DLM / SN margin problem

I currently have a broadband issue, which I am trying to resolve with my ISP.

Would someone kindly tell me......

Is a down SN margin that drops below 6 (2.1 - 4.4) acceptable with DLM, or �Dynamic Line Management� (I don't have fibre)?

The SN margin was always above 10 before recent changeover to DLM.

ADSL (Sync) Summary
System uptime: 1020:43:02
Modem ADSL (Sync) uptime: 80:07:13
Interleaving: On
ADSL mode: ADSL2+
ADSL settings: PVC:0/38
ADSL line speed (kbps): up 1115 down 9213
Line attentuation (dB): up 23.8 down 43.0
SN margin (dB): up 6.2 down 2.1
Total errors seconds: 403

>>

"The technicians who have been looking at your broadband connection have advised that they could see that you had a stable connection for a number of days and have said that the profile is not causing any issues."

A home engineer visit has been offered, but I've already had a number of these in the past.

Edited by deleted (Thu 24-Nov-16 11:58:38)

If it is very close to 3dB immediately after a reconnection, then DLM has decided your line is very stable and has lowered the connection-time margin from 6dB to 3dB. In which case it is reasonable for it to drop as low as 2.1dB with no problem.

If it did make that change, you should have received a significant speed increase at the time. Possibly as much as 1,000kbps.

As Robertos suggests - no problems.

I had an ADSL2+ line with about 46dB of attenuation - so a little longer than yours. It was very stable with a 3dB SNR and would stay in sync for months without a problem and minimal errors. In that time the SNR would vary a little - up to 4dB and down to 0dB.

If the stats are correct with a 80 hour sync and just 403 error seconds - there is certainly nothing to worry about; it is clean and stable. The 9.2Mbps downstream is good too.

Thank you for your help RobertoS.

At times both up and down SN margins have been at 6.0.

I haven't seen any significant speed increases. Reconnections happen in the middle of the night.

What should I do?

Thanks for the information MHC, that makes me feel better.

Do many non BT customers have DLM? Do they have this system in other countries?

Edit: Sorry, to make clearer.....just realised you probably live outside the UK where I am.

I previously read that an SN margin below 6 is bad and can lead to stability problems / loss of broadband connection, that's why I was concerned.

I previously had a long term intermittent loss of broadband connection problem, I don't want that again......it was finally rectified after numerous engineer visits when BT changed the aluminium cable to copper.

Edited by deleted (Thu 24-Nov-16 13:03:23)

If DLM sees a super stable connection, it will eventually do it's final tweak and drop your line down to a 3db target SNR ..........

This will possibly give a little instability, in return for a little more (around 800Kbps) sync.

In my experience some routers don't do so well on this, especially Cisco routers.

Thank you for your help Zarjaz.

I see.

I want stability over speed. I asked for my profile to be put back to what it was pre DLM, was told it couldn't be done.

Is DLM available outside the UK?

My ISP has sent me their new 'Super Router' (not yet used).

Edited by deleted (Thu 24-Nov-16 13:46:27)

I want stability over speed. I asked for my profile to be put back to what it was pre DLM, was told it couldn't be done.

Usually the ISP's want an SFI visit from Openreach, the visiting engineer can then ring and have the target SNR 'hard pinned' to 6db ..... A waste of time and resources, but hey, that's the beaks.

Is DLM available outside the UK?

This is your second mention of this, are you NOT in the UK yourself then ?

If so, then the above is not so useful as the advice I gave is based on a BT Wholesale based ADSL service.

In reply to a post by EloiseJane:

Is DLM available outside the UK?

I'm pretty sure that BT's DLM is their own invention, and not available anywhere else, inside or outside the UK.

However, the principle of dynamically managing the line profile is well understood, with many different techniques built into the standards. The broadband forum has this document describing some of the individual techniques, while the UK's NICC has a couple of documents about their use in the UK: this (2010) and this (2015).

There is one company that implements their own system for dynamic line management is Assia, with their DSL Expresse system. That company was created by one of the original inventors of ADSL, and has been in patent fights with BT over the DLM process.

Vendors of DSL equipment usually have their own network management equipment. Nokia/Alcatel/Lucent appear to have their own dynamic line optimisation tools, for example.

Do you have any stability issues on your line? As Zarjaz says, it will only move to 3dB on a very stable line, if it detects instability over a certain limit it will then revert to 6dB anyway. As I commented before, your uptime is high and very few errors suggesting a stable line.

The best way to be sure is what I said .

