Cisco 837 Noise Margin bug?

Discussion in 'Cisco' started by Mark Knight, Apr 10, 2006.

  1. Mark Knight

    Mark Knight Guest

    Whenever my 837 ADSL router re-trains to a lower speed (tends to happen
    in the evenings), the reported "Noise Margin" goes negative:

    router>show dsl int atm 0
    ATU-R (DS) ATU-C (US)
    Modem Status: Showtime (DMTDSL_SHOWTIME)
    DSL Mode: ITU G.992.1 (G.DMT)
    ITU STD NUM: 0x01 0x01
    Vendor ID: 'ALCB' 'TSTC'
    Vendor Specific: 0x0000 0x0000
    Vendor Country: 0x00 0xB5
    Capacity Used: 100% 54%
    Noise Margin: -51.0 dB 22.0 dB
    Output Power: 20.0 dBm 5.0 dBm
    Attenuation: 29.0 dB 14.0 dB
    Defect Status: LOM None
    Last Fail Code: None
    Selftest Result: 0x00
    Subfunction: 0x15
    Interrupts: 19714 (1 spurious)
    Activations: 3
    Init FW: embedded
    Operartion FW: embedded
    SW Version: 3.9.220
    FW Version: 0x1A04

    IOS version is Version 12.4(7).

    SNMP also reports a negative margin for the download.

    ADSL-LINE-MIB::adslAturCurrSnrMgn.15 = Gauge32: 327170
    ADSL-LINE-MIB::adslAtucCurrSnrMgn.15 = Gauge32: 220

    The line is perfectly stable when the margin does negative so this
    doesn't seem to affect operation. The line typically re-trains from
    approx. 8000 to 6500 to cause this set of figures.

    Am I missing something about Cisco's margin calculation or is this just
    a simple bug? If the modem later re-trains to any faster speed the
    negative margin is cleared (along with the Defect Status). I would
    expect the margin figure to reflect the margin with the current line

    Separately, is there an easy way to force a more conservative maximum
    connection speed to avoid the re-trains that each cause a short outage?
    I've tried altering the "dsl noise-margin" offset without success.

    This is a GB ADSL line on the new BT "Max" service.

    Mark Knight, Apr 10, 2006
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  2. Mark Knight

    Merv Guest


    dsl linerate <kbps>

    kbps - line rate, in kilobits per second, for the DSL ATM interface.
    Allowable entries are 72, 136, 200, 264, 392, 520, 776, 1032, 1160,
    1544, 2056, and 2312.
    Merv, Apr 10, 2006
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  3. Mark Knight

    Mark Knight Guest

    "dsl linerate" does not appear to be a supported parameter on the 837.
    Trying it fails and dsl ? only shows:
    Mark Knight, Apr 10, 2006
  4. Mark Knight

    Merv Guest

    have you tried playing with the 'dsl power-cutback" command ?
    Merv, Apr 10, 2006
  5. Mark Knight

    Mark Balaam Guest


    I have similar issues with stability on the new BT Max services. Though
    this 837 doesn't re-sync, it just suffers packet loss and then disappears
    off the internet.

    By my reconing this in the interface atm0 config might help:-

    dsl noise-margin 3

    Which looks like it will force the router to train with a margin of 9. It
    may also be possible to use the dsl gain-setting tx-offset command to
    further improve things.

    Will let you know how I get on.


    Mark Balaam, Apr 10, 2006
  6. Mark Knight

    chris Guest

    I don't think that it's a bug. I have the same issue and when my Noise
    Margin goes into negative figures the ADSL kit at the ISP shows negative SNR
    to tie up with what I'm seeing. I've just been trialing the 8Mbps service
    but my router would only train up to about 4.9Mbps and then after a few
    hours I would see bucket loads of errors. A hard reset would make it okay
    again for a while.

