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ADSL2+/21CN : Does the DLM always know best?

JayG
Aspiring Pro
Posts: 914
Thanks: 66
Fixes: 2
Registered: 30-10-2011

ADSL2+/21CN : Does the DLM always know best?

I've been on ADSL2+ for about 5 months now and the DLM clearly decided that a 9dB noise margin and interleaving "off" was the best configuration for my line, leaving me with a sync of around 9.5kbps, which was OK but a bit lower than Kitz's prediction for a good line with my attentuation.
The connection was very stable, although it occasionally dropped and re-synched itself, and I sometimes re-synched it myself via the router interface (DG834v4) to see if I could reach the "magic" 10Mpbs figure. What I noticed was that after resynching the noise margin always reported around 9.5, but this would gradually reduce over the next few days to around 7.8 (daytime) and a minimum of  5.5 during the evening peak.
My (complete) guess was that the output power was slowly being reduced to save on power costs and reduce crosstalk with other lines, but by doing so I was being potentially deprived of some speed, and also couldn't understand why in 5 months the DLM didn't appear to have ever tried a 6dB margin (supposedly the default target.)
10 days ago I forced the SNRM down to 6dB via Routerstats, and resynched at 10.4Mbps, which held with no more errors than before with an indicated margin of 6.5 day/4.8 evening.
Last night I powered off the router by mistake so lost the forced margin - on rebooting (9db margin again) and then forcing back down to 6dB this morning, I'm now synching at 10.8Mbps (throughput around 8.5.)  Cheesy
I was pretty wary about forcing a lower SNRM because in theory it could have caused so many errors that the throughput actually got worse, but it appears that with my connection the DLM was too conservative when left to its own devices.
12 REPLIES
jojopillo
Grafter
Posts: 9,786
Registered: 16-06-2010

Re: ADSL2+/21CN : Does the DLM always know best?

Hi JayG,
I'm not a fan of forcing SNR as I've seen it cause issues. Turn it off and let me set it to 6dB for you. Let me know when to go ahead.
Jojo Smiley
JayG
Aspiring Pro
Posts: 914
Thanks: 66
Fixes: 2
Registered: 30-10-2011

Re: ADSL2+/21CN : Does the DLM always know best?

OK Jojo ta (I guess that's a better way of going about it even though it doesn't appear to have caused problems.)
Will reboot modem now so that it defaults back to 9dB.
jojopillo
Grafter
Posts: 9,786
Registered: 16-06-2010

Re: ADSL2+/21CN : Does the DLM always know best?

Hi JayG,
I've requested that now, so we should see it shortly, but it can take up to 24 hours.
Jojo Smiley
JayG
Aspiring Pro
Posts: 914
Thanks: 66
Fixes: 2
Registered: 30-10-2011

Re: ADSL2+/21CN : Does the DLM always know best?

You must have the magic touch Jojo - it's reconnected already at 10846/1183 dn/up with a down margin of 6.3dB
Thanks a lot, have a good weekend.  Smiley
jojopillo
Grafter
Posts: 9,786
Registered: 16-06-2010

Re: ADSL2+/21CN : Does the DLM always know best?

You're welcome. I've just raised your profile to 9500 to allow for your new BT IP profile Smiley
JayG
Aspiring Pro
Posts: 914
Thanks: 66
Fixes: 2
Registered: 30-10-2011

Re: ADSL2+/21CN : Does the DLM always know best?

Very prudent, no doubt!  Roll eyes  Wink
Thanks again.
JayG
Aspiring Pro
Posts: 914
Thanks: 66
Fixes: 2
Registered: 30-10-2011

Re: ADSL2+/21CN : Does the DLM always know best?

After Jojo's intervention which resulted in the 10846kbps/6.3dB downstream sync/SNRM the margin started to drift slowly downwards, to the point where it was below 3dB during the evening peak, and the error rates (CRC/HEC/ES/SES) started to increase dramatically, although the connection did not drop.
Strangely enough that figure is exactly what I achieved with the previous 9dB SNRM set by the DLM (but forced down to 6dB via Routerstats) but doing that also produced a more stable range of SNRM figures, although of course I wouldn't disagree with Jojo or anyone else about the problems sometimes associated with margin forcing!
To see if I could raise the SNRM slightly I rebooted the modem/router on Sunday morning and the sync increased still further to 11357Kbps (Oops!  Roll eyes) with a margin of about 5.5. Needless to say the margin then dipped below 3dB towards evening and the errors started piling up again. The connection still held up though.
Yesterday evening I rebooted again to try to actually produce a lower sync speed and therefore higher range of SNRM’s day/evening, and finished up with what I'm at now which is 10099kbps/6.6dB with a much lower error rate.
I will probably have another go at some point to aim for a sync of around 10.5Mbps, which by my reckoning is around where the best compromise between speed/SNRM/error rates probably lies with this configuration, although how close I get will mainly be down to luck and timing.
On the basis of this little saga I have to suspect that the DLM on its own has not been making the best decisions as far as my connection is concerned.
It all adds to the respect I have for the Digital Care Team who by “remote control” have to routinely try to understand and make their best calls based on all the ADSL variables, many of which they have little or no direct control over.  Smiley
jojopillo
Grafter
Posts: 9,786
Registered: 16-06-2010

Re: ADSL2+/21CN : Does the DLM always know best?

Very interesting results there. Would you like me to try to stabilise it at 6dB and see how it goes?
JayG
Aspiring Pro
Posts: 914
Thanks: 66
Fixes: 2
Registered: 30-10-2011

Re: ADSL2+/21CN : Does the DLM always know best?

Happy to give it a go Jo, although I'm not sure what the difference is betwen trying to stabilise it, and the present situation whereby the DLM does not appear to feel the need to change it from 6dB?
jojopillo
Grafter
Posts: 9,786
Registered: 16-06-2010

Re: ADSL2+/21CN : Does the DLM always know best?

Sorry, I thought you said it was dipping?
JayG
Aspiring Pro
Posts: 914
Thanks: 66
Fixes: 2
Registered: 30-10-2011

Re: ADSL2+/21CN : Does the DLM always know best?

It does dip by up to 3dB between best and worst every day, but after every reboot it now always indicates around the 6dB mark - it's just that around 10,5Mb/6.6dB daytime is enough to see it through the evening peaks without problems, whereas around 11.3Mb/5.5db daytime takes it into escalating error territory during the evening (doesn't look like a big difference but my connection obviously doesn't see it that way!)
Thanks for your trouble Jojo - I'm sure I'll manage to get it to sync at around 10.5Mbps eventually with a robust SNRM and low error rates. At least it's continued to work, and I've learned quite a lot about the relationship between sync/SNRM and errors (at least, I think I have!)  Undecided
jojopillo
Grafter
Posts: 9,786
Registered: 16-06-2010

Re: ADSL2+/21CN : Does the DLM always know best?

OK, well you know where I am if you need me Smiley