SNR margin increased from 6 to 9 dB

Discussion in 'Broadband' started by Geoff Clare, Jul 26, 2008.

  1. Geoff Clare

    Geoff Clare Guest

    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, Jul 26, 2008
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  2. Geoff Clare

    Gary Guest

    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, Jul 26, 2008
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  3. 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 Weston, Jul 26, 2008
  4. Geoff Clare

    Nick Guest

    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?
    Nick, Jul 26, 2008
  5. 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:

    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."
    Flyiñg Ñuñ 2°°8 +, Jul 26, 2008
  6. Geoff Clare

    Guest Guest

    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

    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.
    Guest, Jul 26, 2008
  7. Geoff Clare

    Nick Guest

    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
    If you get the right modem you can.
    Nick, Jul 27, 2008
  8. Geoff Clare

    Guest Guest

    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.
    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.
    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.
    Guest, Jul 27, 2008
  9. 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
    Flyiñg Ñuñ 2°°8 +, Jul 27, 2008
  10. 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.
    Flyiñg Ñuñ 2°°8 +, Jul 27, 2008
  11. Geoff Clare

    Geoff Clare Guest

    Yes, it seems that way, as the problem has now gone away all by itself.
    (Wouldn't you know it - I waited a week to see if it would sort itself
    out, before posting my question, then two days later it does just that.)

    It also seems that my memory of the SNR margin being 6dB before the
    problem was wrong, as the router still reported 9dB SNR margin after a
    reboot this morning, despite now being back at my usual speed.
    Geoff Clare, Jul 28, 2008
  12. Geoff Clare

    James Egan Guest

    James Egan, Jul 28, 2008
  13. That seems about right Eddy, mine appears to be stuck on 9.8dB atm..
    Flyiñg Ñuñ 2°°8 +, Jul 28, 2008
  14. Geoff Clare

    James Egan Guest

    James Egan, Jul 29, 2008
  15. Geoff Clare

    James Egan Guest

    Yes. This is an important point.

    Since the sync rate is set at the exchange based on line
    characteristics supplied by the customer's modem/router that means
    that it is within the customer's control.

    A few months ago, I upgraded the firmware of my (formerly sky) Netgear
    DG834GT router to V1.02.09 - DGTeam Rev. 0743 and the extra settings
    on this allow the SNR margin to be adjusted downwards a bit so that I
    can regularly sync in the higher of two bras profile ranges that I
    used to endure swapping between.

    James Egan, Jul 29, 2008
  16. Oh yes. When my modem was synching at -2dB SNR all the time and falling
    over as a result, BT trained up to 15dB and shoved interleaving on.

    Once I fixed the router, I had to request they go back to saner figures.

    The auto stuff increases it, its a manual job to reset it.
    The Natural Philosopher, Jul 29, 2008
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