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Author Topic: 3dB downstream target SNRM  (Read 8385 times)

Weaver

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3dB downstream target SNRM
« on: January 05, 2016, 08:11:35 PM »

I've been set to a 3dB downstream target SNRM for a week or so. The results have been really great, not too unstable, that I've noticed, and a downstream sync rate of 2700 - 2900 kbps at 65 dB d/s attn. on each of my three lines now that line #a.3 has been set to 3 dB too. That line was ill, but may now have been mended.

Various posters rightly warned me about possible serious instability at 3 dB, but it seems I'm ok thus far. I read that Andrews and Arnold had a trial group of TalkTalk LLU users who were doing this, I didn't realise to begin with that I would be able to choose 3dB when on BTW 21CN, or ADSL2. (Not sure which is the controlling factor?)


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burakkucat

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Re: 3dB downstream target SNRM
« Reply #1 on: January 05, 2016, 09:18:51 PM »

Just to clarify the situation (for anyone speed reading this thread), you are configuring the SNRM target for each of the three circuits via the controls provided by your ISP/CP, A&A, and not by tweaking the configuration via each modem.

{Edited to replace the word post with thread.]
« Last Edit: January 05, 2016, 10:31:14 PM by burakkucat »
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Weaver

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Re: 3dB downstream target SNRM
« Reply #2 on: January 05, 2016, 09:48:30 PM »

Oh sorry. I used the controls on A & As website clueless.aa.net.uk I think there was a screenshot showing the control panel UI in an earlier thread somewhere?

I don't know if there is an SNRM-tweaking facility in this modem, the DLink DSL-320B-Z1.
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Weaver

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Re: 3dB downstream target SNRM
« Reply #3 on: January 06, 2016, 12:43:52 AM »

Another Andrews and Arnold user who has an attn of a mere 49 dB iirc said that he could only tolerate 3 dB d/s target SNRM during the summer, and had to revert to a full 6 dB during the winter months. I'm not amazed at this, but I wonder exactly why?

It's a good job then that I started this experiment in the teeth of winter weather. The weather was worse before and during Xmas with high winds for days. It may be though that with the benefit of multiple lines I will get away with 3 dB even during really bad weather periods. We'll have to see.

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loonylion

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Re: 3dB downstream target SNRM
« Reply #4 on: January 06, 2016, 01:04:57 AM »

I used to get away with 1dB, it depends entirely on the line.
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Weaver

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Re: 3dB downstream target SNRM
« Reply #5 on: January 06, 2016, 01:22:40 AM »

@loonylion - that was what you set? Not what it drooped to? Wow that's good going. What speed did you get and what environment did you have?
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Weaver

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Re: 3dB downstream target SNRM
« Reply #6 on: January 06, 2016, 01:27:14 AM »

I'm not sure what to look out for in assessing reliability. I don't have any direct stats as many of  you do, just the modem and line status reports from A & A's CQM (http://aa.net.uk/kb-broadband-cqm.html) and clueless.aa.net monitoring and control server. It sometimes reports a possible resync explicitly. But I'm not sure I am seeing all resyncs on the CQM graphs though, if there are any that is, because if they are short duration I don't know that they would be picked up.
« Last Edit: January 06, 2016, 01:30:17 AM by Weaver »
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c6em

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Re: 3dB downstream target SNRM
« Reply #7 on: January 06, 2016, 09:07:48 AM »

The summer/winter aspect is due to the solar wind from the sun and how it impinges on the earth in different seasons - and in particular the longer nights of winter. The solar wind also causes the day/night variation.

This 'wind' on the side of the earth facing the sun causes the "Heviside Layer" in the upper atmosphere to rise up.  This is a radio-reflective layer and results in most notably radio stations from further away being receivable during the night time.  It also means a LOT more general noise in the atmosphere on the night time side during the hours of darkness and this effect is multiplied during the winter periods.
Your old AM band radio amply demonstrates this as during the night other stations appear from other continents while the general hiss on the radio on all wavelengths gets much worse.

All this noise is picked up by phone lines which act as gigantic aerials and results in the SNRmargin reducing at night and becoming more variable and jagged.  Interestingly the SNRmargin (with me anyway) usually starts to rise again in the early morning before it has got light in the winter months.
For those lines which are on the margins as to whether they can work on a 3 or 6 dB target margin the extra interference during the winter periods might be enough to tip them over into requiring a 6dB margin.
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sevenlayermuddle

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Re: 3dB downstream target SNRM
« Reply #8 on: January 06, 2016, 09:34:31 AM »

I'd predict that the degree of instability would vary, depending on the services you use.

As the margin reduces, the error rate will increase.   So far as I recall, 0dB margin is simply the margin at which the BER exceeds a standardised threshold, 1 in 10^7 I think(?).  As the BER increases, some non-recoverable errors will take place, leading to data retransmission.

For many services, these retransmissions go unnoticed.   For time-critical stuff like video streaming, it might lead to freezes.   For true IPTV, which does not benefit from TCP retransmissions at all, the lost data would simply lead to picture corruption.

Above is written on the assumption that TCP handles the re-tx.   If using Physical retx, things are not as bad.
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Weaver

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Re: 3dB downstream target SNRM
« Reply #9 on: January 06, 2016, 10:27:07 AM »

Guys is there physical Retx in ADSL2 itself?
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Weaver

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Re: 3dB downstream target SNRM
« Reply #10 on: January 06, 2016, 10:29:03 AM »

I didn't know that 0 dB was some arbitrary level, I never really thought about it to tell the truth, never knew what the definition was in this context.
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Weaver

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Re: 3dB downstream target SNRM
« Reply #11 on: January 06, 2016, 10:35:47 AM »

@c6em thanks for making me aware, I should have thought about that. I have a (bad) degree in Theoretical Physics, so a <doh />  :-[ there  ;D
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Weaver

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Re: 3dB downstream target SNRM
« Reply #12 on: January 06, 2016, 10:38:26 AM »

So 7lm, negative SNRM is entirely possible? I seem to remember some old Netgear modem-routers showing huge positive numbers because of bugs in mishandling of negative numbers
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sevenlayermuddle

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Re: 3dB downstream target SNRM
« Reply #13 on: January 06, 2016, 10:49:00 AM »

I believe it is perfectly possible to maintain a connection with a BER that is worse than the 0dB BER .  But I suspect the modem might be supposed to retrain at 0dB, on the assumption that such a connection was undesirable.

Please note careful, politician-style, choice of wording in above.. 'I suspect', 'I believe', etc.  Deduce that I'm not 100% sure of my facts... and not volunteering to download ITU specs and spend the rest of the week researching to confirm.    ;)

Re PhyR, I thought it was optional though probably rare, in ADSL2.   Somebody might know better. :-\
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Weaver

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Re: 3dB downstream target SNRM
« Reply #14 on: January 06, 2016, 10:54:36 AM »

> volunteering to download ITU specs and spend the rest of the week researching to confirm.

Well that goes for me too. I'm just lazy and hoping the more knowledgeable in our community had gone there before me.
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