Unexplained SNR fluctuations

I have been monitoring noise on my ADSL connection for a couple of
weeks.

The router has kept the speed unchanged at 1.4Mb during all this time
and attenuation is a constant 57dB. The SNR during the day is around
10dB but for a period every night it drops to around 4dB for several
hours. The drop is sudden, as is the recovery. It can start at any
time between 5pm and 11pm and go on for just a few hours or up to 10
hours. The graph of SNR (vertical axis) against time looks like this
every day:

xxxxxxxxxxxxx xxxxxxxxxxxxxx
x x
x x
x x
xxxxxxxxxxxx

I imagine something is causing interference on the line but I know it is
nothing within my building. Is this just random bad luck or could it be
caused by my ISP or BT traffic shaping? I get a lot of errors in the
evening when the SNR has dropped, making surfing very tedious.

If anyone can offer an explanation...

Thanks.
--
Kevin

On 26/03/2010 15:31, Peter Crosland wrote:
> "Kevin Vivian"<subs@[127.0.0.1]> wrote in message
> news(E-Mail Removed)...
>> I have been monitoring noise on my ADSL connection for a couple of
>> weeks.
>>
>> The router has kept the speed unchanged at 1.4Mb during all this time
>> and attenuation is a constant 57dB. The SNR during the day is around
>> 10dB but for a period every night it drops to around 4dB for several
>> hours. The drop is sudden, as is the recovery. It can start at any
>> time between 5pm and 11pm and go on for just a few hours or up to 10
>> hours. The graph of SNR (vertical axis) against time looks like this
>> every day:
>>
>> xxxxxxxxxxxxx xxxxxxxxxxxxxx
>> x x
>> x x
>> x x
>> xxxxxxxxxxxx
>>
>> I imagine something is causing interference on the line but I know it is
>> nothing within my building. Is this just random bad luck or could it be
>> caused by my ISP or BT traffic shaping? I get a lot of errors in the
>> evening when the SNR has dropped, making surfing very tedious.
>>
>> If anyone can offer an explanation...
>
>
> Perfectly normal. It is caused by the differences in medium wave radio
> propagation during the hours of darkness. This means that the noise level
> increases at night by about 6dB. A real pain in the rear but apart from
> getting BT to fit an RF filter to your line there is little you can do about
> it.
>
> Peter Crosland
>
>
Ah, but does the OP mean SNR or SNR *margin*?

Quoting from DSLZone:

"The SNR is the amount of signal your line can see as opposed to how
much noise it can hear. For example, if you have a short line to the
exchange, your SNR is expected to be higher than a line that is a long
way from the exchange, as the signal decreases, and the noise increases
over distance.

The more SNR your line has, the faster you can SYNC at. The less SNR
your line has, the more unstable it can be.

Note that many people confuse the term SNR with SNR margin. The SNR is
as stated above, while the SNR margin is the amount of SNR you have left
on your line, not the SNR itself.

For a line to be stable, it is widely viewed that a minimum SNR margin
of 6 dBis to be observed at all times. If the SNR margin drops below 6
dB, you may begin to have problems with your connection, such as loss of
SYNC or lots of errors. This is where the DLM (described above) can kick in.

If your line begins to lose SYNC frequently because of a low SNR margin,
the DLM should pick this up and increase the target SNR margin. This
means the stability of the line should be increased. Note that a higher
target SNR margin means a lower SNYC rate".

In other words, the lower the SNR margin, the more chance of
disconnections, due to instability. However, this with a bonus of a
higher synch rate (speed).

George

Kevin Vivian <subs@[127.0.0.1]> wrote:

>I imagine something is causing interference on the line but I know it is
>nothing within my building.

Your SNR should drop during the evening anyway. A step change is someone
turning something on or off. It could be something faulty causing
electromagnetic radiation which your line picks up, it could be something
faulty injecting noise into phone lines which run next to yours.

It is most likely just someone else turning their router on and off. ADSL
signals on adjacent phone lines interfere with each other.

On Fri, 26 Mar 2010 14:52:02 +0000, Kevin Vivian wrote:

>The router has kept the speed unchanged at 1.4Mb during all this time
>and attenuation is a constant 57dB. The SNR during the day is around
>10dB but for a period every night it drops to around 4dB for several
>hours. The drop is sudden, as is the recovery. It can start at any
>time between 5pm and 11pm and go on for just a few hours or up to 10
>hours.

Hi Kevin,

Assuming it's SNR margin it's normal for it to drop at night as others
have said. Now it's that magic number, if you can keep it at or above 6
things may improve. What's your internal wiring like? Having a filtered
faceplate at the master socket with the modem connected to it, all the
other extensions connected to the filtered side of it and with the bell
wire disconnected is best. If you are not wired like this it's possible
that a redo of the master socket could boost your SNR margin just
enough to improve things for you.

