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Author Topic: 3dB Target SNRm Live?  (Read 22226 times)

ejs

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Re: 3dB Target SNRm Live?
« Reply #180 on: March 20, 2017, 12:39:59 PM »

In the ITU G.993.2 document, that SNRGAP term only appears in the ATTNDR formula for use when the modem is in "loop diagnostic mode". So it's for when the modem is in a special diagnostic mode, not actually trained up. Normally the ATTNDR wouldn't need to be calculated that way.

With the basic ATTNDR method, the modem is free to choose the amount of FEC data, interleaving depth and other framing parameters for maximum bandwidth (within the maximum delay). Whereas for the actual line rate, it will have to set the amount of FEC data, interleaving depth and other framing parameters to achieve the required minimum INP value specified.

Last time this was discussed, I think I had difficultly explaining or convincing people about the "coding gain" aspect, that FEC+interleaving could be used to give a higher net data rate than without. I'll try explaining it again.

The modem connects at the best speed it can constrained by having to meet the error rate (and target SNRM, but SNRM is defined relative to the error rate).
So when you add error correction capabilities, that would lower the expected error rate, therefore allow a greater bandwidth to be achieved for the same error rate. That's the coding gain.
Of course the FEC data takes up some of the bandwidth, which has to be taken into account to give the net data rate. But it's possible to gain more than you lose.

It was easier to see on ADSL2, when switching on FEC+interleaving can give you a higher net data rate than without.
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kitz

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Re: 3dB Target SNRm Live?
« Reply #181 on: March 20, 2017, 02:42:35 PM »

I think we are singing from a similar hymn sheet.

ADSL has always used an 'SNRGAP' figure during the process of calculating bit allocation and ultimately the sync speed.  Its basically the difference between the SNR margin and the true SNR.   It its simplistic form and in early days of DSL it covered an allowance for BER of 107

A more recent definition of SNRGAP is
The signal-to-noise ratio (SNR) gap to capacity, or SNR gap
for  short,  is  a  function  of  the  coding  gain,  noise  margin  and
target average BER


I do get what you are saying about coding gain and am not disputing that,  it became apparent on some lines after G.INP.
Like you say it may also be apparent on ADSL2+ which was when INP was introduced.

-----------

What I was saying in my above post was... what if the reason why the ATTNDR calculation goes skewy on lines which have INP applied is because some modems are using the actual 'SNRGAP' figure calculated for bit allocation, rather than one with no coding gain and INP=0.

It would appear amendment 6 in 2015 to the ITU G.993.2 seeks to clarify that when calculating ATTNDR then you must use SNPGAP with BER 10-7, no coding gain & INP=0


If BCM was using the 'actual SNRGAP' figure for ATTNDR rather than the clarified baseline figure, then IMHO it is a highly possible reason why max attainable goes way out if the line has INP applied.
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ejs

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Re: 3dB Target SNRm Live?
« Reply #182 on: March 20, 2017, 03:28:37 PM »

The paragraphs and formula referring to SNRGAP are the same in the 2011 edition of G.993.2 as they are in the 2015 edition of G.993.2, which includes whatever was in amendment 6.

What was added in the 2015 edition is the whole section with the improved attainable net data rate method. The improved method includes more conditions, including having to use the actual minimum INP requirement. It does not necessarily require INP=0 to be used, although that is an configurable option.

what if the reason why the ATTNDR calculation goes skewy on lines which have INP applied is because some modems are using the actual 'SNRGAP' figure calculated for bit allocation, rather than one with no coding gain and INP=0.

I'm not sure what you're saying makes sense. If the modem were using the actual SNRGAP figure to calculate the ATTNDR, the same as what it used for the actual bit allocation and the ACTNDR, then why would the ATTNDR become much higher than the ACTNDR? Surely an ATTNDR figure calculated using the actual parameters used on the line would give a value similar to the ACTNDR.

That bit about the SNRGAP=9.75 for the usual BER, no coding gain and INP=0 applies when the ATTNDR has to be calculated while in loop diagnostic mode. I think it's saying that's a less accurate method it has to use when in loop diagnostic mode, because the bits and gains table has not been calculated when entering loop diagnostic mode.

