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Standard User meditator
(fountain of knowledge) Sun 23-Dec-12 12:11:20
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Setting a high SNR with the Billion 7800


[link to this post]
 
I've been using a Billion 7800 in a G.dmt setup for some six months now - and generally speaking I've been quite pleased with the 7800's performance. It's one of the few around where you can set up the line SNR from the user interface. That's essential for me, as I'm on a long line and need to have the downstream SNR running at between 12dB and 15dB in order to keep the line maintained steady.

Setting the SNR is done via an extension to the router's address in the Web address field, which then takes you to a sidelined part of the 7800 user interface where all you need do is enter a decimal value that corresponds to a required SNR value, eg. entering 50 is reckoned to get you a target line SNR of 3dB, ie. 3dB below BT's default SNR. Various websites, and including within one or two forums on this one, have published a table of these values for entering.

However, none of the tables indicate the values you need when you want to go positive in SNR. Instead, the tables are designed primarily for those users wanting to increase their downstream sync by lowering their SNR. Despite this, some months ago I was passed some values by one or two helpful souls for increasing the SNR. These didn't seem to work properly but after many attempts I managed to obtain the sort of SNR I needed pretty much by trial and error.

Since then my line's SNR has dropped a small amount and occasionally now I'm losing sync, particularly in streaming applications. So I've tried bumping up the SNR by another 3dB (to around 15dB total). The point is that the enterable values for increasing the SNR that I was given don't actually fit with the results. I've just made around a dozen attempts to home in on 15dB but none of the values I tried - which included 150, 200, 250 - would seem to get near 15dB. I got 18dB. I got even 21dB. And at the other end of the scale I got 6dB and 3dB, all with sensible corresponding sync speeds. But I just couldn't go from 12dB to 15dB. I'd have thought that entering the value 150 would have done the trick but it certainly didn't.

For the positive SNR values above 6dB there seemed to be no consistency. For example, I'd have thought that entering 100 would have produced a 0dB change, 150 a 3dB positive change, 200 a 6dB positive change, and 250 a 9dB positive change, but this was not what I found. Instead, the results swung from one extreme to the other, and seemed almost arbitrary. One would think that the target SNR values here would be relative to 6dB but since that's not how it worked out, I thought that perhaps instead the target value would be with respect to the last SNR value. But no, that didn't fit either.

So, has anyone managed to fathom a set of values that actually work for going positive in SNR, rather than negative? And if so, are the values always with respect to 6dB or instead with respect to the last SNR line value achieved?

As matters stand at present I'm running at an SNR of 15.7dB, giving me around 4.6M bps sync, but I only got that after a dozen attempts with all sorts of values, as the recommended ones just didn't seem to work properly. I'll now leave it at that, but if I ever need to tweak the SNR again it'd be nice to know which values will actually render the required SNRs.

Edited by meditator (Sun 23-Dec-12 12:13:28)

Standard User RobertoS
(sensei) Sun 23-Dec-12 12:44:30
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Re: Setting a high SNR with the Billion 7800


[re: meditator] [link to this post]
 
Decibels are measured using a logarithmic scale. Which is why you are getting nowhere.

It is sheer coincidence that the "50" setting, which is in fact 50%, that you cite gives (about) 3dB result, as it would intuitively give if the scale were linear. 3dB happens to be the figure that actually double or halves the effect. I've not explained that very well, but a couple of examples should do it.

Starting again at the 6dB, a setting of 200% will give about 9dB result - that being double 6dB. Conversely if your line has been set to 9dB by BT's DLM then the 50% setting would reduce it to 6dB, not 4.5dB. If the DLM has set it to 12dB, then 50% will reduce it to 9dB. 25% would reduce it to 6dB from 12dB.

300% from 6dB will give 12dB.

Have a browse around a couple of these google results.