    So, a negative Noise Margin would suggest a line issue. Most likey you are
    pushing the bandwidth to more that the line can handle.

    chris, Apr 10, 2006
  7. Mark Knight

    Mark Knight Guest

    I tried this; it certainly helps. However I think I could do with it
    being set a little higher (still getting approx. 1-2 disconnects a day).
    Sadly the upper limit seems to be 3 :(

    As for the negative Noise Margin I mentioned, this only seems to occur
    when "dsl enable-training-log" is in the running-config.

    Mark Knight, Apr 24, 2006
  8. Mark Knight

    Mark Knight Guest

    Doesn't that only affect the upstream and default to the most
    conservative setting?
    Mark Knight, Apr 24, 2006
  9. Mark Knight

    anybody43 Guest

    Noise Margin: -51.0 dB 22.0 dB

    Just in case this is not obvious.

    -ve dB is not less than zero.

    It is less than one and greater than zero

    i.e. fractional. Think of it as referring to the position of the
    decimal point. (or octal point, or...)

    3 -> 1000
    2 -> 100
    -2 -> 0.01
    -51 -> well, quite small.

    -51dB would seem to me to be a _very_ small number
    if the value is sometimes positive.

    Hoping I recall correctly;-)
    anybody43, Apr 24, 2006
  10. Mark Knight


    May 12, 2006
    Likes Received:
    Cheshire UK
    I believe your description is likely to cause more confusion than it solves.

    Firstly, while you rightly define a -ve SNR as not implying a negative power, it does tell you that the noise power is greater than the signal power. Not good, if measured in the same bandwidth as the active signal (or in the case of adsl, signals plural). So it is unlikely that a system (with simple coding) that is maintaining sync and has a lowish error rate, really has a SNR of less than about 6dB.

    Secondly, your translation of 3 to 1000 is not too helpful either, for these are deciBels, not Bels. A ratio of two powers is expressed in deciBels (dB) as 10 * Log (base 10) (Power 1 / Power 2). Note the factor of 10.

    So 3dB is a power ratio of about 2, 10dB is a power ratio of 10, 20dB is a power ratio of 100, 30dB is a power ratio of 1000 etc.

    -50dB signal to noise means that the noise has 100,000 the power of the signal, so if the signal was 1mW (0dBm) the noise would light a 100W light bulb.:shock:
    jtevans, May 12, 2006
  11. Mark Knight


    May 29, 2006
    Likes Received:
    I too own a Cisco 837 and am using DSLMax... we also see marginal noise margin values, often below the recommended 6db and we too see it falling to very low figures, like -40db.

    I have contacted cisco and have some information.

    Firstly, we have not tried this yet, but if you have a BT NTE5 face plate on the master socket, try plugging the adsl filter into the test socket, if the line conditions improve slightly, it maybe worth investing in a filtered socket like the XTE-2005 from ADSLnation...

    I too have used the dsl noise-margin 3 command with some success, however, still seeing errors, although no disconnects.

    I have updated firmware for the modem, version 3.9.221 and have made it available here.

    Please rename it to "adsl_alc_firmware.bin", and copy it to the flash. The trainup may be a slightly longer than using the original firmware.

    After it, the show dsl interface atm 0 output should looks like this:



    Init FW: embedded

    Operartion FW: embedded

    SW Version: 3.9.220

    FW Version: 0x1A04



    Init FW: ASW_init_3_9_221.bin

    Operartion FW: ASW_R3_9_221.bin

    SW Version: 3.9.221

    FW Version: 0x1A04

    Then, if this FW upgrade not provide the expected results, we can remove some bits from the bins, with the command 'dsl max-tone-bits' under interface ATM0 configuration command. This will maximize the coded bits per each frequencies, so it can limit the speed. It is similar than 'flattening' the lineshape. Default value for this command is 14, so you are looking to lower it to 13 in increments of 1 to the value of 2.


    I found the dsl max-tone-bits command made no difference, but, the dsl gain-setting tx-offset 3 command gives 3db less attenuation upstream, which is a good thing.
    Last edited: May 31, 2006
    some1, May 29, 2006
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