reducing your actual bandwidth.

The ISP has it wrong. If you'd suffered an IP profile change then that might get corrected in 3 days but a change in noise margin will take at least 14 (unless things have altered from when I was on ADSL).

High error counts and noise margins are more often problems with the internal wiring than the line itself. Have you read any of the guides on ring wire removal/isolation?

Unless they've requested a reset, and are saying three days to allow system to kick in.

Anyway IP Profile is upto five days

In reply to a post by MrSaffron:

The masking can often mean, not a corrected packet, but a retransmit of a data cell, thus reducing your actual bandwidth.

I'd count that as a "hard" error... perhaps I should have qualified what I said with "assuming interleave is on" so that there's a better chance that an error can be corrected locally and not require a re-transmit.

In reply to a post by iwish:

10697 crc in that time

I can't remember what my CRC error rate was when I was on ADSL, but that sounds rather high... though I'm quite prepared to be contradicted!

It's possible to get rather higher and still have a usable link.

I have a long line - attenuation approx 62dB but relatively stable noise levels so I force the connect SNRM down from its 9dB default to around 3dB (probably drops to 2dB or just under at its worst point in the day). However that does produce some frightening figures.

Current uptime just over a month (2835457 secs) ES: 55687, SES: 1888; HEC errors: 151046

Generally 3-5% of packets need error correction, about 0.01%-0.05% are uncorrectable but TBH the wider Internet probably looses more than this.

I'm afraid my philosophy is rather that if you're getting few errors then FEC is a waste of bandwidth and I don't really have it to waste (sync ~3600Kbps even @ 3dB SNRM). I do realise that I' failrly lucky in having a line with a stable noise profile so that playing this sort of game is feasible.

In reply to a post by mr_bean:

It's possible to get rather higher and still have a usable link.

It's not me you have to convince, it's BT's DLM

if you're getting few errors then FEC is a waste of bandwidth

Of course it is, you don't need error correction if you're not getting many errors. But if you are, it's a distinct advantage.

It's not me you have to convince, it's BT's DLM

In my case it's C&W LLU (Demon).

Interestingly my default SNRM has drifted down from 15dB after a particularly bad spell (result of a dodgy modem lead) to 9dB so C&W must apply some sort of monitoring - possibly it looks a stable length of sync (as I said I've currently been synced for 33 days without a break) more than error rate.

However if the OP can get hold of and use a Netgear DG834v4 then the initial SNRM can be tweaked to whatever you want so it's relatively easy to ignore BT's DLM.

Of course it is, you don't need error correction if you're not getting many errors. But if you are, it's a distinct advantage

Very true - I'd probably have to run somewhere around 1-1.5Mbps otherwise.

In reply to a post by mr_bean:

so it's relatively easy to ignore BT's DLM.

That's unlikely to be true.

I didn't think many ISPs got that done. Pretty pointless anyway if the underlying problem isn't fixed because the DLM will just slowly push it back up.

Tweakable routers allow you to override the margin set by the DSLAM so in effect you can ignore BT's settings by manually overriding them.

In reply to a post by kwikbreaks:

Tweakable routers allow you to override the margin set by the DSLAM so in effect you can ignore BT's settings by manually overriding them.

BatBoy being BatBoy I think you should read a little more into what he said .

It is easy to override the DLM in the short-term. But if the line is unstable in the first place, which this one may still be, then the result could be disastrous. If the DLM's hiked settings do not stabilise the line, due to user tweaking that it will not take into account, then it will choose even more restrictive settings.

Hence in this case it is not easy to ignore the DLM.

Its my extension that has become damaged
So have got myself 2 wireless network cards and will stick the router at the adsl nation faceplate
Then once the errors have shown to stop being as bad, will tweak the snr until bt DLM kicks in to lower it or [censored] off to LLU

defeats the purpose of interleaving

Hence in this case it is not easy to ignore the DLM.

I'm with BatBoy on this - the DLM, or at least the initial SNRM can be ignored with the right router - even if it winds up at 15dB.

You are correct, of course, in that if the DLM is pushing the target margin for a reason then that underlying reason may well bite you anyway - just because one can ignore the DLM in this regard doesn't always make it wise to do so.

However it sounds like iwish has identified the root cause of his problem so a bit of judicious SNRM tweaking will get him back to sensible figures/sync speeds in minutes rather than the weeks which it will take DLM to go from 12dB to 6dB

In reply to a post by mr_bean:

a bit of judicious SNRM tweaking

The operative word there is "judicious"... if the DLM becomes sufficiently irritated with a non-decreasing error rate it's quite capable of putting you on a banded profile, and there's no tweak that will get around that.

I know, I've been there

it's quite capable of putting you on a banded profile

Ah, didn't know that - tread carefully indeed

EDIT: Having said that how much is the BT DLM affected by errors - as opposed to drops & re-syncs?

I know it's C&W but my target SNRM has dropped despite relatively high error rates (see previous post) but sync itself is rock solid (perhaps it's keeping me from dropping back the final 3dB).

I don't recall the details from when Demon used the BT ADSL offering but I have always needed to tweak the SNRM to get a decent speed and IIRC the target SNRM was stable (but can't recall whether it was 6dB or 9dB, the latter I think).

Like iwish mine shot up due to a dodgy cable (in my case the modem lead), so it was to get the margin back to give me a decent speed rather that waiting for the DLM to do it (it took months in my case).

Edited by mr_bean (Tue 18-Jan-11 20:35:28)

Hard to tell.

The BT Wholesale DLM applies to all ISPs providing their services. But LLU DLMs are entirely down to the individual supplier's views on how they want it to operate.

Actually that isn't quite true. It also depends on the programmers' ability to implement those requirements.

In reply to a post by RobertoS:

It also depends on the programmers' ability to implement those requirements.

It's not so bad now (the one used on FTTC actually seems quite reasonable if you accept the necessity for the damn thing in the first place), but at one time I'd have said that it depended on the ability of the latest work experience kiddie to write programs

the latest work experience kiddie to write programs

I have found Indians with such things in their CV with a bit of Googling

In reply to a post by yarwell:

Indians with such things in their CV

Just after buffalo hunting and paleface scalping?

Actually, if you'd said "cowboys" I'd have understood it.

Edited by billford (Tue 18-Jan-11 23:19:20)

well my line has reduced the crc rate over the past 24 hours to less than half of what i would get
SNR is still at 12 but bt being the idiots they are have stuck me on a 1mb profile now
My isp have no idea why bt are playing up with my line, and say my line is looking good and clear
I have now lost all hope with bt and the almighty DLM

Its my extension that has become damaged

so there was an actual problem at the root of all this ?

we pulled the extension out and found no damage
But we still put the router into the master socket just incase
line stats from master face plate
Total Uptimes (From SF counts):
WAN: 1 days, 01:16:54
LAN: 1 days, 01:16:54


CRC: 313
LOS: 0
LOF: 0
ES : 163

could just be how the cat 5 cable was run but don't see how
my last stats from said cable was about 185 crc errors a day if you work it out over that 6 day period

Edited by deleted (Thu 20-Jan-11 12:47:18)