My broadband basic info/help site - www.robertos.me.uk | Domains,website and mail hosting - Tsohost.
Connection - Plusnet Extra Fibre (FTTC). Sync ~ 54.0/14.9Mbps @ 600m. - BQM

"Where talent is a dwarf, self-esteem is a giant." - Jean-Antoine Petit-Senn.
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Allergy information: This post was manufactured in an environment where nuts are present. It may include traces of understatement, litotes and humour.
Standard User RobertoS
(sensei) Sun 23-Dec-12 12:48:04
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Re: Setting a high SNR with the Billion 7800


[re: meditator] [link to this post]
 
In reply to a post by meditator:
So, has anyone managed to fathom a set of values that actually work for going positive in SNR, rather than negative? And if so, are the values always with respect to 6dB or instead with respect to the last SNR line value achieved?
Rather than edit my previous post, as this is a separate issue, in the case of using BT Wholesale circuits the percentage is always in relation to whatever the DLM has set for your line. The DLM tries to impose that, and your percentage setting then overrides it.

My broadband basic info/help site - www.robertos.me.uk | Domains,website and mail hosting - Tsohost.
Connection - Plusnet Extra Fibre (FTTC). Sync ~ 54.0/14.9Mbps @ 600m. - BQM

"Where talent is a dwarf, self-esteem is a giant." - Jean-Antoine Petit-Senn.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Allergy information: This post was manufactured in an environment where nuts are present. It may include traces of understatement, litotes and humour.


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Standard User baby_frogmella
(experienced) Sun 23-Dec-12 12:50:17
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Re: Setting a high SNR with the Billion 7800


[re: meditator] [link to this post]
 
I'm no longer using a 7800N (using a Draytek 120/Cisco EA4500 combo instead) but if you've got the time & patience read this.

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Standard User RobertoS
(sensei) Sun 23-Dec-12 14:14:18
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Re: Setting a high SNR with the Billion 7800


[re: baby_frogmella] [link to this post]
 
Except it doesn't say the values are based on the DLM/DSLAM setting.

My broadband basic info/help site - www.robertos.me.uk | Domains,website and mail hosting - Tsohost.
Connection - Plusnet Extra Fibre (FTTC). Sync ~ 54.0/14.9Mbps @ 600m. - BQM

"Where talent is a dwarf, self-esteem is a giant." - Jean-Antoine Petit-Senn.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Allergy information: This post was manufactured in an environment where nuts are present. It may include traces of understatement, litotes and humour.
Standard User b4dger
(knowledge is power) Sun 23-Dec-12 16:28:33
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Re: Setting a high SNR with the Billion 7800


[re: meditator] [link to this post]
 
FWIW 4.6meg sync with a 15db TSNRM doesn't sound like a "long line" to me! tongue

Standard User XRaySpeX
(eat-sleep-adslguide) Sun 23-Dec-12 19:57:09
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Re: Setting a high SNR with the Billion 7800


[re: meditator] [link to this post]
 
Those numbers you enter the interface are percentages but the SNRs/NMs are measured in deciBels on a logarithmic (base 10) scale. E.g. +3 dB = +0.3 Bels = log (2) Bels = log (200%) Bels. Is that clear?

So enter the percentage as 10^((desired NM -6)/10) *100.

To have a 12 dB NM when the DLM has set a Target NM of 6 dB enter:
10^0.6 * 100 = 400%

To have a 15 dB NM when the DLM has set a Target NM of 6 dB enter:
10^0.9 * 100 = 795%

1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 19 Meg WBC
Standard User meditator
(fountain of knowledge) Mon 24-Dec-12 12:53:49
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Re: Setting a high SNR with the Billion 7800


[re: XRaySpeX] [link to this post]
 
Roberto, XRay, et al,

Yes, as a retired professional engineer I was well aware that the values you enter into the configuration box in the 7800's interface are decimal values representing percentages but that the SNR is measured in dBs, which is a logarithmic scale.