Assuming you are in the UK then with most ISPs DLM has been present for many years. Sky and TalkTalk being the main exceptions, with the other one being if you were on a fixed downstream speed 512kbps or 1Mbps or 2Mbps circuit.

If you disconnect and reconnect - rebooting your modem or modem/router is simplest and safest, then read the stats immediately and see if the downstream SNR(M) is within one or two tenths of 3dB or 6dB, then we can we more sure of what is happening. Post the stats again if you do that .

In reply to a post by WWWombat:

I'm pretty sure that BT's DLM is their own invention, and not available anywhere else, inside or outside the UK.

I've seen it outside of the UK.

Telstra in Australia certainly do it but like TTB/Vodafone, allow you to turn it off.

Matt

Thank you very much to all of you, you've been a great help.

Covering several posts...

I am in the UK, but don't know where you are all based......I have a curious mind and wanted to know more about DLM, not heard of it until recently.

I am with TalkTalk. They've offered to send one of their technicians (non BT) to my home if I am still concerned.

As mentioned earlier there were long term stability issues on this line once, but not since BT changed the aluminium cable to copper, which was some time ago. Now when I look at my router summary screen and see that the Modem ADSL (Sync) uptime has renewed, I don't know if it's because the connection has dropped during the night or the profile has been changed.

"As Zarjaz says, it will only move to 3dB on a very stable line, if it detects instability over a certain limit it will then revert to 6dB anyway. As I commented before, your uptime is high and very few errors suggesting a stable line."

Seems I don't have anything to worry about, which is what I need to know.

Stats after rebooting, had to log in again so couldn't see them immediately I'm afraid (tried twice):


ADSL (Sync) Summary
System uptime: 0:01:56
Modem ADSL (Sync) uptime: 0:00:50
Interleaving: On
ADSL mode: ADSL2+
ADSL settings: PVC:0/38
ADSL line speed (kbps): up 1139 down 9214
Line attentuation (dB): up 23.8 down 43.0
SN margin (dB): up 6.2 down 3.7
Total errors seconds: 0

I am with TalkTalk. They've offered to send one of their technicians (non BT) to my home if I am still concerned.

One of their 'Bright Spark engineers' .... a chocolate tea pot springs to mind.

Interesting figures there, but what is noticeable is that it is nothing to do with overnight. There was a power down and up to the internal electronics (not a mains power off) 1 minute 56 seconds before you took the stats. That was the reboot which reset the uptime and of course the sync time. It took 1 minute 6 seconds from the reboot for it to sync with the exchange. (The second and third lines).

The margin at 3.7dB looks to be some strange setting - possibly 4dB at the moment it sync'ed. Your sync itself has not significantly changed.

You said at the start you had just been moved onto DLM (Dynamic Line Management). Did TalkTalk tell you that? If so, they have changed their system. They used not to have a DLM. Basically for that reason everything we have all said earlier about detail is to do with the BT Wholesale DLM - unless I'm mistaken none of us knew TT now have one. Sky probably still doesn't. In broader terms, you've been given some links to the general principles of DLM, which of course are still valid.

I don't think you have anything to worry about. If you see it below, say, 1dB, then it might get dodgy. It is supposed to hold sync to 0dB and some modems/routers will even hold to -1dB. I know that sounds impossible, but all a negative means is that the estimate your modem made at the time iy sync'ed was a bit out. Like opinion polls!

Edited by RobertoS (Thu 24-Nov-16 20:19:41)

unless I'm mistaken none of us knew TT now have one.

NAFAIK either.

There's a notice on the TT Community page that says they are moving over to DLM but it doesn't give a timescale.

The exact text is 'Did you know - We are moving to a new DLM system, this means that in the future OCE's will not be able to select individual profiles anymore'

I do wonder how they can manage up to 4million routers manually (number taken from current subscriber info, could be way out but still millions).

I believe the OP has a HG533 router as the stats are presented in exactly the same way as mine.

In reply to a post by RobertoS:

Sky probably still doesn't.

Sky have been using Assia's kit since 2008
http://www.ispreview.co.uk/news/EkkuEVAAyFuhUufScx.html

In reply to a post by EloiseJane:

I am in the UK, but don't know where you are all based......I have a curious mind and wanted to know more about DLM, not heard of it until recently.

Almost everyone here is in the UK too.

Really appreciate you looking at that RobertoS.

I rebooted through my computer Summary Screen without touching the router.....it said it would take around 2 minutes.

For a while both up and down SN margins were around 6 after the changeover to DLM.

