Variable Noise Margin - causes?

Discussion in 'Broadband' started by Grant, Oct 3, 2004.

  1. Grant

    Grant Guest

    I had ADSL installed last month - a managed HH>ADSL conversion. The BT
    engineer supplied a filtered face plate. The line is used exclusively for
    ADSL. I'm quite a way from the exchange - I'd estimate 6 or 7km by the
    shortest likely route. My downstream attenuation reported by my 3Com
    modem/router is a consistent 63 or 64dB.

    However, the downstream noise margin is highly variable - during the day it
    manages to maintain a steady 11 or 12dB, maybe reaching the heady heights of

    Right now (9.10pm), I'm watching it vary between 1dB and 4dB with 2 being
    the most usual figure. This results in effectively dead internet access - it
    appears that very low impact traffic can still get in and out (ICMP, NTP
    etc.) but anything I'd like to use (NNTP, HTTP) is dead - it may take me
    some time to post this message.

    A similar episode has happened for the last couple of days - fairly OK
    during the day, dire at night.

    I'm interested to know what factors could cause the reported noise margin
    figure to vary by so much?

    My biggest concern is BT say 'sorry, we'll have to take it away...'.

    Current operational data:
    Upstream Downstream
    Noise Margin 24 dB 2.5 dB
    Output Power 12 dBm 15.5 dBm
    Attenuation 31 dB 63 dB
    Grant, Oct 3, 2004
    1. Advertisements

  2. Grant

    Dan Wood Guest

    Interesting phenomenon, and one that people have mentioned before. My own

    1. Increased local use of TV's, computers, lighting (including street
    lighting) etc. in the evenings when folk come home from work, generating
    local RF noise.

    2. The lack of sunlight reaching the 'D' region of the ionosphere after
    dark. Basically this means that radio signals (not local) in the same
    frequency range as ADSL are stronger after dark. (The D-region absorbs these
    signals in the day.)

    Try tuning around on a standard AM radio in the day and again after dark.
    You should see what I mean. In fact, you might find some link between the
    number of stations you can hear, and the attenuation / noise figures on your
    line... If you do, could you let me know?

    Dan Wood, Oct 3, 2004
    1. Advertisements

  3. Grant

    Nigel D Guest

    Another possibility is crosstalk in the cable between yourself and other
    'evening only' ADSL users.

    This would certainly be true if some of the other ADSL users are only
    turning their modem on during the eveing when they are on the internet.

    It is possible that even if they do not turn their modem off during the day,
    there is more crosstalk in the eveing when they are on the internet because
    their activity is creating 'noisier' ADSL signals which are more likely to
    cause problems when they couple to adjacent cables.

    Crosstalk is more likely if your cable is not 'balanced', ie the capacitance
    to ground on the two wires is not matched. I don;t know whether it is
    possible to improve things by adding a small amount of capacitance to ground
    on one or other leg to improve the balance.

    Nigel D, Oct 4, 2004
  4. One other rather more mundane reason can be moisture in the cable. This
    could be anywhere between you house and the exchange but overhead parts of
    the route are favourite. Assuming you have tried disconnecting everything
    else from the line it would be worth asking you ISP to do some tests.
    Peter Crosland, Oct 4, 2004
  5. For interest read the "Regular loss of service around 8pm?" thread in the

    "BTwholesale ADSL Implementation" forum on

    I suspect that you best hope is to complain to your ISP and ask them to get BT
    to try and fix the line if they can. AIUI BT will make a reasonable effort to
    do this. You should check on ADSLGUIDE if BT still insist on taking poor lines
    away or whether you now days have an option of keeping the line even if it is
    not very good.

    Michael Chare
    Michael Chare, Oct 4, 2004
  6. Grant

    Dave Guest

    BT were superb last week for me. My modem wouldn't even attempt to sync
    with the exchange. BT chappy came out and put me onto a shorter piece of
    cable. He also said that DSL has to work for BT so you may well find BT
    make a decent effort to fix your problem.
    Dave, Oct 4, 2004
  7. Grant

    Reg Edwards Guest

    Noise, by its very nature, is purely random and variable. It can't help

    Noise sources include neighbour's computers and domestic flourescent lights,
    including the long-life variety. Individual light bulbs differ between
    themselves. Electric light dimmer switches sometimes generate continuous
    terrible noise. Also domestic central heating and clock-controlled gas hot
    water systems. Thermostats are notorious noise sources.

