MaxDSL and noise margin

Having been unable to get broadband until MaxDSL was introduced
because I am around 9Km from the exchange I was finally connected by
Zen to their Home 250 service on MaxDSL this week. The connection has
remained stable according to my experience and the Netgear DG834v2
statistics. (Downstream 606Kbps and Upstream 384). Marvellous for
someone who has only had Dial-up 'til now. It is the Downstream Noise
margin that is worrying me. It has peaked at 6 and at the moment has
actually gone down to 1. According to what I have read in the
newsgroups anything below 3 is unusable yet my line still seems to be
working. How significant is this Noise Margin and will it make a
difference when the BT Management Software comes to set my service
level at the end of the 10 day "training" period?


Chris

> Having been unable to get broadband until MaxDSL was introduced
> because I am around 9Km from the exchange I was finally connected by
> Zen to their Home 250 service on MaxDSL this week. The connection has
> remained stable according to my experience and the Netgear DG834v2
> statistics. (Downstream 606Kbps and Upstream 384). Marvellous for
> someone who has only had Dial-up 'til now. It is the Downstream Noise
> margin that is worrying me. It has peaked at 6 and at the moment has
> actually gone down to 1. According to what I have read in the
> newsgroups anything below 3 is unusable yet my line still seems to be
> working. How significant is this Noise Margin and will it make a
> difference when the BT Management Software comes to set my service
> level at the end of the 10 day "training" period?

You need about 6dB noise margin for a sustainable connection.

Peter Crosland

Chris wrote:
> Having been unable to get broadband until MaxDSL was introduced
> because I am around 9Km from the exchange I was finally connected by
> Zen to their Home 250 service on MaxDSL this week. The connection has
> remained stable according to my experience and the Netgear DG834v2
> statistics. (Downstream 606Kbps and Upstream 384). Marvellous for
> someone who has only had Dial-up 'til now. It is the Downstream Noise
> margin that is worrying me. It has peaked at 6 and at the moment has
> actually gone down to 1. According to what I have read in the
> newsgroups anything below 3 is unusable yet my line still seems to be
> working. How significant is this Noise Margin and will it make a
> difference when the BT Management Software comes to set my service
> level at the end of the 10 day "training" period?

It looks like the way your netgear shows/mis shows margin - if it works
and doesn't keep dropping then I don't think the BT software will care.

http://forum1.netgear.com/support/vi...c.php?p=114284

Andy.

On Sat, 29 Apr 2006 14:32:03 +0100, "Peter Crosland"
<(E-Mail Removed)> wrote:

>> Zen to their Home 250 service on MaxDSL this week. The connection has
>> remained stable according to my experience and the Netgear DG834v2
>> statistics. (Downstream 606Kbps and Upstream 384). Marvellous for
>
>You need about 6dB noise margin for a sustainable connection.

The noise margin, as reported by my DG842v2, has varies between 8dB
and 1dB on my rock solid line synced at 8mb:

http://dipsy.microsign.co.uk/mrtg/arnoldsnr.html
--
Andy Norman
http://www.gigajam.com/
Learn to Play Guitar, Bass, Keyboard and Drums.
It’s easy with our Lessons and Software!

In an earlier contribution to this discussion,
Chris <chris@no_spam.chriswillsher.co.uk> wrote:

> Having been unable to get broadband until MaxDSL was introduced
> because I am around 9Km from the exchange I was finally connected by
> Zen to their Home 250 service on MaxDSL this week. The connection has
> remained stable according to my experience and the Netgear DG834v2
> statistics. (Downstream 606Kbps and Upstream 384). Marvellous for
> someone who has only had Dial-up 'til now. It is the Downstream Noise
> margin that is worrying me. It has peaked at 6 and at the moment has
> actually gone down to 1. According to what I have read in the
> newsgroups anything below 3 is unusable yet my line still seems to be
> working. How significant is this Noise Margin and will it make a
> difference when the BT Management Software comes to set my service
> level at the end of the 10 day "training" period?

You need 6db - but it's BT's measurement which matters, and that may well be
different from the figure reported by your router.
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

On Sat, 29 Apr 2006, Andrew Norman wrote:

> On Sat, 29 Apr 2006 14:32:03 +0100, "Peter Crosland"
> <(E-Mail Removed)> wrote:
>
> >You need about 6dB noise margin for a sustainable connection.