Roberto, I seem to recall that +6dB is a doubling of LEVEL but +3dB is a doubling of POWER. So, the values you need to enter might depend on whether the router works with voltage levels or instead with power levels. If those quoted figures on that australian forum are to be believed - and I reckon that those are the same figures that I was given originally - then we are talking about levels here, and so an entered decimal value of 200 (percent), let's say, should have increased the SNR by 6dB from its default value. As far as I'm aware, the default value on BT lines is usually 6dB. Similarly, 100 would be 0dB change, and 250 (two-and-a-half times) would produce an increase of 9dB from the default SNR. Be it actually right or wrong, that was my understanding of the mechanism (taken from others), when the other day I tried to bump up my SNR by 3dB.

My starting point was in fact just over 12dB. But I didn't get 15dB when I entered the value 150. Instead, I got over 18dB (which just doesn't seem to make any sense at all). At that stage I thought that maybe you have to always return the situation to the line having an SNR of 6dB and perhaps then and only then put in the requisite value. But doing that didn't work out either. In the end, I just kept using values in the range 50 to 200 until I finally achieved 15dB or thereabouts. It took me about a dozen frustrating attempts before I happened to land on 15dB.

If the decimal figures represent 'percentage changes of the dB values', then the figures I used should have been correct. But it's obvious that they're weren't. Those australian figures surely can't be correct? Or at the very least they must be couched in a misleading way. There seems to be some confusion over 'percentage of what?' (if you see what I mean).

Incidentally, the 7800 always has a default value of its own sitting in the SNR configuration box in its Web interface, that being 50 (even after using the facility).

Assuming that this mechanism in the 7800 does work properly when aiming in the positive SNR direction, what also needs clarifying is whether the figures used are always with respect to 6dB SNR or instead can work with any existing SNR. For example, in XRay's sample numbers it's been assumed that you subtract 6dB in the 'raised-to-the-power'. But what if your starting point is 12dB SNR? Using XRay's formula, if I'm starting from 12dB and I'm wanting 15dB, the decimal value you seem to need to enter is 200, ie. 10 to the power of 0.3. Is this correct, XRay?
Standard User RobertoS
(sensei) Mon 24-Dec-12 13:05:16
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Re: Setting a high SNR with the Billion 7800


[re: meditator] [link to this post]
 
In reply to a post by meditator:
My starting point was in fact just over 12dB. But I didn't get 15dB when I entered the value 150. Instead, I got over 18dB (which just doesn't seem to make any sense at all).
The point is whether the 12dB was the noise margin at that point in time, or the noise margin being set by the DLM at sync time. The DLM sync time value is what the percentage is applied to, not the running value.

Furthermore, given a lot of bit-swapping and possible frequency "write-offs" by the modem, results can change with a modem reboot, due to those being brought back into play.
At that stage I thought that maybe you have to always return the situation to the line having an SNR of 6dB and perhaps then and only then put in the requisite value. But doing that didn't work out either. In the end, I just kept using values in the range 50 to 200 until I finally achieved 15dB or thereabouts. It took me about a dozen frustrating attempts before I happened to land on 15dB.
There's a high chance that in that process you caused the DLM to think the line was unstable, and bump its sync time value up by 3 or 6dB anyway. That could explain your getting inconsistent results.

My broadband basic info/help site - www.robertos.me.uk | Domains,website and mail hosting - Tsohost.
Connection - Plusnet Extra Fibre (FTTC). Sync ~ 54.0/14.9Mbps @ 600m. - BQM

"Where talent is a dwarf, self-esteem is a giant." - Jean-Antoine Petit-Senn.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Allergy information: This post was manufactured in an environment where nuts are present. It may include traces of understatement, litotes and humour.
Standard User meditator
(fountain of knowledge) Mon 24-Dec-12 13:25:34
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Re: Setting a high SNR with the Billion 7800


[re: RobertoS] [link to this post]
 
Before you endeavour to change the SNR, how can you determine what the DLM's SNR value is?

