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Author Topic: SNR reduction, giving DLM a real chance...  (Read 9561 times)

sevenlayermuddle

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SNR reduction, giving DLM a real chance...
« on: August 05, 2009, 09:35:33 AM »

As is well documented for anybody with time to read my posts, I've now been waiting nearly a year for DLM to reduce my target margin from 15dB.

We had a power cut earlier in the week.  When the power was restored I got a low sync as I always do following a power cut, presumably because everything electrical in my own and neighbours houses is all springing to life at exactly the same time.  And also, of course, because my router loses it's SNR tweak.  I know, installing DGTeam f/w would help, but for various reasons I've not done so.

So, I'm currently synced at 1920 with a 18dB margin, vs my normal >4000 @9dB.  In just over 48 hours (175,000 AS), I've had a total of 57 ES, 74 HEC and 39 CRCs.

I'm not planning any activity that needs high speed access, so I'm going to leave it as it is for a few weeks, and will report back whether DLM then reduces my margin.  I'm predicting that it won't, but will be happy to eat my words if it does. :-\
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roseway

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Re: SNR reduction, giving DLM a real chance...
« Reply #1 on: August 05, 2009, 09:48:53 AM »

There does seem to be a lottery aspect to this process. As I recorded several months ago, my target noise margin did come down from 15 dB to 6 dB in three fortnightly steps, but it didn't last long. Leaving it untweaked as you're doing is obviously giving DLM the best opportunity to work, and it will be interesting to see how it goes.
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kitz

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Re: SNR reduction, giving DLM a real chance...
« Reply #2 on: August 06, 2009, 08:23:03 AM »

>> with a 18dB margin

 :no:


>> at 1920 with a 18dB margin, vs my normal >4000 @9dB

fits very nicely that on average each 3dB is costs approx 800kbps of speed.
- not good for you though :(

>> but will be happy to eat my words if it does.

Be nice if it does.. but its never seem to have shifted before for you.   
Would be good if the power cut actually kick started the DLM to work how the theory says.
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sevenlayermuddle

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Re: SNR reduction, giving DLM a real chance...
« Reply #3 on: August 06, 2009, 10:14:42 AM »

My theory is that in return for a low sync speed I will get very low error rates, and that's what may convince DLM my line's not so bad.

But wouldn't it be nice if BTw would tell us more about how DLM actually works?
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HPsauce

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Re: SNR reduction, giving DLM a real chance...
« Reply #4 on: August 06, 2009, 11:48:55 AM »

18dB is one hell of a margin!

I had a "strange occurence" on my line yesterday (ADSL2+ with Be) resulting in no or minimal throughput despite an apparently normal (around 20mbps) sync. After a lengthy call to support, who could see lots of errors, my target was raised to 9dB and interleaving re-enabled (I'd been on 3dB Fastpath for over a year with no problems) so I'm having to "live with" only 16mbps  :angel:.

The idea is to run for 24 hours at 9dB then revert to standard if the error rates are OK. After 15 hours of that I have 0 (zero) absolutely NO ERRORS of any description whatsoever using a DG834GT with DGTeam firmware!
I can only assume there was some temporary interference that has now gone, but if it re-appears I'll know what to do, and that it can be done quickly!

The contrast with the BT system is enlightening.
Firstly, the willingness & ability of Be support to check, investigate and adjust in real time - they rang me back on my mobile to facilitate line tests. Really helpful.
Secondly, the flexibility of NOT being a slave to BT's DLM systems.  :P

My Netgear will usually hang on reliably down to about 1dB margin before giving up.
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sevenlayermuddle

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Re: SNR reduction, giving DLM a real chance...
« Reply #5 on: August 06, 2009, 12:08:13 PM »


The contrast with the BT system is enlightening.
Firstly, the willingness & ability of Be support to check, investigate and adjust in real time - they rang me back on my mobile to facilitate line tests. Really helpful.
Secondly, the flexibility of NOT being a slave to BT's DLM systems.  :P

Sounds good to me, I think I'm envious.

Actually, I'd like to think that BTw might come under political pressure to overhaul the DLM algorithms in pursuit of the goal of getting everybody in the country up to a sensible minimum speed.

Of course, there are cirumstances in which DLM will want to adjust the target margin to stabilise a line, but the existing DLM does seem a little inclined towards a sledgehammer approach of 'raise it forever, at the slightest excuse'.
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orainsear

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Re: SNR reduction, giving DLM a real chance...
« Reply #6 on: August 06, 2009, 12:20:05 PM »

Actually, I'd like to think that BTw might come under political pressure to overhaul the DLM algorithms in pursuit of the goal of getting everybody in the country up to a sensible minimum speed.