I'll try a reboot again tomorrow.

Yes, TalkTalk did tell me that; I was offered a new package at a lower price. >>>

"I'm sorry we can't manually override DLM, the profile is set by an automated process, DLM monitors connection stability and errors and sets a line profile to give a fast speed with stable connection and low error count."

Yes, a Bright Spark engineer. Have to say though that TalkTalk have helped me a lot since I joined. I go straight to the top.

Oh ok that's good, almost everyone here in the UK.

In reply to a post by EloiseJane:

Really appreciate you looking at that RobertoS.

For a while both up and down SN margins were around 6 after the changeover to DLM.

That's DLM doing its job . It started with a fairly safe 6dB that is almost always high enough to stop frequent disconnections. After a while it decided your line was having low volumes of errors and decided it could lower it - giving you more speed. Absolutely nothing to worry about - that's what it's supposed to do.

If your line had been getting a very high number of errors it would have raised it to 9dB, slowing you down.

Have a read of this page. Now a bit out of date and only talks about BT Wholesale DLM, (that's what most people are on who aren't on Sky or TalkTalk), but the basic logic is the same.

I'll try a reboot again tomorrow.

I wouldn't bother. It looks to be fine. You were worried because of what you read about 6dB. It used to be the BT Wholesale default and they would only ever go higher. Now they allow 3dB.

Over the years I've seen lots of ill-informed posts saying your line had to stay at 6dB or else disaster. You have probably seen some of those.

Once you've read the linked page you'll see why that is rubbish. It's a margin! As long as the drop caused by random noise is less than the margin then there isn't a problem. You still have a margin. Mine used to drop by 3.5dB every night and recover to 6dB in the morning. So I needed 6dB setting otherwise I would have had an overnight reconnection at a lower speed. (DLMs don't normally re-sync once the margin has risen).

If you see the margin shooting up and down by several dB many times a day, than that suggests either a line problem or somebody with a very electromagnetically noisy piece of equipment close to your line somewhere between you and the exchange.

Edited by RobertoS (Fri 25-Nov-16 01:06:51)

RobertoS,

Thank you for all the useful information.

There's a problem my end with that link - 404 Not Found?

Will be back here soon, have a busy day tomorrow.

Yes, currently using a HG533 router 'vitrocmax'.

So glad I found this site.....

Oops! Sorry .

I always check links work, in the Preview screen, except - once in every 2,816 posts I forget. Then there's always something wrong with it. In this case the link double-pasted into the link format.

Fixed and tested now.

TalkTalk may not have a DLM like the dynamic BT Wholesale ones, but they do have different line profiles available but are usually manually controlled, i.e. no licence cost for DLM system

The seeking to a target noise margin is of course the usual xDSL behaviour.

I have a fixed 6dB S.N.R.M. it def. helps my >6km E.O.line to cope......just!!

With TalkTalk.

Agreed that's what they had Andrew, but this sounds like they are moving to a true dynamic one. That's what the OP appears to have been moved to, and see this key post. Unfortunately it doesn't have a link.

Edited by RobertoS (Fri 25-Nov-16 11:11:20)

In reply to a post by vitrocmax:

There's a notice on the TT Community page that says they are moving over to DLM but it doesn't give a timescale.

Any chance of a link please? I've been posting relying on this since you posted it, but being able to read the original is always a help .

Thanks for the new link RobertoS....

Will check that out this weekend.

Well I wanted sound independent advice and I certainly got that here.

Just received an email from the TalkTalk CEO handling my case.....

"Our technical department have looked at your connection again and have said they checked the line stats via their new Dynamic Line Management system and it has dropped the SNR margin down to 3dB as the line was deemed stable enough to handle it. The only recent drops that they can see are in the early hours of the morning and this is when the system has been amending the profile. It is showing stable any other time.

The technicians have said that if a line has an SNR margin of 3dB the system will set that level if the line can handle it which this one can and it is normal for it to fluctuate around 1dB either way across the day and that is why you would have seen it move from 2 and 3.9dB.

The reason it was previously set to 10dB was because our technicians could set it themselves but the new system constantly monitors the line and if it becomes unstable the SNR margin will automatically amend to a more appropriate level."

So seems DLM did make that final tweak 'Zarjaz' spoke of earlier.....

But why only on the down and not up, which is still at 6dB it seems?

I'm wondering.......is this TalkTalk's own DLM or BT's? I'll ask.

It is TalkTalk's own (or purchased from elsewhere as has been suggested). Almost certainly not a copy of BT Wholesale's.