From what you say, it would seem impossible for the user to set the line's SNR.

Back in the days when I used the graphical utility DTM, I never had this kind of problem, so my view is that what the DLM does has a minimal effect.
Standard User systemx
(experienced) Mon 24-Dec-12 13:28:48
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Re: Setting a high SNR with the Billion 7800


[re: meditator] [link to this post]
 
I am sure this has been mentioned before, and there was probably a good reason for your actions, but why not just reset the Billion back to its factory settings, set it up again using the inbuilt wizard and see what happens.

Most people want to adjust the margin because they are unhappy and feel they can get more speed from their connection but you appear to be happy with the speed you have so why not let the line sort itself out.
Standard User RobertoS
(sensei) Mon 24-Dec-12 14:25:30
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Re: Setting a high SNR with the Billion 7800


[re: meditator] [link to this post]
 
In reply to a post by meditator:
Before you endeavour to change the SNR, how can you determine what the DLM's SNR value is?
Some routers report the stats at sync time in their log. if they don't, you just read the stats ASAP after connecting. IT will normally be within 0.2, and as the values are DLM-set in 3dB steps, (on BT Wholesale connections), it's easy.
Back in the days when I used the graphical utility DTM, I never had this kind of problem, so my view is that what the DLM does has a minimal effect.
I think you mean DMT. That works exactly as I describe. The vertical slider is a percentage setter, based on what is detected at sync time. But only works in 10% steps, which isn't ideal.

My broadband basic info/help site - www.robertos.me.uk | Domains,website and mail hosting - Tsohost.
Connection - Plusnet Extra Fibre (FTTC). Sync ~ 54.0/14.9Mbps @ 600m. - BQM

"Where talent is a dwarf, self-esteem is a giant." - Jean-Antoine Petit-Senn.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Allergy information: This post was manufactured in an environment where nuts are present. It may include traces of understatement, litotes and humour.
Standard User meditator
(fountain of knowledge) Mon 24-Dec-12 15:55:14
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Re: Setting a high SNR with the Billion 7800


[re: RobertoS] [link to this post]
 
Yes, I meant DMT. Finger trouble.

What do you mean by "at sync time"?

I'm not guessing what my starting SNR is (when I'm moving from 12dB to 15dB). I'm reading the 12dB figure from the router's interface. Thus, if the router's doing its job, it should be showing - at the point just prior to my inputting a value - the line's true SNR.
Standard User RobertoS
(sensei) Mon 24-Dec-12 16:38:57
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Re: Setting a high SNR with the Billion 7800


[re: meditator] [link to this post]
 
In reply to a post by meditator:
What do you mean by "at sync time"?
When your router synchronises with the DSLAM. In ither words, connection time.
I'm not guessing what my starting SNR is (when I'm moving from 12dB to 15dB). I'm reading the 12dB figure from the router's interface. Thus, if the router's doing its job, it should be showing - at the point just prior to my inputting a value - the line's true SNR.
Yes it is showing the SNRM, (calling it SNR, which many routers do, is simply incorrect), which varies continuously as you yourself have described. That figure is not what the percentage is applied to. You asked about that in the opening post and it has been answered more than once.

To repeat, the value at the time you apply the percentage is not what the percentage is applied to, nor is it necessarily 6dB that is the starting point. The starting point is whatever the noise margin is set to for your line in the BT Wholesale BRAS system. The values that sets are 3dB, 6dB, 9dB, 12dB and 15dB. On the first ever connection of the 10-day training period it will be 6dB. From then on it can be changed. Down to 3dB on very stable lines, up on less stable ones. These adjustments can go on for ever - it is an ongoing process, but is not the same as the minute by minute, hour by hour variation that also goes on for ever.

http://www.robertos.me.uk/html/noise_margin-snr-snrm...

