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.

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.

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.

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...

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.

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 deleted (Mon 24-Dec-12 17:36:18)

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.

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.

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.

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.