ADSL Noise Question

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Can anyone explain this?

I have a Netgear DG834Gv3 Modem Router connected to an AdslMax service. The
typical Noise Margin reported is 6dB. During the late afternoon and evening
the Noise Margin deteriorates, often to the extent that I lose Sync - which
tends to occur when the Noise Margin falls below 0dB.

The odd thing is that if I power down the Modem Router and then power back
up, the system resumes at a better noise figure. Typically if I power down
when the Noise Margin is 1, plus/minus 1dB, the system returns at 3,
plus/minus 1dB. Last night it didn't lose sync, so I left it alone, but this
morning it was reporting 2dB before I did a power off-then-on, and a steady
6dB afterwards.

In all cases, the Sync tends to be in the range 3.8 to 4.4Mbit/sec and the
Line Attenuation is 52 or 53dB.

Why the jump in reported Noise Margin? Is it something in the Netgear Noise
Margin computation algorithm, some effect from the BT side of things - or


Re: ADSL Noise Question

John A wrote:
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It is all down to the way ADSL trains.  Each time the connection syncs, it does
so by measuring the noise at various frequencies, and selecting a set of
frequencies that provide the highest available speed at a target noise margin.
The normal target for BT Max is 6dB*, but is increased on lines where there are
wide variations in noise levels causing repeated syncs.  You can ask your ISP to
increase your noise margin, usually in 3dB steps, if your line is re-syncing too
often, but this will affect your speed.  Swings and Roundabouts....

Some cisco kit allows you to specify your own offset to the Rx target, forcing a
wider margin results in a lower initial speed, but more stability.  However,
such features are not universally available on consumer broadband products.

*This may have changed;  Tiscali LLU lines have a target of 12dB

Re: ADSL Noise Question

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Which is useless because my SNR is at best 8db !
It fluctuates at random resulting in frequent disconnections, usually when I
am doing something time critical...
Any suggestions ?

Re: ADSL Noise Question

In an earlier contribution to this discussion,

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Well, I've had an interesting experience recently which just might be

For a period of several weeks I was suffering frequent disconnections (about
5 per day on average) from PlusNet while using my ZyXEL router. This was
consistently synching at about 3.6 Mbps withn a noise margin of around 6db.

I eventually got fed up with this and decided to try my 3Com router instead.
[This is the one mentioned in threads of many months ago which refuses to
work with the 3Com-supplied switch-mode power supply, but works ok with a
conventional transformer-based supply - albeit synching at a lower rate].

The 3Com synchs at about 2.4 Mbps with a noise margin of about 12db - and
has been rock solid for the last 10 days or so since I started using it. I'm
actually quite happy with this - and, despite the lower synch speed, the
line doesn't feel any slower subjectively.

I don't really understand what happening (Kraftee?). My understanding is
that MaxDSL will keep cranking the speed up until the SNR comes down to
about 6db - but mine isn't doing that. Is is remembering all the
disconnections which occurred when it *was* doing that, and playing safe?

Why would you get a different synch speed and SNR by changing kit, anyway?
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Re: ADSL Noise Question

Roger Mills wrote:
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Booo, nice to see someone has missed me.....

The difference in your findings can be easily explained away by saying
different kit, different responces, it would be very simplistic but could be

You've also got to remember that the results given by your kit is subjective
& may not be the true reading seen by the DSLAM in your exchange.

After saying all that you could be 'stuck' as it can happen, but seeing that
your connection is now stable, perhaps you should leave things be, you could
try forcing a reboot (suggest that you do this in the evening) a couple of
times but there isn't much more you can do without trying to get PN

Re: ADSL Noise Question

In an earlier contribution to this discussion,

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I'd rather have a stable connection than a faster one, do I don't really
want to do anything about it. As I said earlier, the line is no slower
subjectively even though the synch rate is lower.

I was just puzzled as to why the DSLAM isn't disconnecting and trying a
higher rate - even though, as I say, I don't particularly want it to. Could
you explain what you mean by 'stuck'?
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Re: ADSL Noise Question

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How would it be useless?  Your current SNR may be 8db at best, but
as you say it fluctuates randomly causing disconnections.  Get BT
to increase the target SNR, the line will sync at a slower speed and
higher SNR.  The random fluctuations will then hopefully not reduce
the SNR to a point where the line disconnects.

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Either remove the source of the noise (not usually possible), or shield
the wiring from the source of the noise (only really possible if the
noise is being picked up by your extension wiring), or deal with the noise
(and connect a little slower).

David Taylor

Re: ADSL Noise Question

David Taylor:
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Nope. Even when I was on fixed 512kb/s the SNR never went above 8db. We then
had 1Mb/s which was reasonably stable, but were told our line would not
support 2Mb/s (49.5db attenuation).
Now with Max the router usually synchs at about 2Mb/s and 6-7db, then
disconnects, creeps up a bit, repeating that every 20 mins until it gets to
about 2600kb/s, then goes all the way down to ~1.3kb/s (or even 800kb/s).
It will stay at low speed for up to 24hrs (unless rebooted), then starts
again at 2Mb/s.
I am reasonably certain that 2Mb/s would be sustainable, but how to persuade
Max not to exceed that speed ?

(My Vigor 2600 router can't limit the speed, and I have checked all the
wiring and tried at the main socket with nothing else connected).

Re: ADSL Noise Question

Jim, please see the response I've made beyond the hijack

Re: ADSL Noise Question

John A wrote:
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I don't know anything about the technology, so I started by reading a
few web pages.

Where I am, I'm not on a "Max" service. Which means my rate is fixed, and
my noise margin would vary with time.

In your case, the implication of the "Max" part, is that when you power on
the modem, the unit has defined margins set of 6dB in one direction and 3dB
in the other. The modem attempts to train for the highest data rate it can
achieve, with that amount of noise margin. Then, it tries to maintain the
datarate it started with, for the rest of the session.