My broadband basic info/help site - www.robertos.me.uk | Domains,website and mail hosting - Tsohost.
Connection - Plusnet Extra Fibre (FTTC). Sync ~ 54.0/14.9Mbps @ 600m. - BQM

"Where talent is a dwarf, self-esteem is a giant." - Jean-Antoine Petit-Senn.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Allergy information: This post was manufactured in an environment where nuts are present. It may include traces of understatement, litotes and humour.
Standard User XRaySpeX
(eat-sleep-adslguide) Mon 24-Dec-12 16:43:47
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Re: Setting a high SNR with the Billion 7800


[re: meditator] [link to this post]
 
In reply to a post by meditator:
I'm not guessing what my starting SNR is (when I'm moving from 12dB to 15dB). I'm reading the 12dB figure from the router's interface.
Yes, you are guessing! You are taking the current SNR and trying to calculate what is needed to change from it.

As Roberto keeps telling you your starting SNR should be the Target set by DLM at sync time which you can only see by reading the stats immediately after reconnection.

1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 19 Meg WBC
Standard User meditator
(fountain of knowledge) Mon 24-Dec-12 17:24:38
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Re: Setting a high SNR with the Billion 7800


[re: XRaySpeX] [link to this post]
 
Sorry if I'm missing a trick here. Let me see now. You both seem to be saying that "sync time" is the occasion when the router is first powered on and achieves initial sync with the DSLAM. Sorry for seeming to be so thick, but have I got that right? Well, this isn't that time, this is at some time later. In fact, many months later.

Normally that initial SNR is 6dB, which my line can't handle and which simply leads to lots of re-syncs over many days. In fact, way way back (this is going back some years now, when different routers were in use) I used to get so fed up with waiting for the whole thing to stabilise out (it in fact never ever reached the likes of 12dB or 15dB by itself) that I reached for the use of DMT. In other words, I never bothered to wait ever again and just simply 'forced' the line to run at somewhere between 12dB and 15dB.

These days I don't use DMT. Instead, I use the SNR-changing facility in the 7800's Web interface to achieve the SNR I want. It's just that the recommended values for use in that interface don't seem to produce the required SNRs, at least not when setting the SNR above 6dB. I've even tried 100, which is the value that's supposed to return the SNR to 6dB, but without success.

Are you saying that the only way I can get this right (in future) is to power off the router, then power on again and achieve sync and check to see if the initial SNR is 6dB, and then enter the required value? My understanding was that the 7800 doesn't lose the SNR if it's turned off. I normally leave the router permanently powered on.

Given what you've stated here and at your website, Roberto, I guess that the figure of 15dB that I've been aiming for is an 'instantaneous line signal-to-noise ratio'. In other words, it's a snapshot value after the line's been running for some time.

Edited by meditator (Mon 24-Dec-12 17:36:18)

Standard User XRaySpeX
(eat-sleep-adslguide) Mon 24-Dec-12 17:59:50
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Re: Setting a high SNR with the Billion 7800


[re: meditator] [link to this post]
 
Spare us the ancient history!

Did you notice the prefix "re" in my "reconnection"?

Every time the router syncs (or re-syncs if you like) with exchange the DLM sets the Target NM; that's the one you need to base your calculations on.

You usually don't need to reboot the router to force a re-sync; you can often do a Disconnect/Connect in the GUI. But in your case it would be best to reboot so as to get rid of any tweaking of the NM you've done and start afresh.

1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 19 Meg WBC
Standard User RobertoS
(sensei) Mon 24-Dec-12 20:58:59
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Re: Setting a high SNR with the Billion 7800


[re: meditator] [link to this post]
 
I've even tried 100, which is the value that's supposed to return the SNR to 6dB
So what does that give you? That is what the DLM is setting, and is your base figure for your logarithmic calculation.

The base figure can be any one of the values I listed in my previous post. 6dB is the figure the first time it is ever connected, as I said. If the DLM doesn't like the way the line behaves on that setting, it alters it. Every time you connect after that it uses the new setting. Not 6dB. Until it decides it is still misbehaving, or has restarted so doing. It then alters it again.

