SNR margin increased from 6 to 9 dB

Recently my ADSL line speed dropped to about half its previous value
(which had been consistently around 3 Mb/s for the last two years).

I'm not entirely sure, but I think my router (Draytek 2600) used to
report a SNR margin of 6dB after a reboot/reconnect, and now it is
reporting 9dB. Would that increase in SNR margin explain a halving
of line speed? If so, what could have caused the target SNR margin
to change, and is there any way I can get it changed back?

--
Geoff Clare <(E-Mail Removed)>

"Geoff Clare" <(E-Mail Removed)> wrote in message
news:8verl5-(E-Mail Removed)...
> Recently my ADSL line speed dropped to about half its previous value
> (which had been consistently around 3 Mb/s for the last two years).
>
> I'm not entirely sure, but I think my router (Draytek 2600) used to
> report a SNR margin of 6dB after a reboot/reconnect, and now it is
> reporting 9dB. Would that increase in SNR margin explain a halving
> of line speed? If so, what could have caused the target SNR margin
> to change, and is there any way I can get it changed back?
>
> --
> Geoff Clare <(E-Mail Removed)>

It could be that your line has had a burst of noise on it and this caused
the SNR to be lowered, which in turn has likely reduced the line speed.
ADSL is supposed to be rate adaptive and hopefully if no more noise crops up
on the line your SNR will go back to its initial value, and the speed will
increase again. It may be some days before it happens though, as I am not
sure of the timescale for this. But that is what is suppsed to happen.

"Gary" <00@00.00> wrote in message news:(E-Mail Removed)...
>
> "Geoff Clare" <(E-Mail Removed)> wrote in message
> news:8verl5-(E-Mail Removed)...
>> Recently my ADSL line speed dropped to about half its previous value
>> (which had been consistently around 3 Mb/s for the last two years).
>>
>> I'm not entirely sure, but I think my router (Draytek 2600) used to
>> report a SNR margin of 6dB after a reboot/reconnect, and now it is
>> reporting 9dB. Would that increase in SNR margin explain a halving
>> of line speed? If so, what could have caused the target SNR margin
>> to change, and is there any way I can get it changed back?
>>
>> --
>> Geoff Clare <(E-Mail Removed)>
>
> It could be that your line has had a burst of noise on it and this caused
> the SNR to be lowered, which in turn has likely reduced the line speed.
> ADSL is supposed to be rate adaptive and hopefully if no more noise crops
> up
> on the line your SNR will go back to its initial value, and the speed will
> increase again. It may be some days before it happens though, as I am not
> sure of the timescale for this. But that is what is suppsed to happen.

There's a lot of this going on lately, judging by reports on this and other
forums (conspiracy theory starts here, folks!).
It happened to me too about a month ago, exactly similar to the OP - speed
suddenly reduced from 2.5 Meg to 1 Meg and stayed there.
My ISP got BT to do tests and they made a few adjustments but these only
resulted in fewer drop-outs with no increase in speed.
The only way I've managed to increase the speed - but not quite to previous
levels - is to fit an i-plate, which brought the speed back up to 2 Meg.
I've also now fitted a new router and will wait a few days and see if that
makes any difference.
What I - and presumably many others would love to know is, why are many
people suddenly experiencing a halving of their speed?

George

George Weston wrote:

> What I - and presumably many others would love to know is, why are many
> people suddenly experiencing a halving of their speed?
>

I would presume it is a change in BT operating policy. If a line looks
in anyway dodgy it is easier to reset the target SNR than deal with
customer queries.

I wonder has anyone ever had their target SNR reduced without
specifically asking for it to be done?