Telephone signals are carried in bundles at various points in the telephone
system. That means there can be crosstalk from one wire pair to another. The
increase in noise, later in the day, could be non-commercial users firing up
DSL gear, when they get home from work. This could be adding crosstalk, which
degrades the noise margin.

So the solution is simple - power down the modem, then power up, *during* the
time of the day when the noise is at a peak. By doing so, you'll have 6dB
margin at the noisiest part of the day (when the modem is trained), and the
noise margin can then only improve during the quieter parts of the day. In
other words, it might read 8dB late at night, and degrade to "only" 6dB margin
at the noisiest part of the day (5PM say).

If, on the other hand, you train the modem at 5PM, and the noise margin
continues to degrade or get worse, than what you think is the noisiest
part of the day, at that point I would bring into question, the algorithm
used to calculate noise margin.

"Train at 5PM, be happy all day" - view in Courier font...

  0 db margin        ----------------- 5PM -----------------
  (loss of sync etc)

  6 db margin                        +------+
                         __         /        \__
  8 db margin        ___/  \___---_/            \__________

By doing this, naturally your sync will be low, but you can also stop
staring at the noise margin :-)

Just a guess,

Re: ADSL Noise Question

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Thanks to those who have replied to my OP.

Jim - thanks for the explanation of MaxDSL syncing. I now understand what
happens when I disconnect and reconnect - at least when I do it from 6AM to
4PM. What about the 4PM to 11PM cases where I only achieve a 3 to 4dB Noise

Paul - powering up and down is just what I've been doing in to avoid
subsequent loss of sync - except that I tend to to it at about 6PM - but it
is not quite where I want to be! As an experienced engineer, I'm well aware
of how the noise is getting in! Having worked extensively with RF, I'm
impressed that anything but noise gets from one end to the other of such a
4km-long cable.

All - The downside to doing nothing is that my connection profile degrades
and I lose my service for a minute or so whenever the ADSL loses sync. Last
week it was set at 1500k for three days - with no greater immunity to
early-evening problems. All my ISP (PlusNet) has done is to suggest that
they get BT Wholesale to check the line at (their figures) a cost to me of
£46 + VAT minimum. They haven't suggested that the Noise Margin Calibration
Figure be changed. Any thoughts why not?

The profile, BTW has been set at 1500, 2000, 2500, 3000, 3500 and 4000k
since I've been on AdslMax. The maximum I actually can achieve is 2800k so
profiles above 3000k seem to be a waste of time. If  I manage to get the
Calib Figure raised from 6dB to 9dB, might I still be offered 3000k or will
rates collapse dramatically?

Thanks again.

John A

Re: ADSL Noise Question

John A wrote:

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I'm surprised you are being billed for line maintenance. If BT was your
ISP, would they bill you for line maintenance as well ? Isn't PlusNet
buying a "service" from BT ? The only time I would get billed here, is
if telco people do service inside my house. Everything outside the house
is free.

I did have some trouble with my ADSL, but I resolved it by ripping out
my inside house telephone wiring, and making a direct connection to the
demarc in the basement. (We use the "splitterless" method here, where you
place filters before each telephone.) On further examination of my floor
mounted telephone jacks (next to carpeting), I found corrosion on the
pins of the jack. Do voice calls over the phone lines have any problems that
you can hear ? Maybe you could get the line buzzed for free, if you
complain about voice call problems to BT :-)

Obviously, as you raise the noise margin figure, the sync rate will
drop. The 6dB figure was intended for "normal" lines with "normal"
impairments. Are you by chance, too far from the central office ?
There have been cases of people having ADSL installed and running,
only to be contacted by the telephone company a month later, that
"ADSL service was not possible" and the service disabled.
They are supposed to know how far you are from the C.O., and only
connect service for people who meet the reach requirement.


Re: ADSL Noise Question

John A wrote:
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I expect the increase in noise (and hence decrease in margin) is down to hordes
of USB-modem users getting home and getting on-line.  Users who connect using
routers tend to leave them on during the day, so the number of ADSL connections
in the bundle of cable stays relatively constant while everyone is out at work,
and increases as people power up equipment at night.

Certainly, the uptake of broadband has had an effect on my speeds;  I was
originally 'nailed' to 8128k, but over the past year, the speeds have slowly
dropped and I am currently at 6816k - I got an upward BRAS increment to 7392k
yesterday, but it lasted all of 20 hours....

Time             Up     Down     MSR     FTR     Mode
15/03/2007 02:49:24     -     6816     2272     1590     Fast Opt Out
14/03/2007 05:05:36     448     7392     2272     1590     Fast Opt Out
13/03/2007 18:50:30     -     6816     2272     1590     Fast Opt Out
13/03/2007 12:16:14     -     7392     2272     1590     Fast Opt Out

(I still do not know why my upstream BRAS shows 0 most of the time)
At some point, I plan to do some surgery on my internal wiring, removing things
like ring wires, and filtering at the demarc only.  Perhaps I'll get back to
8128?  Although I'm not that bothered, to be honest.  6M is sufficient for the
most part.

Re: ADSL Noise Question

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  When I was seeing only 6 dB on one ADSL modem, we had DSL. But it
would drop to zero - no DSL.  Putting a POTS phone on that wire, it
was obvious that the wire was broken.  DSL was passing across a
capacitor 'created' by that broken wire.  Once we finally got a
lineman, he immediately traced out the broken wire.  Wire was punched
down on a connector block with two broken ends held adjacent by wire

  My experience is that 6 dB is woefully low.  Signal should exceed 10
dB often.  With the broken wire corrected, signal at that location
exceeded 14 dB always and stayed at a consistent 16dB.

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