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[GigabitEthernet]

Hello everyone,

As many of you are aware, I am on BT 20CN with BT's horrible DLM system! Now, here's what happened and why I am frustrated: I connected yesterday (03/06/12) at a sync speed of 6784Kbps at an SNRM margin of about 6.5dB (I connected at about 8PM). The connection stayed up all night dropping to a minimum SNRM of about 4dB. Great I thought! Well no, because at about 6:35AM, the connection dropped and synced back at an SNRM margin of 10dB due to DLM increasing the target SNRM to 9dB.

What I would like to know is, why did DLM increase the target SNRM? The connection didn't drop once during the night and it was up for about 9 hours, the error seconds count was 50 when it disconnected, hardly any. The CRC error count was low as well (I don't have an exact figure but it was pretty close to the error second count). There was one severed error second but I am pretty confident it occured because the connection dropped. So, why did DLM increase the target SNRM?

Thank you.

[roseway]

There's no publicly available description of how DLM works, although various early BT documents and patent applications can be found. So we have to guess, but I think the most likely explanation is that an increase in the target noise margin is the result of a combination of things over a period of time. So although your connection was well behaved last night, DLM would remember things which had happened somewhat earlier, and the eventual response occurred when it did.

I totally agree that it's frustrating - you feel that you're being punished for something you haven't done. The only comfort I can offer is that DLM on 21CN-WBC (when you get it) is somewhat more benign to the user, although you have less control over its functioning.

[GigabitEthernet]

Thank you Roseway, for offering a good explanation. I believed that on 21CN DLM, there was much more cusomisability available?

I do have some good news, I asked Zen (my ISP) on Friday about permanently setting a target SNRM. They said they couldn't do it but they did say they could change the stability profile on my line. So on Thursday the line is being changed to "super stable". Do you (Roseway) or anybody else know exactly what the stability profiles actually do?

Thank you again, Roseway.

[Bald_Eagle1]

I do have some good news, I asked Zen (my ISP) on Friday about permanently setting a target SNRM. They said they couldn't do it but they did say they could change the stability profile on my line. So on Thursday the line is being changed to "super stable". Do you (Roseway) or anybody else know exactly what the stability profiles actually do?

One way to introduce "super stability" is to cap sync speeds at a really low level.
This would effectively increase SNRM to really high levels so that even really high noise interference levels would not cause a disconnection.

When DLM has occasionally capped my FTTC service at 15Mb (& lower), my connection was very stable, but obviously noticeably very slow in use.

[roseway]

I think the above is correct. "Super stable" makes DLM more likely to increase the target noise margin to improve stability, which is probably the opposite of what you want.

[burakkucat]

No comfort to those who have to suffer Beatie's 20CN technology but this is an example of where a LLU'd service has a benefit. I and other TalkTalk service users have had the DLM disabled on our lines by just making the request.

[GunJack]

No comfort to those who have to suffer Beatie's 20CN technology but this is an example of where a LLU'd service has a benefit. I and other TalkTalk service users have had the DLM disabled on our lines by just making the request.

Absoluteley !!!  No DLM plus tweakable router = happy user

[Blackeagle]

I do have some good news, I asked Zen (my ISP) on Friday about permanently setting a target SNRM. They said they couldn't do it but they did say they could change the stability profile on my line. So on Thursday the line is being changed to "super stable". Do you (Roseway) or anybody else know exactly what the stability profiles actually do?

One way to introduce "super stability" is to cap sync speeds at a really low level.
This would effectively increase SNRM to really high levels so that even really high noise interference levels would not cause a disconnection.

When DLM has occasionally capped my FTTC service at 15Mb (& lower), my connection was very stable, but obviously noticeably very slow in use.

Can't comment on Zen's profiles over BT, but certainly on Talktalk's LLU, the most stable profile would have a target downstream SNRM of 15dB, with interleaving on.  Their profiles then step down in 3dB steps to 6dB and each interleaved profile has a corresponding fastpath version.  TT have some experimental profiles I believe, that set target SNRM at 3dB but AFAIK they are not available to customers.  I might also add that TT's DLM works in a different way to BT's and that there has been some discussion in the past regarding cutomers being able to change their own profiles via 'my account'.  Whether this will actually come to fruition though is anybodys guess 

[GunJack]

But then it's ALWAYS going to be a trade-off between snrm and sync speed, with "super-stable" being high snrm/low sync...UNLESS they can put you on a fixed speed, but IIRC most ISPs can only go .5, 1 and 2 meg fixed...

Many factors involved in that equation, limited by what's possible with your particular ISP and package, and whether the override is BT if on an IPStream product....

[snadge]

just too add, this "SUPER STABLE" setup may also incorperate aggressive Interleaving and Inpulse Noise Protection aswell as raised TSNRM

sky's implementation of DLM seems to be able to use 'fractions' of INP values, or at least DMT Tool reports them as so, I have read that some Dslams (such as Broadcom) allow changing of the amount of DMT symbols per RS FEC codeword, this allows the higher INP settings to be used without sacrificing the sync rate - I was wondering if this is why DMT reports my INP as 1.21 or 1.08 etc..??