Edit: Now that I've managed to download Amd 6 to G.993.2 2011, it turns out that it has very little to do with ATTNDR at all (only inserting SNRM_MODE=5 in existing lists of SNRM_MODE numbers, one of which happens to be in the ATTNDR section). The improved ATTNDR calculation was in the 2011 edition of G.993.2, but support for it is optional.
« Last Edit: March 20, 2017, 03:56:47 PM by ejs »
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vic0239

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Re: 3dB Target SNRm Live?
« Reply #183 on: March 25, 2017, 07:56:13 AM »

Just had a re-sync on one of my lines (vic0239-2), SNRM now 4.3 ~4Mbps increase. And another as I type?
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displaced

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Re: 3dB Target SNRm Live?
« Reply #184 on: March 25, 2017, 09:23:46 AM »

My line's stubbornly holding at 6dB +/- 0.3 dB.  Booo! :)
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Balb0wa

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Re: 3dB Target SNRm Live?
« Reply #185 on: March 25, 2017, 11:28:14 AM »

My lines still on 9db!!! think its stuck
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michty_me

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Re: 3dB Target SNRm Live?
« Reply #186 on: March 25, 2017, 04:03:11 PM »

I was just about to create a ne wthread as my line has been doing some funny things lately but it could be this.

Firstly, G.INP was removed from my line for approx 1 week. I think errors were roughly 12/hour on an interleave level of 1/1.

A week later, It was re-applied then a few days later I had a DLM remote indicator 1 where my new sync was 2.4dB down and 6.4db up. I rebooted the router a few days later where it reverted to 6.3db down and 6.4db up. Just a few days ago, It had another DLM indicator 1 and it is now sitting at 4.3db down and 5.9db up. Gained a good chunk of speed and it seems happy with 0.06/6.5 ES/hour.
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vic0239

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Re: 3dB Target SNRm Live?
« Reply #187 on: March 26, 2017, 09:28:41 AM »

Just had a re-sync on one of my lines (vic0239-2), SNRM now 4.3 ~4Mbps increase. And another as I type?
Same again this morning with the other line.  Errored seconds don't look too bad, so maybe DLM will be kind and offer me a little more.  :)
« Last Edit: March 26, 2017, 03:02:16 PM by vic0239 »
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michty_me

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Re: 3dB Target SNRm Live?
« Reply #188 on: March 26, 2017, 02:46:51 PM »

Same here. Now sitting at 3.4db and a down sync rate of 73.6mbps
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Xup

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Re: 3dB Target SNRm Live?
« Reply #189 on: March 26, 2017, 11:01:10 PM »

Re-sync today at 2 PM have dropped from 6db to 5db(been 6db for years), let see how it goes and if it drops to 4db in the coming weeks.
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NewtronStar

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Re: 3dB Target SNRm Live?
« Reply #190 on: March 27, 2017, 01:45:52 AM »

Have to say I'm apprehensive if this 6-3dB target goes live on my circuit for the extra wee bit of sync as it's not that important what is important is line stability and kind of know what my line is capable of and 6dB looks the best for me.

Have notice others saying it won't be active on lines who have stability issues and wondering what these are IE errored seconds or SNRM changes or retrains or all three
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vic0239

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Re: 3dB Target SNRm Live?
« Reply #191 on: March 27, 2017, 10:01:24 AM »

Further drop this morning:

Reason: 1 Remote Defect Indicator/DLM

Current Downstream Sync Rate is now 72806kbps @ 3.4db SNRM
Current Upstream Sync Rate is now 10491kbps @ 6.3db SNRM
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burakkucat

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Re: 3dB Target SNRm Live?
« Reply #192 on: March 27, 2017, 05:48:38 PM »

Have to say I'm apprehensive if this 6-3dB target goes live on my circuit for the extra wee bit of sync as it's not that important what is important is line stability and kind of know what my line is capable of and 6dB looks the best for me.

In your case you know the typical, average, DS synchronisation speed that is obtained with a target 6 dB SNRM.

If it appears that the DLM process is experimenting on your circuit by dropping the target SNRM, then you could counteract that change by manually capping the DS synchronisation speed to what is currently obtained with the 6 dB target.
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NewtronStar

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Re: 3dB Target SNRm Live?
« Reply #193 on: March 27, 2017, 08:28:38 PM »

That is a good idea and for some reason forgot about manually capping the modem back to 6dB unsure what the DLM would make of it, MR N*STAR we the DSLAM sent you a target margin of 5 and then 4dB but for some reason your circuit is ignoring are commands  :D
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displaced

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Re: 3dB Target SNRm Live?
« Reply #194 on: March 27, 2017, 08:46:22 PM »

Looking at Vic's stats with my untrained eye, it seems reasonably similar to mine - slightly higher attenuation and roughly the same SNRM levels and hlog graphs. (Although your snrm's much smoother than mine - stats quantisation by the Zyxel modem perhaps?)

So I'm still holding out hope that I get an snrm drop sooner or later!
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