Gary wrote:
> "Geoff Clare" <(E-Mail Removed)> wrote in message
> news:8verl5-(E-Mail Removed)...
>> Recently my ADSL line speed dropped to about half its previous value
>> (which had been consistently around 3 Mb/s for the last two years).
>>
>> I'm not entirely sure, but I think my router (Draytek 2600) used to
>> report a SNR margin of 6dB after a reboot/reconnect, and now it is
>> reporting 9dB. Would that increase in SNR margin explain a halving
>> of line speed? If so, what could have caused the target SNR margin
>> to change, and is there any way I can get it changed back?
>>
>> --
>> Geoff Clare <(E-Mail Removed)>
>
> It could be that your line has had a burst of noise on it and this
> caused the SNR to be lowered, which in turn has likely reduced the
> line speed. ADSL is supposed to be rate adaptive and hopefully if no
> more noise crops up on the line your SNR will go back to its initial
> value, and the speed will increase again. It may be some days before
> it happens though, as I am not sure of the timescale for this. But
> that is what is suppsed to happen.

I've had this happen to me a couple of times over the last few years. On
these occasions my downstream synch rate was reduced from around 6800kbps to
just under 6000 kbps for an increase of target SNR from 9dB to 12dB. This
didn't return to the 9dB value automatically so I opened a ticket on both
occasions with my ISP Plusnet and eventually they managed to get BT to reset
the SNR to 6dB and now syynch rates are back to between 6700 and 7000 kbps.

Bob Pullen of Plusnet posted this comment in another thread earlier this
week. It could help if you happen to have a compatible router.

"Have you heard of DMT? If not have a look here and see if you have one
of the compatible routers:

http://www.kitz.co.uk/routers/DMTv8.htm

If you do then you could force your kit to connect at a lower SNR.
Having said that though, you could make things worse, especially given
the fact that your line is still dropping out."

--
Heard melodies are sweet, but those Unheard are sweeter
(E-Mail Removed) FN 2°°8 +, Mungo Brandybuck of Buckland

Geoff Clare <(E-Mail Removed)> wrote:

>Recently my ADSL line speed dropped to about half its previous value
>(which had been consistently around 3 Mb/s for the last two years).
>
>I'm not entirely sure, but I think my router (Draytek 2600) used to
>report a SNR margin of 6dB after a reboot/reconnect, and now it is
>reporting 9dB. Would that increase in SNR margin explain a halving
>of line speed? If so, what could have caused the target SNR margin
>to change, and is there any way I can get it changed back?

The BT exchange decides what the *target* SNR is with 6db being default. If
you line is unstable with frequent disconnects and retrains (I think the
limit is something like 10 retrains per hour) the exchange increases the
target SNR in 3db steps until it thinks your line is stable.

It is a bugger trying to get BT to reduce the target SNR even if you think
the cause of instability has been removed.

3db more target SNR didn't halve your connect speed. Something happened to
the exchange equipment, your modem, or anything in between which increased
the noise and/or reduced the signal.

It is a bugger finding out what caused the problem. The internal wiring and
modem are your problem, the rest is BTs but they won't accept there is a
problem unless the connect speed falls below an MSR (Min? Max? Stable
Rate) which is established during the first 10 days of a new ADSL
connection.

Halved speed might be enough to convince BT there is a problem, you will
have to approach them through your ISP anyway. You should confirm the
problem is not yours - connect a known good modem directly to the master
socket with all other wiring disconnected to prove it is something the
other side of the master socket.
--

(E-Mail Removed) wrote:
>> It's MY line, I am paying for it, and I should be able to choose what
>> stability / speed compromise I am happy with !
>
> Exactly the point I made, it's not rate adaptive, running faster
> when there is little noise, slower if there is a noise burst, then
> recovering quickly when conditions improve.

I guess it is a question of support. Users complain if their line
disconnects, a support call cost BT money.

I suspect there has recently been a policy change to raise the SNR
target of any dodgy line in order to minimise disconnections and
consequent support costs. The thing that surprised me was my line had
been connected for a month immediately prior to getting its SNR target
raised.

> Even better would be if users could set their own figure from
> 4db upwards, provided they accept that if they set it too low
> there may be some loss of stability.