The upstream I don't think I have ever seen changed. It basically isn't necessary to do so. It can be affected quite badly by a line fault, but then the downstream is shot to pieces anyway and the fault gets dealt with.

Edited by RobertoS (Fri 25-Nov-16 13:52:18)

I see.......thanks for the info.

I've yet to answer their email, will do so at the weekend and try to find out more.

Interesting, and after so many used to like the no DLM system seems it has maybe arrived at TalkTalk, the trick will be now seeing how it works in practice.

I've (and I suspect others) to be sceptical of descriptions as Chinese whispers play a big part, but the explanation you've had sounds perfectly reasonable.

Sorry for the delay, busy day today.

Here is the link to the TT Community - unlimited broadband (ADSL) page that details the text I copied to my original answer ref DLM;

and my question on there that I asked the community ref DLM is here

Thanks for those two links .

Yes MrSaffron, the explanation I've had does sound perfectly reasonable and is what I've been told here too, so I'm no longer concerned about the SN margin. Now I understand DLM and know that it will take care of any problems that may arise. I was pleased to hear that my line is good, after the problems I've had with it in the past.

RobertoS, very good article. I noticed the SN margin dropped at night and wondered why......you've explained. >>

"On virtually all lines the background noise rises as the sun falls and falls again as the sun rises. This mainly due to the effect the sun has on the radio-reflecting layer of the atmosphere, particularly medium-wave signals. So the noise margin (SNRM) will fall near dusk and not fully recover till well after dawn. Other causes are covered in Miscellaneous Nasties on the Troubleshooting pages"

I asked the TalkTalk CEO >>

Is this TalkTalk's own Dynamic Line Management system, BT Wholesale's or purchased from elsewhere? Answer today >>

"As far as I am aware it is TalkTalks own system and not BTs."

Need to understand total errors seconds. Would someone kindly tell me...

What is the threshold......what is an acceptable level and what is not?

Why are the total errors seconds almost double what they were when I first posted......do they normally fluctuate like this? >>

I copied this this morning:

ADSL (Sync) Summary
System uptime: 85:34:36
Modem ADSL (Sync) uptime: 85:32:51
Interleaving: On
ADSL mode: ADSL2+
ADSL settings: PVC:0/38
ADSL line speed (kbps): up 1136 down 9206
Line attentuation (dB): up 23.8 down 43.0
SN margin (dB): up 6.2 down 2.8
Total errors seconds: 793

Edited by deleted (Wed 30-Nov-16 14:32:05)

In reply to a post by EloiseJane:

I asked the TalkTalk CEO >>

Is this TalkTalk's own Dynamic Line Management system, BT Wholesale's or purchased from elsewhere? Answer today >>

"As far as I am aware it is TalkTalks own system and not BTs."

It could be that your exchange is not TalkTalk LLU'd and/or you are not on MPF, i.e. not fully unbundled.

The proper name is an "errored second". Each errored second is one second-long period where an error has been seen in the bitstream.

That means you can accumulate a maximum of 60 errored seconds per minute.

ES's only ever accumulate, though they'll start at zero after a power cycle.

In BT's DLM, they are the primary statistic that is used to determine, alongside the count of resyncs, whether your line is "low quality". BT allows for different thresholds for DLM intervention, but their least-stringent setting allows for an ES count of 2880 over 24 hours - though that is on FTTC rather than ADSL. ISPs can choose more severe thresholds are 1440 and (I guess) 720 for more stability.

On an old line, I'd get 600-800 per day. On the current line, with G.INP, I get zero.

No idea what TT will have for thresholds.

Honestly you are really really over analysing it.

DLM is there to run 24/7, it never stops running. If the line is unstable and the errors are too high believe me DLM will raise the noise margin automatically and/or increase interleaving as required. It is designed to run permanently and kick in and make adjustments as and when is necessary.

The best thing you can do is just relax and stop monitoring it.

DLM only applies 3db noise margin on very stable lines, hence your line is one of the better ones and you have nothing to worry about. There are plenty of people with 15db noise margins as their line is that bad! To get a 3db noise margin you must have sustained uptime without drop outs and a low error build up over a long period of time.

This is why when you posted here everyone was asking whether you actually had stability issues, as it's hard to believe a line would ever get to a 3db noise margin if it was unstable.

Overall stop worrying it's perfect and have a beer

Thank you very much to all of you for explaining and helping to put my mind at rest.

4M2 - I had been wondering if this was TalkTalk's own DLM or if it had been acquired from elsewhere. Was told yesterday it is their own.