Connection, reconnection, sync are the same thing for this purpose. For most purposes.
Are you saying that the only way I can get this right (in future) is to power off the router, then power on again and achieve sync and check to see if the initial SNR is 6dB, and then enter the required value?
No, we are not. We are saying see what the value is. If it is 9dB then that is already double 6dB. If you want then to try 12dB you would use 200, to double the 9dB to 12dB.
My understanding was that the 7800 doesn't lose the SNR if it's turned off.
Your understanding is wrong. The "target" noise margin, to be applied at the time of connection, is held by the DSLAM and its associated databases. The actual noise margin showing in the router GUI immediately before switch-off is not remembered anywhere.

In the example I just gave of the base figure, the setting held in the databases is 9dB.
Given what you've stated here and at your website, Roberto, I guess that the figure of 15dB that I've been aiming for is an 'instantaneous line signal-to-noise ratio'. In other words, it's a snapshot value after the line's been running for some time.
I'm not at all sure what you mean there. The figure you see in your GUI is always a snapshot. There are programs that graph it for many routers - RouterStats and RouterStats Lite are excellent - I prefer the Lite version if it.

It is impossible for you to keep your noise margin at a fixed level, though it can remain steady for reasonable periods. You can only reset it by re-sync'ing. Re-sync'ing frequently can upset the DLM, as I suggested in an earlier post might have happened because of what you were doing.

I'm beginning to think you are focussing on the noise margin as a solution to whatever problem your line has. I feel you would do better to start a thread in General Broadband Chatter explaining the problem and asking for help to find and cure it. But to do that you would probably be best to stop tweaking and present things as they are.

My broadband basic info/help site - www.robertos.me.uk | Domains,website and mail hosting - Tsohost.
Connection - Plusnet Extra Fibre (FTTC). Sync ~ 54.0/14.9Mbps @ 600m. - BQM

"Where talent is a dwarf, self-esteem is a giant." - Jean-Antoine Petit-Senn.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Allergy information: This post was manufactured in an environment where nuts are present. It may include traces of understatement, litotes and humour.
Standard User XRaySpeX
(eat-sleep-adslguide) Mon 24-Dec-12 22:57:18
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Re: Setting a high SNR with the Billion 7800


[re: RobertoS] [link to this post]
 
In reply to a post by RobertoS:
My understanding was that the 7800 doesn't lose the SNR if it's turned off.
Your understanding is wrong. The "target" noise margin, to be applied at the time of connection, is held by the DSLAM and its associated databases. The actual noise margin showing in the router GUI immediately before switch-off is not remembered anywhere.
He can only mean by this that the router remembers the tweak level. Dunno this router but with my experience of others, they remember the tweak thro' a re-sync, but not thro' a reboot.

1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 19 Meg WBC
Standard User RobertoS
(sensei) Mon 24-Dec-12 23:27:26
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Re: Setting a high SNR with the Billion 7800


[re: XRaySpeX] [link to this post]
 
Some firmwares can remember, through a reboot, the percentage tweak to apply to the DSLAM figure they receive at connection time. The DGTeam mods to Netgear firmwares being a well-know case in point. Not the specific noise margin.

My broadband basic info/help site - www.robertos.me.uk | Domains,website and mail hosting - Tsohost.
Connection - Plusnet Extra Fibre (FTTC). Sync ~ 54.0/14.9Mbps @ 600m. - BQM

"Where talent is a dwarf, self-esteem is a giant." - Jean-Antoine Petit-Senn.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Allergy information: This post was manufactured in an environment where nuts are present. It may include traces of understatement, litotes and humour.
Standard User XRaySpeX
(eat-sleep-adslguide) Mon 24-Dec-12 23:39:14
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Re: Setting a high SNR with the Billion 7800


[re: RobertoS] [link to this post]
 
Then that's what OP must be referring to in that quote, not the SNRM.