Perhaps some sort of more advanced learn/adapt fuzzy logic systems or neural network, but anything would be a truly enormous task.
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toulouse

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Re: SNR reduction, giving DLM a real chance...
« Reply #7 on: August 06, 2009, 12:30:54 PM »

from orainsear -> 'but anything would be a truly enormous task'

But SOMETHING needs to be done about the way it works at the moment. The phrase 'using a sledgehammer to crack a nut' sum it up nicely I think.

TTFN


toulouse
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sevenlayermuddle

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Re: SNR reduction, giving DLM a real chance...
« Reply #8 on: August 06, 2009, 12:43:44 PM »

From what I understand of it, one weakness is that I think it uses the FTR as one parameter in the decision to raise the target.  Basically, in my understanding, it won't raise the target if doing so would breach FTR - and that should operate as a safety valve to prevent a long line that's capable of, say, a ssteady 2mbps at a 6dB marging being reduced tenfold to 0.2mbps with a 15dB margin (assuming 600kbps for each 3dB).

However,  FTR isn't ever re-calculated after the initial training period, despite the fact that the initial FTR becomes innapropriate after improvements, such as filterred sockets, better routers, etc.   I do strongly suspect that's part of my own specific problem, since my typical speeds have improved at least three-fold since my iniital training, thanks mainly to chopping off the bell wire and getting a better router.

As always, I have to stress that's only my understanding, based on what I've dug up form my own research.  I could be wrong.

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jeffbb

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Re: SNR reduction, giving DLM a real chance...
« Reply #9 on: August 06, 2009, 03:40:38 PM »

Hi
Quote My theory is that in return for a low sync speed I will get very low error rates, and that's what may convince DLM my line's not so bad.

I wouldn't hold my breath.I had some problems 3 o4 months ago .Some contractor part chopped through the undergroung cable . I ended up with 15db target SNR . There were a few hundred errors during that particular incident.Since then My errors have been  very low .

Total Up Times (From SF counts):
 WAN:   10 days, 20:41:17
 LAN:   11 days, 03:53:08
 
CRC:  12
LOS:  0
LOF:  0
ES :  12

Noise Margin:     11.9   dB
Connection Rate:  6592  Kbps
Line Attenuation: 31.0  dB
Power:            19.8  dBm
Max Rate:         7392  Kbps



  The above are for the last 11 days . I rebooted then when my snr margin was at its highest to get slightly better synch speed.
These figures are typical for the last 3 to 4 months .At the moment I am tweaked to 12db.
Similar to Roseway ,before the last problem my Target had come down in stages to 9db from 15. in about 6 weeks.Now no sign of movement.
Regards Jeff

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sevenlayermuddle

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Re: SNR reduction, giving DLM a real chance...
« Reply #10 on: August 06, 2009, 03:58:39 PM »

 :'( :'(
Thanks for that Jeff, in view of these comments I'm thinking I'm wasting my time. 

But I'll give it a few weeks anyway, or at least until the next time I need to download something big, like a DVD .iso image.  For the things that I do, that's the only time dismal speed really hurts much.

The next thing I'm tempted to try is to re-instate my bell-wire, and reconnect my crappy old router, which I suspect would take my resultant speed down close to, or maybe below, my FTR.   But (a) I don't know if I'm willing to make that sacrifice, and (b) it would be hard to conclude anything from it unless I give low error-rates a chance first.  There's also a (c) that if I do go below FTR, I might upset DLM even more. 
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toulouse

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Re: SNR reduction, giving DLM a real chance...
« Reply #11 on: August 06, 2009, 04:23:06 PM »

...I might upset DLM even more


Well, we wouldn't want that to happen. She (DLM) is a bit a bit of temperamental bitch isn't she ?
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People tell me that I ought to get out more. But in the words of the great Homer J Simpson, "Yeah, but what ya gonna do ?"

jeffbb

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Re: SNR reduction, giving DLM a real chance...
« Reply #12 on: August 06, 2009, 08:13:56 PM »

quote  She

you're living dangerously  :lol:

Regards Jeff
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toulouse

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Re: SNR reduction, giving DLM a real chance...
« Reply #13 on: August 06, 2009, 08:24:44 PM »

The (only) reason I used SHE, is because there is a lady in my life who, very much like the DLM system is virtually totally unpredictable, I just NEVER know what she's gonna come out with next


Apologies to any of the nice ladies on here who may become offended by such remarks.


TTFN


toulouse
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People tell me that I ought to get out more. But in the words of the great Homer J Simpson, "Yeah, but what ya gonna do ?"

kitz

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Re: SNR reduction, giving DLM a real chance...
« Reply #14 on: August 07, 2009, 01:47:45 AM »

>> Basically, in my understanding, it won't raise the target if doing so would breach FTR - and that should operate as a safety valve to prevent a long line that's capable of,

From the document you found a while back.. that was my understanding too.
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