The ADSL Nation faceplate seems to get a lot of recommendations:

http://www.adslnation.com/products/xte2005.php

--
Regards - Rodney Pont
The from address exists but is mostly dumped,
please send any emails to the address below
e-mail ngpsm4 (at) infohitsystems (dot) ltd (dot) uk

Kevin Vivian wrote:
> I have been monitoring noise on my ADSL connection for a couple of
> weeks.
>
> The router has kept the speed unchanged at 1.4Mb during all this time
> and attenuation is a constant 57dB. The SNR during the day is around
> 10dB but for a period every night it drops to around 4dB for several
> hours. The drop is sudden, as is the recovery. It can start at any
> time between 5pm and 11pm and go on for just a few hours or up to 10
> hours. The graph of SNR (vertical axis) against time looks like this
> every day:
>
> xxxxxxxxxxxxx xxxxxxxxxxxxxx
> x x
> x x
> x x
> xxxxxxxxxxxx
>
> I imagine something is causing interference on the line but I know it is
> nothing within my building. Is this just random bad luck or could it be
> caused by my ISP or BT traffic shaping? I get a lot of errors in the
> evening when the SNR has dropped, making surfing very tedious.
>
> If anyone can offer an explanation...
>

Ham radio nearby?


> Thanks.

nospam wrote:
> Kevin Vivian <subs@[127.0.0.1]> wrote:
>
>> I imagine something is causing interference on the line but I know it is
>> nothing within my building.
>
> Your SNR should drop during the evening anyway. A step change is someone
> turning something on or off. It could be something faulty causing
> electromagnetic radiation which your line picks up, it could be something
> faulty injecting noise into phone lines which run next to yours.
>
> It is most likely just someone else turning their router on and off. ADSL
> signals on adjacent phone lines interfere with each other.
>
Thats unlikely to cause such a big drop.

In article <(E-Mail Removed)>,
(E-Mail Removed)d says...
>
> Kevin Vivian <subs@[127.0.0.1]> wrote:
>
> >I imagine something is causing interference on the line but I know it is
> >nothing within my building.
>
> Your SNR should drop during the evening anyway. A step change is someone
> turning something on or off. It could be something faulty causing
> electromagnetic radiation which your line picks up, it could be something
> faulty injecting noise into phone lines which run next to yours.
>
> It is most likely just someone else turning their router on and off. ADSL
> signals on adjacent phone lines interfere with each other.

The step change is characteristic of some routers which sometimes jump
abruptly from one relatively stable figure to another 2 or 3dB lower (or
higher). My Draytek 2820 does this, as did the 2800 before it.

My guess is that the bits per bin changes and the displayed SNR margin
jumps. The modem and the DSLAM can do this "bitswapping", without having
to do a full resync AFAIK. I certainly see a different bit per bin
distribution following these step changes.

The bitswapping is an attempt by the modem/DSLAM to cope with a changing
noise spectrum distribution. Quite normal - no problem.

"The Natural Philosopher" <(E-Mail Removed)> wrote in message news:hoipf1$mv7$(E-Mail Removed)...
> Kevin Vivian wrote:
>> I have been monitoring noise on my ADSL connection for a couple of
>> weeks. The router has kept the speed unchanged at 1.4Mb during all this time
>> and attenuation is a constant 57dB. The SNR during the day is around
>> 10dB but for a period every night it drops to around 4dB for several
>> hours. The drop is sudden, as is the recovery. It can start at any
>> time between 5pm and 11pm and go on for just a few hours or up to 10
>> hours. The graph of SNR (vertical axis) against time looks like this
>> every day:
>>
>> xxxxxxxxxxxxx xxxxxxxxxxxxxx
>> x x
>> x x
>> x x
>> xxxxxxxxxxxx
>>
>> I imagine something is causing interference on the line but I know it is
>> nothing within my building. Is this just random bad luck or could it be
>> caused by my ISP or BT traffic shaping? I get a lot of errors in the
>> evening when the SNR has dropped, making surfing very tedious.
>>
>> If anyone can offer an explanation...
>>
>
> Ham radio nearby?

Unlikely to be "key down" for up to 10 hours though.
(I am aware some would say "keyed up" and mean the same thing. Isn't language is a strange thing?)

--
Graham.

%Profound_observation%

The Natural Philosopher <(E-Mail Removed)> wrote:

>> If anyone can offer an explanation...
>>
>
>Ham radio nearby?

Who broadcasts continuously for several hours?

In article <hoitip$572$(E-Mail Removed)>, (E-Mail Removed)
says...
>
> "The Natural Philosopher" <(E-Mail Removed)> wrote in message news:hoipf1$mv7$(E-Mail Removed)...
> >
> > Ham radio nearby?
>
> Unlikely to be "key down" for up to 10 hours though.
> (I am aware some would say "keyed up" and mean the same thing. Isn't language is a strange thing?)

Most ham bands are well above the ADSL spectrum. Therefore unlikely to
cause problems unless the antenna is VERY close to the ADSL line.