Sudden speed increase but poor reliability

Discussion in 'Broadband' started by Alex Fraser, Feb 19, 2015.

  1. Alex Fraser

    Alex Fraser Guest


    Recently, my ADSL connection (Plusnet) has become very problematic, but
    the symptoms seem unusual, and I wonder whether anyone else has
    experienced similar and/or can suggest a cause.

    Before problems started, downstream was around 6-7 Mbit/s with 3dB noise
    margin. Interleaving is on, with some uncorrectable errors but not
    enough to be noticeable.

    I was aware of problems on Friday morning, which I subsequently
    established coincided with the end of scheduled maintenance in my area.
    Later investigation showed downstream was now 8.5-9 Mbit/s, but with
    about 0dB margin(!). And as you might expect, many uncorrectable errors
    and (sometimes lengthy) disconnections - an unusable connection, more
    often than not.

    On Saturday, after plugging the router directly into the test socket and
    finding no difference after a few hours, I raised the matter with
    Plusnet who did a "broadband reset" (not sure if this is their term or a
    general one, but it sounds like the same "calibration" as happens when a
    line is first activated; they said "may take up to ten days"): problem
    almost instantly solved. I checked at various times and downstream was
    in fact somewhat faster (around 7.5 Mbit/s) with noise margin varying
    between 3dB and 6dB (part of the process Plusnet had initiated?),
    settling on 3dB.

    Great, I thought... until yesterday (Wednesday), when the behaviour that
    had started on Friday returned.

    Has anyone come across similar? Any thoughts as to what is going on?

    Thanks for reading,
    Alex Fraser, Feb 19, 2015
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  2. Alex Fraser

    Graham J Guest

    Can you repeat the problem with another router? Both a similar make &
    model and something quite different?

    If so, ring Plusnet again and they will send out an Openreach technician
    to investigate.

    The first thing this technician will do is to try to repeat the problem
    using his own equipment. If he can't then he goes home and Plusnet
    sends you the bill. So following Plusnet's instructions and testing
    with another router (perhaps one they supplied to you) is essential.
    Graham J, Feb 19, 2015
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  3. Alex Fraser

    Woody Guest

    Don't worry, Uncle Bob will be along shortly to fix it 'from the
    Woody, Feb 19, 2015
  4. It's relatively trivial for me to submit another SNR reset (I'd just
    need some method of identifying your account). It is fairly odd that
    things seem to be going south over time rather than north though.

    If it comes to it, we can always try fixing the SNR/line attributes.
    Plusnet Support Team, Feb 20, 2015
  5. Alex Fraser

    Martin Brown Guest

    Odd that your line settles to 3dB noise margin. I though the target was
    6dB and on difficult lines steps up in 3 (mine has been up to 15dB!).

    Increased noise margin gives a falling speed around 500k per dB as a
    rough heuristic and vice versa.
    My noisy line (not Plusnet) sometimes crashes the router overnight and I
    end up with a near zero noise margin and a sync light that remains on
    even though there is no routing or any IP address shown in advanced
    diagnostics. The external health lights on the modem claim all OK.
    Depending on how much continental MW interference you get you may be
    able to game the exact SNR by either syncing just after midday to get
    the fastest speed or in the late evening to get the higher noise margin
    and therefore a bit more stability. You can see a diurnal variation in
    the SNR especially if you run routerstats lite for a while.

    I started a thread about it here a while back when I was experiencing
    dry joint/cable fracture noise problems on my rural line.
    Martin Brown, Feb 20, 2015
  6. Alex Fraser

    Alex Fraser Guest

    On 20/02/2015 10:47, Plusnet Support Team wrote:
    Question 99568466 relates to the modify order failing - though when I
    queried it I was advised it did go through a couple of hours later. This
    remains a mystery as it does not seem to be in effect, but is no longer
    desirable (see below) so not worth further action beyond curiousity value...

    Despite reasons to suspect changes at the far end causing the issue, and
    having no reason to suspect my router, given Graham's comment (thanks
    Graham) I arranged to test both ways with another router and connection
    (to the same exchange). It was a completely different router: mine is a
    D-Link, the other a Netgear.

    In short, I found that my router behaved poorly on both connections, but
    particularly on my line, while the other router was consistent.

    Here are the stats, as a table (NM = noise margin (downstream), dB; DS =
    downstream sync, Mbit/s; US = upstream sync, Mbit/s):

    My connection Other connection
    My router 0.0 8.7 0.44 1.0 18 1.0
    Other router 3.0 8.7 0.44 3.0 20 1.2

    Both routers reported the same attenuation (45dB and 20dB respectively)
    for my and the other line.

    Based on this alone, it obviously looks like my router is broken (though
    the other facts give some doubt that this is the full story). Given
    this, and being keen to get a reliable connection, I bought a new
    router, which I am now using. It seems to be working OK and behaving
    similar to the Netgear router (currently 3.1dB margin, 8.3Mbit/s

    Alex Fraser, Feb 20, 2015
  7. Thanks for the detail Alex.

    You're right, doesn't look like the attempt to fix the SNR worked.

    I notice that interleaving has been applied to your circuit though so
    I've placed an order to have it removed as per your ticket request.
    Plusnet Support Team, Feb 23, 2015
  8. Alex Fraser

    Alex Fraser Guest

    Based on what my router was telling me ("Channel: FAST" but non-zero
    "RSCorr"/"RSUnCorr" counts - and equivalent with the old D-Link router),
    I had FEC but not interleaving. Yes, I know that's not normal.

    Anyway: no interleaving or FEC now, 1.1Mbit/s up, and the connection
    seems basically stable, although the router reports downstream CRC
    errors at a rate of around one per minute - I'm not sure at what point
    this is worth addressing by increasing target SNR (no obvious problem,
    ie packet loss, though, so I guess the answer is "a higher rate than that").

    Thanks for your help,
    Alex Fraser, Feb 24, 2015
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