WWWombat - Sorry should have made clearer. I run 2 separate 80 hour (approx) ADSL syncs, which resulted in 2 very different errored seconds rates (see my first post for original stats). I was wondering why the second one had almost doubled and if this normally fluctuated. Errored seconds have now gone back to original rate.

Not heard of G.INP, just been reading about it - you get zero errors with it - I want it, will it be coming to everyone?

ukhardy07 - When you have had the long term loss of broadband connection problem I have had with numerous BT and TalkTalk engineer visits to your home, new parts and equipment over several years and you're not in the know, believe me you are going to analyse a new system and be concerned with errors and when you see the SN margin you were previously told should be at least above 6dB drop below it.

If I don't ask I won't understand and know in future...

The down SN margin has been dropping as low as 0.7 in the last few days, but now I know that this shouldn't concern me with DLM.

As I mentioned earlier I have a curious mind and I need to make an informed decision before I answer this question I was asked by the TalkTalk CEO handling my case yesterday:

"Are you happy for me to close your complaint now with full and final resolution? (concerns are labelled complaints)

Edited by deleted (Thu 01-Dec-16 13:17:11)

I do not know if I agree with DLM, as if often masks a fault.

E.g. when a line drops continuously at 6db noise margin, DLM increases the noise margin to 9db and the speed drops, this consequently reduces the likelihood of the line dropping.

If it carries on dropping, we commonly see the noise margin go to 12db and then even 15db.

With a 15db noise margin you often lose a lot of Mbps, say 5Mbps speed on a short line!

Here, the underlying fault is masked and the user just has far slower broadband, but no engineer callout is required and the customer can use their broadband. Unfortunately customers often just switch ISP when they have a fault, and commonly the new ISP has stricter DLM so the fault appears to be "fixed" when they move, but their speeds are actually slower and the fault is masked. Due to consumers switching rather than reporting faults DLM has become almost necessary.

Also commonly people have very poor internal setups IE a 25m poundland extension cable, and here DLM has to intervene to keep the line stable, whereas really the consumer should be told this cable needs to be binned. The issue is if the consumer used this cable on BT, where DLM was used, and moves to a provider where DLM is not used (as talktalk used to be), they will blame the ISP not their wiring, so every provider ends up using DLM.

Edited by ukhardy07 (Thu 01-Dec-16 13:41:29)

In reply to a post by EloiseJane:

4M2 - I had been wondering if this was TalkTalk's own DLM or if it had been acquired from elsewhere. Was told yesterday it is their own.

Thank you for raising this issue on this forum - if TalkTalk are changing their kit at LLU'd exchanges then the very steady downstream 6dB SNR margin that I enjoy could be messed up. On one occasion after a resync during day light hours I did connect with a 3dB margin, for some unknown reason, and the sync speed increased but the line appeared to become unstable.

I don't have full LLU, only partial LLU on a TalKTalk enabled exchange and the issue that you have raised could be significant in terms of the line's stability.

UKhardy07 - I see, those are very good points you make......DLM is not without potential problems.

Yes, the internal set up needs to be looked at too.

One of the first things TalkTalk did when I kept losing my internet connection was test the line, send me a new ethernet cable plus several new routers - they were very good to me. They also arranged for changes to be made at the exchange. BT gave me a new master socket, removed the star wiring and extension I had (which I miss)......I had many BT engineer visits! It later transpired to be a fault with the outside cabling to the exchange, which should have been changed much earlier I was told.

4M2 - Sorry, I know nothing about LLU, need to look it up. Perhaps you should raise a concern directly with TalkTalk, get them to look at your line?

In reply to a post by EloiseJane:

Perhaps you should raise a concern directly with TalkTalk, get them to look at your line?

Fortunately that is not necessary since my broadband account is not with TalkTalk but with an ISP named xilo who provide a TalkTalk Business broadband only service (partial LLU - SMPF) using TalkTalk kit at the exchange

Anyway thanks for mentioning any potential issues, if there is a change to my router reported "Remote Vendor ID BDCM" (Broadcom) and as a result there are problems with stability I will contact xilo.

Edited by 4M2 (Thu 01-Dec-16 16:32:55)

Oh I see 4M2.

I hope you don't get any problems with your broadband connection.

In reply to a post by EloiseJane:

Oh I see 4M2.

I hope you don't get any problems with your broadband connection.

Well my preference would be for FTTP but until that happens (if ever) then I have to put my faith in old buried copper wires