WannabeMKII, yes that's a very pertinent question and one that I've doted on myself for quite some time. I can't give you an answer but my assumption all along has been that the downstream SNR value that we read in our routers or in utilities such as DMT is specifically a composite value. In other words, the user's router and the DSLAM may very well have their own 'programmed' SNR values for the line but, in practice, the two devices continually negotiate a combined SNR value for the line. I've therefore always wondered how JinxterX2, for example, has been able to declare that his DSLAM's SNR was 15dB. Did he really mean that, or was his 15dB figure, in fact, already a composite figure?

And what are the basic rules, anyway? Is it always the device negotiating with the lowest SNR value that effectively sets the agenda, or what? My gut feeling is that the composite SNR value is something that's not easily arrived at, and that, in fact, by trying to tweak the overall SNR from the user end, you're actually trying to tweak a moving target (since the SNR changes with network occupancy, local interference, etc).

It seems to me that the SNR tweakers amongst us need to adjust the value at the user end in such a way as to take account of the dynamic range of the SNR, under Max. Typically, this could be 3dB but, in some cases, could be considerably more. I think that, for instance in my own case, this is now as much as 6dB (12dB, down to 6dB). So, if my understanding about DMT is correct, getting an optimised SNR for the line - one that would guarantee the best speed but without fear of disconnecting - might well take weeks and weeks of experimentation. Am I right?

Edited by deleted (Sat 22-Jul-06 15:54:21)

by default I think the router is set to allow the other end to set the target snr, when tweaked it overides the dslam.

wether or not BT have the ability to have the dslam overide the router I have no idea.

Re your first sentence, an interesting theory, Chrysalis. Trouble is, from the very moment you connect the router to the line, the router will presumably be applying its own default SNR value. After all, I've not seen or heard of any telnet command, for example, that turns the router's SNR specifically on or off. But you could well be right - the ruling you've suggested could well be workable. Presumably, though, that would mean that, once you've used the router to set the line SNR, you can never get back to the DSLAM being in control?

In essence, you're saying that the SNR value for the line is, in fact, NOT a composite value at all. It's an either 'router' or a 'DSLAM' value. Right?

Edited by deleted (Sat 22-Jul-06 16:30:26)

I happen to have a router (Proline 6100) which always initially syncs at the DSLAM set target margin, so I just go by that.

I think I agree, I dunno, if I had my 10 days over again...I'd probably tweak, but that's just me

You can work it out because all MAX lines start off on a 6db target in the DSLAM (which my Proline 6100 confirmed when I first regraded to MAX), as time went on it was obvious from the router stats that the margin was being increased in 3db steps in the DSLAM.

I guess I'm lucky at the moment then, as my SNR is currently sitting at about 5.5dB

I'm also lucky in that this doesn't vary by more than +/-1dB, so it's pretty stable.

It's interesting to read the difference in stat's from router to router though.

My router (DrayTek 2600VG) reports attenuation of 44.5dB and SNR of about 5.5dB and give's me about 4MB down.

Yet, my current router (Netgear DG834PN - DrayTek is in for repairs) has an attenuation of 51dB and 5.6dB SNR, yet I get 5MB down!

What's that all about?

Yes, it was abundantly clear to me that my own initial target SNR had become 6dB, when I first changed over to ADSL Max. But, Jinxter, I thought you'd been saying that you yourself had started from a figure of 15dB? If it's the case that the DSLAM initially sets the target SNR and thereafter it's possible to override that with a user-configured value in the router, then either your particular DSLAM must have been initially wrongly set or you yourself must have increased the line's SNR at your end.

Having had some time to reflect on what Chrysalis and others have said, about which end takes command in terms of SNR, I'm inclined to agree with them. Everything's now kinda falling into place.

Actually, what happened in my case was that my initial 6dB margin was simply too low for my line to support and I therefore experienced very frequent disconnections in the evenings (like, every other minute). This was maddening and continued for some months, with the margin rising only to 8 or 9dB at best. Contrary to theory, the DSLAM didn't automatically bump up the SNR, to get me to a stable position. My ISP continually ignored requests from me to have the DSLAM's value raised by special request. It wasn't until the beginning of July that my ISP finally conceded, from disconnection statistics made privy to them, that my wholly unstable situation had been real and certainly hadn't been just a figment of my imagination. My ISP nevertheless continued to say nothing. However, from that day, my line then suddenly stabilised and, for almost a month now, I've not experienced a single disconnection. I still check my target SNR at different times of the day and, whereas before it only ever see-sawed between 6dB and 8dB, it now ranges from 6dB to 12dB, with it being mostly around the 9dB mark. I still don't know whether interleaving's been applied; my ISP won't tell me and my router doesn't show it. Possibly, it has. Instead of the hitherto ridiculous swings in sync speed, I now operate at between 4M bps and 4.7M bps, which I regard as being sensible sync speeds for my line, consistent with a high degree of stability.

I still plan to get myself a Speedtouch 546v6, though, as I'm keen to have something that'll perform well on my relatively long line (about 3.5km and 46 - 48dB attenuation), as well as having a facility to not only see whether interleaving is being used but also to set the target SNR from the router end, should that become necessary.

PS. Blow me down, I've just had a disconnection! Well, that's the first in a long time!

same experience as me my snr was raised manually, I can probably explain why other people are maoning that their snr was raised too easy and why mine was never raised.

In another thread it appears me and one other person are not having all their resynchs logged by BT so the line is looking much more stable then it is, so if RAMBO thinks their isnt many resynchs it thinks the line is stable and doesnt adjust the target SNR appropriatly.

It is possible to go back to the dslam set target SNR after you tweaked by pressing the set/save default button, it has to reboot the router to do this.

In reply to:

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But, Jinxter, I thought you'd been saying that you yourself had started from a figure of 15dB?

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Yup, correct. But that's because I never got hold of a ST516v6 (and started tweaking) until after the DSLAM was set to 15db, which took about 2 months from the moment of regrade.