What is DLM and when does it happen?

[Saturday]

Note: the following applies to O2's LLU products only (Standard, Premium and Ultimate). O2 Access is a rebadged BT product and which utilises a different type of DLM which is permanently running and can not be turned off by O2.


What is DLM?

DLM stands for Dynamic Line Management.


What is it for?

When O2 first connect your line they turn a simple form of DLM on (more accurately this is more "line testing" than DLM - see note at end). The system then monitors your connection and if it finds your line is unstable, automatically adjusts it to an optimum level to improve stability.


How long does it last for?

Up to 10 days but will be shorter if no line problems are found. At the end of this period O2 will contact you by email to tell you your line has finished testing.


Does it then stay on?

No. Any changes made stay in place but DLM is turned off.


What's the downside?

Well that depends on your perspective. If DLM finds line problems then the net effect is likely to be that your router sync is lowered to give a higher noise margin and better stability. On all lines, with ADSL there is always a trade off between speed and stability. All other things being equal (e.g. line length), a poor quality line just can't hold the same speed as a good quality line.


What if I'm not happy with the changes DLM makes?

If you feel you'd rather have the maximum speed possible and can put up with disconnections from time to time OR if you are able to subsequently improve your line, then you can call O2 and ask for the changes to be reversed.


New info:

A couple of new bits of info about O2's DLM have cropped up: the first is that DLM appears to react instantly to any changes you make to your connection i.e. if you improve it then DLM sees and responds to it instantly; the other is that O2 say that DLM (if it detects a problem) will lower the sync/raise the noise margin significantly (usually in 3db increments I believe) and then gradually raise the sync/lower the noise margin over the coming days until the optimum level is determined.

If you have cause to call O2BB support, you'll probably find that the first level support are adamant that O2 do not use DLM on their LLU products. This is because they are trained in BT's IPstream product as well, which uses a fairly complex and permanently running DLM process which is very different from the DLM like line testing process used for LLU services.

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[Mike8]

Good information on DLM, never came across it before. I joined on Friday (O2 Home Premium), and initially stabilised out at 17675/att. 23dB/sn. 6.10dB and 1325/10/6.70 router to dslam connection. Did plenty of speed tests and average thruput was around 14500. This seemed stable to me. Got the 'email' last night from O2 and they have fixed me at 16376/23dB/6.8dB and 1325/10dB/7.4dB. Guess the DLM errs on the side of safety. I will leave it for now, but was a bit annoying. But I have taken off my NTE5 faceplate and gone straight into service connector. Any one know of the best ADSL Filter faceplate to buy. Im now using Belkin filters rather than the O2 ones.

Also as an interesting point to note, when I was on ADSL 8MB Max with freenetname I was getting downstream line atten 20dB and noise margin
12.8dB at 7616kbps. Should there be this disparity between ADSL and ADSL+.

[Saturday]

Number of points you raise there:

I don't think that's DLM that has dropped your sync slightly.

When O2 first connects you the connection is completely uncapped i.e. whatever package you're on, you'll sync at the maximum your line can support.

You're on Premium which is "up to" 16meg. Your line is capable of slightly more than that (17meg). All they've done is cap you down to your package at the end of the testing. Had your line synced at and been capable of holding more than 17meg they'd probably have contacted you and asked you if you wanted Ultimate. As it happens your max was only a touch over 16 so they didn't bother.

Opinion is divided on who makes the best filtered faceplates. ADSLnation are very popular but some say the extra gadgetry (esp the transistors) is detrimental. Other brands are Clarity and Solwise plus of course BT - though you can't buy the BT one.

ADSL2+ uses much higher frequencies than ADSL1/2. Attenuation is a measure for a specific frequency so it is normal for the attenuation to apparently change when on different ADSL modes. Also, different routers will report the attenuation differently as there is no standard measure that routers use.

Lastly, attenuation, sync and noise margin are linked. Attenuation is fixed (for a particular set of frequencies) so therefore the others change in relation to each other i.e. raise the noise margin and you lower the sync, raise the sync and you lower the noise margin.

[Mike8]

Thanks for the explanation, what a splendid chap you are

[brian69]

Before Dave from Run-it-Direct sees this, I'll point out that they supply the same filter faceplate as used by BT.

[Saturday]

New info added to the first post:

A couple of new bits of info about O2's DLM have cropped up: the first is that DLM appears to react instantly to any changes you make to your connection i.e. if you improve it then DLM sees and responds to it instantly; the other is that O2 say that DLM (if it detects a problem) will lower the sync/raise the noise margin significantly (usually in 3db increments I believe) and then gradually raise the sync/lower the noise margin over the coming days until the optimum level is determined.

[Saturday]

More new info added to the first post:

If you have cause to call O2BB support, you'll probably find that the first level support are adamant that O2 do not use DLM on their LLU products. This is because they are trained in BT's IPstream product as well, which uses a fairly complex and permanently running DLM process which is very different from the DLM like line testing process used for LLU services.

In other words, we call it DLM but O2 don't. They call it line testing.

[andy66]

i know you get notified by o2 when testing is finished,is there anyway to tell by looking at your stats??
over on skyuser i keep seeing posts where people say "i can see by your stats your going through dlm "

[Saturday]

O2's DLM is completely different to Sky's:

• Sky's kicks in immediately by setting a reduced sync speed (which raises the noise margin above the default 6.9db) then raises it steadily over a period of days until errors are seen then it locks to that sync/noise margin. DLM stays on permanently.
• O2's starts immediately but just watches the connection in background with the connection untouched. If problems are seen then the noise margin is raised by 3db and it's watched again. This continues until error levels are below their target (and I don't know what the target is). If no errors are found (or they're low) then nothing is done to the connection. DLM is turned off at completion of the exercise.

To answer your question, the only way to know whether DLM is active on your line with O2 (i.e. it has changed your noise margin) is to reboot the router and see what the noise margin is. If it is around 6db then DLM hasn't made any changes.

[andy66]

nice one saturdaygood bit of info there!!!