Indeed. The Max seems to be adjusting my line rate to achieve about
6 - 7dB margin. I don't know what basis it uses for doing that, but
the ADSL line is working fine[1]

> The noise margin, as reported by my DG842v2, has varies between 8dB
> and 1dB on my rock solid line synced at 8mb:

I've just been reading a long thread which indicates that the SNR
margin figures from the DG834 (various models and firmware versions)
are bogus^Wuntrustworthy. From what I was reading: after a restart,
the value may very well make sense. But, the longer the box stays up,
the less plausible the numbers become. There was a reference to:
http://forum1.netgear.com/support/vi...c.php?p=114284

Could it be that your box is subject to the same misbehaviour?

My own (BT Voyager 205) router almost always shows 6 - 7dB local SNR,
whenever I look. Just occasionally I've seen less (once as low as 3)
or more (but no more than about 8.5). Sorry, I'm not running MRTG
against it here ;-)

h t h

[1] the actual throughputs achieved are disappointing, but the ADSL
line speed itself seems fine. I suspect a throttle in the BTw
network...

On Sat, 29 Apr 2006 21:34:02 +0100, "Alan J. Flavell"
<(E-Mail Removed)> wrote:

>I've just been reading a long thread which indicates that the SNR
>margin figures from the DG834 (various models and firmware versions)
>are bogus^Wuntrustworthy. From what I was reading: after a restart,
>the value may very well make sense. But, the longer the box stays up,
>the less plausible the numbers become. There was a reference to:
>http://forum1.netgear.com/support/vi...c.php?p=114284
>
>Could it be that your box is subject to the same misbehaviour?

Could well be, however it appears to be consistent with itself at
least, it has been doing the 8dB-1dB cycle for several weeks since Max
started. It has been rebooted several times in that period.
--
Andy Norman
http://www.gigajam.com/
Learn to Play Guitar, Bass, Keyboard and Drums.
It’s easy with our Lessons and Software!

Roger Mills wrote:

> You need 6db - but it's BT's measurement which matters, and that may well be
> different from the figure reported by your router.

two misconceptions here, firstly you don't need 6 dB as the figure is
an SNR margin where "margin" means "more than necessary" and zero is
defined as achieving a certain error rate. You can have an SNR margin
below zero and the line works, but with more errors. Modems usually
retrain at zero to try to get a better margin.

The other misconception is that BT make measurements independently of
your kit. The only conceivable way they know what the SNR margin is in
your house is for your equipment to tell them. It may give them a
different (correct) number to that displayed on a poncy GUI, but the
number has to come from your equipment.

Phil

In an earlier contribution to this discussion,
PhilT <(E-Mail Removed)> wrote:

> Roger Mills wrote:
>
>> You need 6db - but it's BT's measurement which matters, and that may
>> well be different from the figure reported by your router.
>
> two misconceptions here, firstly you don't need 6 dB as the figure is
> an SNR margin where "margin" means "more than necessary" and zero is
> defined as achieving a certain error rate. You can have an SNR margin
> below zero and the line works, but with more errors. Modems usually
> retrain at zero to try to get a better margin.
>
I used the word "need" in the context that that appears to be the value that
BT look for when fixing the rate under MaxDSL. Sure, it will *work* with a
lower margin - but probably not very reliably!

> The other misconception is that BT make measurements independently of
> your kit. The only conceivable way they know what the SNR margin is in
> your house is for your equipment to tell them. It may give them a
> different (correct) number to that displayed on a poncy GUI, but the
> number has to come from your equipment.
>
Where do you get your facts from? With the wide range of end-user equipment
in use, I very much doubt whether it is all capable of reporting line stats
to the remote end - in a standardised way, if at all. I'd be amazed if BT
didn't have an independent way of measuring line quality.
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

Roger Mills wrote:

> Where do you get your facts from? With the wide range of end-user equipment
> in use, I very much doubt whether it is all capable of reporting line stats
> to the remote end - in a standardised way, if at all. I'd be amazed if BT
> didn't have an independent way of measuring line quality.

prepare to be amazed then.

To know what the SNR is at your end of the circuit, BT collect this
information from your equipment.

Think about it - how can they know a) the signal level that you are
receiving and b) the prevailing noise level at your end of the line.
They can't, but your CPE is fully aware of this and signals it back to
the BT DSLAM as part of the ongoing communmication defined in G.dmt.

The issue of "wide range of end-user equipment " is easily addressed by
standards. It all has to be capable of talking to the the DSLAM to set
up the signal in the first place and all this is tightly defined. If
your kit can't measure and transmit SNR data, it ain't going to work.

The only independent way, for example, of measuring attenuation is to
put a signal generator on your line at the exchange and have an
engineer drive out to your place to measure the received signal level.

Phil