1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 19 Meg WBC
Standard User XRaySpeX
(eat-sleep-adslguide) Tue 25-Dec-12 01:16:33
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Re: Setting a high SNR with the Billion 7800


[re: meditator] [link to this post]
 
In reply to a post by meditator:
I seem to recall that +6dB is a doubling of LEVEL but +3dB is a doubling of POWER. So, the values you need to enter might depend on whether the router works with voltage levels or instead with power levels.
Yes, there are 2 ways of measuring the ratio of the actual physical quantity before taking logs; the power ratio (double = 3dB) or the amplitude ratio (double = 6dB). I was assuming the former. It may be that we should consider the latter, in which case a square root should be applied to my formulae, thus:

So enter the percentage as SQRT[10^((desired NM -6)/10)] *100.

To have a 12 dB NM when the DLM has set a Target NM of 6 dB enter:
SQRT[10^0.6 ]* 100 = 200%

To have a 15 dB NM when the DLM has set a Target NM of 6 dB enter:
SQRT[10^0.9] * 100 = 282%

In either case, that Australian forum & the usual suspect: http://www.kitz.co.uk/routers/dg834GT_targetsnr.htm do not agree with either and do not involve logs at all, which is strange considering that the dB is a logarithmic unit. They exhibit a purely linear relation:
Percentage = 16.7 * (desired NM -6) + 100
But what if your starting point is 12dB SNR? Using XRay's formula, if I'm starting from 12dB and I'm wanting 15dB, the decimal value you seem to need to enter is 200, ie. 10 to the power of 0.3. Is this correct, XRay?
Yes, I was using the normal Target NM of 6dB in my examples, believing that, as you were tweaking NMs, you would know this fact grin. I did say " when the DLM has set a Target NM of 6 dB".

However you can't start on this exercise until you have ascertained the Target NM awarded you immediately after the the last re-sync, as we have both suggested.

1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 19 Meg WBC
Standard User meditator
(fountain of knowledge) Wed 26-Dec-12 16:09:42
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Re: Setting a high SNR with the Billion 7800


[re: XRaySpeX] [link to this post]
 
Hmm, between you, you can't seem to agree. This has left me rather confused.

I came across a printout I'd made about a year ago of the kitz table when I was sorting some papers last night, and kitz's figures are the same as the ones quoted on that australian forum, and the same as I used.

I think you must be right, Roberto, because the resultant dB figures I was getting agreed neither with the starting point of 6dB assumed in the kitz table nor with the starting point being my then snapshot SNR.

Given that, despite all the trouble, I've actually got the SNR I was aiming for, namely 15dB, I think for the present I'll just leave things as they are. The DSLAM seems to be happy with that. But if, over some time, things slip back I'll refresh my memory as to what to do (the Roberto approach) by coming back to this thread.
Standard User XRaySpeX
(eat-sleep-adslguide) Wed 26-Dec-12 22:07:19
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Re: Setting a high SNR with the Billion 7800


[re: meditator] [link to this post]
 
In reply to a post by meditator:
Hmm, between you, you can't seem to agree.
As it happens, Roberto's figures mainly agree with my 1st formula given here: http://forums.thinkbroadband.com/dslrouter/t/4192227... , but I'm beginning to have doubts about that. Neither of us agree with the Kitz linear table.
In reply to a post by meditator:
I think you must be right, Roberto, because the resultant dB figures I was getting agreed neither with the starting point of 6dB assumed in the kitz table nor with the starting point being my then snapshot SNR.
The starting point is never the instantaneous snapshot NM, as we always keep telling you and it is a waste of time even considering it as such. It is always the Target NM set at the previous re-sync. Kitz does not assume a starting point of 6dB or anything else for the matter; he always works on the change or delta from the Target NM.

1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 19 Meg WBC
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