    A factory using a battery of electric-arc welding machines can spread severe
    continuous noise over hundreds of yards. One arc-welder in a local garage
    is bad enough. Although, legally, the welder's user/owner is guilty of
    polluting the radio waves, there's no hope in asking him to shut up shop to
    allow you to use your own computer.

    It may not occur to you the problem you are experiencing is due to
    intermittent or continuous noise. In that case there's no hope of locating
    the source. The danger lies in resorting, quite unnecessarily, to
    re-formatting your hard drive.

    Thunder storms cause noise spikes over many miles. A single strong noise
    spike, from whatever cause, can disable internet synchronisation until it is
    manually reset. (Switching off and back-on the power to your router
    sometimes works.) Such events can give a false impression that the
    offending noise is continuous and so handicap attempts to trace it to its

    Every time anybody throws an electrical switch, on or off, a strong noise
    'spike' is radiated from the house wiring over distances of 100's of feet.
    The more neighbours you have the more frequently it occurs. A group of
    densely populated tower blocks are worst. City centres and industrial areas
    are many times worse than isolated farm houses.

    The only noise picked up on phone cables is internet crosstalk from other
    pairs in the same cable. Crosstalk noise is difficult to cope with because
    it has just the same electrical characteristics as the wanted signals. It is
    crosstalk which limits the distance from exchange to sunscribers. It
    increases in direct proportion to line length.

    As the months go by, and the number of used internet pairs in the cable
    increases, the noise level increases as the wanted signals decrease.
    Service quality is progressively degraded until the cable is 'full',
    subscribers being furthest from the exchange, who originally received a
    satisfactory service, suffering first.

    Sending stronger signals from the local phone exchange is no remedy - the
    signal-to-crosstalk power ratio remains the same.

    Overhead lines and dropwires from poles into domestic premises collect both
    locally generated noise and radio waves. Long-wave, medium-wave and short
    waves are all received. But ISP's do their economical technical best to
    avoid use of frequency bands used by legitimate high-power broadcasting
    transmitters and nearby radio amateurs.

    The ultimate solution is to provide a light fibre, plus additional optical
    hardware at both ends, from the nearest tele-exchange into everybody's
    living room. But this won't occur until competing international service
    providers have extracted all possible revenues from the present chaotic,
    ill-managed, incomplete, set of stop-gaps.

    Sorry to be so pessimistic. Even greater problems lie in incompatibilities
    and faulty ineractions between the many thousands of software programs,
    their add-ons and their continual updates. Looks like calling in the
    Chinese, junking Windows and going back to square one.

    Worse than changing the gauge of a railway system from 4 feet, 8.5 inches to
    a standard but more efficient 8 feet. Or driving on the right side of the
    Reg Edwards, Oct 4, 2004
  8. Grant

    Reg Edwards Guest


    Michael, you must be new around here.

    You are in danger of forgetting it is avaricious service providers,
    defective software writers, defective equipment manufacturers and dishonest
    sales departments who are responsible for causing all the many problems in
    the first place.

    Next week, if your particular problem doesn't re-occur, the chances are you
    will be afflicted with several different more time-wasting ones, not so
    simply cured, which will just have to be lived with as best as you can.

    "Reliabilty" is a meaningless word.

    But your present elation is quite understandable. I wish you the best of
    luck. We all need it - many of our livings (excluding games players and
    porn pushers) depend on it!
    Reg Edwards, Oct 4, 2004
  9. Grant

    jas0n Guest

    i had the same sort of symptoms at one of my remote sites ... i swapped
    2 routers and still had intermittant problems, tried 3 different
    filters, etc .. all were known to work on other adsl connections but on
    this one would appear very flaky at best.

    the isp got bt out to the exchange who may or may not have tweaked
    something, nobody knows but that same night i also bought a linksys
    wap54g and put that in and everything has been ok since.

    this connection wasnt the sort i could mess with and take my time over
    making sure exactly what had caused it/resolved - the client just wanted
    it working and wouldnt understand the vaugories of all the companies
    next door and a few doors down from them working fine when their's didnt
    ..... daft buggers wouldnt even listen but anyway, it was either the bt
    tweak or a more compatible router .... or just plain damn luck!
    jas0n, Oct 4, 2004
    cyberframe722, May 27, 2013
  11. its such a interesting topic...i will thought about it
    cyberframe722, May 27, 2013
  12. Grant

    Andy Burns Guest

    Why don't you piss off and take your spam with you?
    Andy Burns, May 27, 2013
  13. The Natural Philosopher, May 27, 2013
  14. Grant

    Graham. Guest

    Graham., May 27, 2013
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.