Unexplained SNR fluctuations

Discussion in 'Broadband' started by Kevin Vivian, Mar 26, 2010.

  1. Kevin Vivian

    Kevin Vivian Guest

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

    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

    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...

    Kevin Vivian, Mar 26, 2010
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  2. 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 Weston, Mar 26, 2010
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  3. Kevin Vivian

    Guest Guest

    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.
    Guest, Mar 26, 2010
  4. Kevin Vivian

    Rodney Pont Guest

    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:

    Rodney Pont, Mar 26, 2010
  5. Ham radio nearby?
    The Natural Philosopher, Mar 26, 2010
  6. Thats unlikely to cause such a big drop.
    The Natural Philosopher, Mar 26, 2010
  7. Kevin Vivian

    Gaius Guest

    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.
    Gaius, Mar 26, 2010
  8. Kevin Vivian

    Graham. Guest

    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., Mar 26, 2010
  9. Kevin Vivian

    Guest Guest

    Who broadcasts continuously for several hours?
    Guest, Mar 26, 2010
  10. Kevin Vivian

    Gaius Guest

    Most ham bands are well above the ADSL spectrum. Therefore unlikely to
    cause problems unless the antenna is VERY close to the ADSL line.
    Gaius, Mar 26, 2010
  11. You might say that, but it only takes a bit of rusty fence to do enough
    of a job down converting to in-band.

    RF is freakish stuff.
    The Natural Philosopher, Mar 26, 2010
  12. Might leave the carrier on.
    The Natural Philosopher, Mar 26, 2010
  13. Which takes me back many years ago to the time when on HM's Service, I
    used an old metal bedstead - the type with the springs in the middle to
    support the mattress - as a hasty HF antenna whilst colleagues were
    erecting a proper mast. It worked. we got through successfully to UK
    from mainland Europe.
    The signal was not much better when the mast was erected and the proper
    antenna connected!

    George Weston, Mar 27, 2010
  14. Kevin Vivian

    Graham J Guest

    In the ham radio world, a metal coat hanger was an informal "unit" of
    Graham J, Mar 27, 2010
  15. Is there anything special about 612 kHz apart from it being in the
    middle of the ADSL frequencies? Surely noise at any frequency
    from around 100 to 1000 kHz - pretty much anywhere on medium and
    long wave - could cause problems?

    -- Richard
    Richard Tobin, Mar 30, 2010
  16. Correct.

    My graphs show period daily fluctuations, weather related fluctuations
    and stuff that's probably correlated with sunspots. Or something. Its a
    broadband aerial that picks up anything from what - 60Khz-1.5Mhz or so .

    You probably wont be using much over 1Mhz on long lines though. Its
    simply not usable.

    As I understand it DSL divides up the spectrum into channels, and
    selects those that show good noise performance, and discards those that
    have excessive interference, thus stepping around MW and LW broadcasts.
    From time to time, the selection needs changing and some sort of
    resynch happens, usually accompanied by a speed change and a step change
    in SNR.

    So step changes in SNR are very common, but associated with no resynch
    and same synch speed *exactly*, I have not seen.
    The Natural Philosopher, Mar 30, 2010

  17. SNR margin=SNR in practical terms.

    But it is not the same as total noise.

    I am wondering if the OP really is seeing a (forced or otherwise) change
    in synch speed that reflects in a change of SNR as well. I've seen such
    step changes when BT, in their infinite wisdom, decided that I should be
    on a 12db margin instead of 9db. And where a resynch happens because of
    noise in one particular channel, that then stays that way until
    something else forces a further resynch.

    Ive even ended up - lord knows how - with a massive drop in upstream SNR
    and an upstream synch speed of less than 200K, whilst still having a
    downstream at 3Mbps +..no idea what THAT was.

    The algorithms that decide what channels are used, are quite
    complex..and its not impossible that they are also in some kit or
    combinations of kit, buggy.
    The Natural Philosopher, Mar 31, 2010
  18. Kevin Vivian

    John Guest

    My Demon provided Speedtouch 510 and both the Netgear routers I have,
    and do, use all report SNR up and down as well as the attenuation up and
    John, Mar 31, 2010
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