If you get the right modem you can.

eddy@127.0.0.1 (Eddy) wrote:

>Can anyone explain the difference between the SNR Target and the SNR margin?
>I think these two terms get mixed up sometimes.

The higher the connect rate the more signals the line must carry. The more
signals there are the smaller they must be to 'fit' in the line. For the
same amount of noise the smaller the signals are the lower the ratio of
signal to noise.

>I always assumed
>the higher SNR Margin was better as it gives a better sync rate, at least whilst
>the line is quiet.

The reported SNR is the signal to noise ratio at that moment and gives no
indication of line quality without also specifying the connection rate. For
the modem to see the signals they must be bigger than the noise and to
accommodate some variation in noise a margin of 6db is typically required.
While establishing an ADSL connection the exchange tells your modem the
target SNR (6db usually) and it is up to the modem to connect at a rate
which gives that SNR.

>How do you find out what the SNR Target is, rather than the SNR Margin as reported
>by the router? I cannot see that anywhere on the 2600 router stats.

Immediately after establishing a connection the reported SNR should be a
good indication of the target. Thereafter the reported SNR will vary as
noise on the line varies. The exchange sets the target from 6db upwards in
steps of 3.
--

nospam wrote:
> eddy@127.0.0.1 (Eddy) wrote:
>
>> Can anyone explain the difference between the SNR Target and the SNR
>> margin?
>> I think these two terms get mixed up sometimes.
>
> The higher the connect rate the more signals the line must carry. The
> more signals there are the smaller they must be to 'fit' in the line.
> For the same amount of noise the smaller the signals are the lower
> the ratio of signal to noise.
>
>> I always assumed
>> the higher SNR Margin was better as it gives a better sync rate, at
>> least whilst the line is quiet.
>
> The reported SNR is the signal to noise ratio at that moment and
> gives no indication of line quality without also specifying the
> connection rate. For the modem to see the signals they must be bigger
> than the noise and to accommodate some variation in noise a margin of
> 6db is typically required. While establishing an ADSL connection the
> exchange tells your modem the target SNR (6db usually) and it is up
> to the modem to connect at a rate which gives that SNR.
>
>> How do you find out what the SNR Target is, rather than the SNR
>> Margin as reported by the router? I cannot see that anywhere on the
>> 2600 router stats.
>
> Immediately after establishing a connection the reported SNR should
> be a good indication of the target. Thereafter the reported SNR will
> vary as noise on the line varies. The exchange sets the target from
> 6db upwards in steps of 3.

Yep mine's targeted at 9dB but after connection the SNR margin can lie
anywhere between 8 and 11dB although it appears to average around 10 dB
--
Heard melodies are sweet, but those Unheard are sweeter
(E-Mail Removed) FN 2°°8 +, Mungo Brandybuck of Buckland

Eddy wrote:
> On Sun, 27 Jul 2008 11:47:00 +0100 Flyiñg Ñuñ 2°°8 +
> <(E-Mail Removed)> wrote:
>
>> Yep mine's targeted at 9dB but after connection the SNR margin can
>> lie anywhere between 8 and 11dB although it appears to average around
>> 10 dB
>
> How do you know it is targeted at 9db?
> Where does that info come from?
>
> Eddy.

When BT had upped it my ISP: Plusnet _told me_ it had been increased from 9
top 12 dB, I requested that it be changed back to 9dB since the only effect
that had been achieved was the lowering of my synch rate (and hence IP
profile and measured d/l speeds) by around 800 kbps. No increase in
connection stability had been gained. So BT reduced it back to 9dB. If you
monitor your SNR margin it will hover around the target. Mine varies
between 8 and 11 dB but mostl;y around 9.5 dB.
--
Heard melodies are sweet, but those Unheard are sweeter
(E-Mail Removed) FN 2°°8 +, Mungo Brandybuck of Buckland