SNR margin & router questions

Discussion in 'Broadband' started by Graham J, Oct 21, 2008.

  1. Graham J

    Graham J Guest

    I'm about 5km from the local exchange and use a Vigor 2600VGi router (price
    about £150), with the ADSL service provided by Zen. I need the Vigor for
    its VPN capability.

    I see typically:-
    448k Up Speed : 1440k Down Speed : 12.5 SNR Margin : 62.5 Loop Attenuation

    I accept that the 62.5 dB attenuation is fair for the distance.

    I tried a different router - an Edimax AR-7084A (price about £20) This
    typically shows:-
    448k Up Speed : 2464k Down Speed : 5.5 SNR Margin : 63.5 Loop Attenuation

    Clearly the lower SNR margin allows a much greater download speed. So why
    do the two routers behave so differently on the same line?

    At ths same location I have another line with the ADSL service provided by
    Demon. I see much the same performance difference between the two routers
    on that line.

    I've tried more modern versions of the Vigor router - for example a V2800
    and V2820 - both these show much worse performance, typically:-

    448k Up Speed : 800k Down Speed : 18.0 SNR Margin : 62.5 Loop Attenuation

    Can anybody explain why a premium price router should behave so poorly
    compared with an economy product?

    In March this year BT broke some wires leading into the village and a good
    many people were without any phone or ADSL service for several days. Before
    that time, my Vigor router would report:-

    448k Up Speed : 1900k Down Speed : 6.0 SNR Margin : 63.5 Loop Attenuation :

    When BT fixed their wires, they did something which appeared to affect the
    performance of the Vigor routers. Can somebody explain this for me?

    Graham J, Oct 21, 2008
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  2. Graham J

    Guest Guest

    You can't make such simple comparisons. The modem syncs at a speed which
    gives an SNR margin set by the exchange. The default margin is 6db but the
    exchange will increase it if the line looks unstable (frequent re-syncs)
    which is often the case on longer lines.

    The modem stays at that sync speed and as noise varies the SNR margin also
    varies unless the margin gets so low that too many errors occur and the
    modem will resync at a lower rate to restore the margin set by the

    You can only make comparisons by looking at sync rate and SNR margin
    immediately after the modem has synced and with minimum delay between the
    comparison in the hope that noise levels have not changed. Both modems
    should sync with about the SNR margin set by the exchange and the
    comparison will be in the sync rate achieved. From the SNR margins you
    quoted it looks like you didn't do this.

    That said the design and/or ADSL chipset used in the Drayteks isn't very
    good on poor lines tending to sync at lower rates and resync more often
    than some other cheaper modems.

    If you really need the features in the Draytek I think they do a router
    (2910?) that uses external modems, not sure what the requirements are for
    suitable external modems.

    It would be worth browsing the Draytek uk support forum where you will find some discussions about
    modem performance and probably some talk about the router.
    Guest, Oct 22, 2008
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  3. Graham J

    Grumps Guest

    I have a Linksys router that used to sync at 10M using Nildram LLU. I
    changed ISP to O2 and the same router immediately sync'd at 8M with a 13dB
    margin. I swapped to the supplied O2 box and it managed a connection of 13M
    with a 6dB margin. Is there something that the O2 equipment does differently
    with the O2 box than with other ADSL2+ routers? I can't explain why the
    Linksys won't drop the margin to get a better rate, even when it has been
    left on for a few days.
    Grumps, Oct 22, 2008
  4. I also have a long noisy line and have tried several routers including a couple of draytechs.

    By far the best results are with a BT2700 HGV (aka 2Wire). This out performs everything else I have
    tried on my line. They are dead cheap on ebay.

    See here fro much discussion on this router:

    HTH - Adam
    Adam Lipscombe, Oct 22, 2008
  5. Graham J

    PeterC Guest

    Is there an equivalent ADSL modem that's wired? The 2700 looks good, but I
    don't want wireless.
    PeterC, Oct 22, 2008
  6. Graham J

    Graham J Guest

    Two years ago the SNR margin would often drop to 2dB (similar attenuation
    at 62dB) and although the router would claim to have a connection it would
    not carry any significant traffic - even pinging the Zen default gateway
    would fail most of the time. Rebooting would always cure the condition, and
    the SNR margin would stay at about 5dB for a few days. Speed was typically

    So I installed the Long Line firmware in the self-same V2600VGi router;
    performance then became more reliable. The SNR margin varied between 4dB
    and 8dB, attenuation was reported as 63.5 dB, and speed dropped slightly to

    The only change since then is that BT broke the wires in March. And since
    then the SNR margin has been 12dB to 14dB (loss 63 dB) and speed only about

    I tried converting the router to standard firmware and now see no difference
    whatever between the two versions of firmware.
    Graham J, Oct 22, 2008
  7. Graham J

    Graham J Guest

    Thanks. Looked through the forum back to mid 2005, seacing for "set snr
    margin" :::

    The topic at:

    suggests that you can't set the SNR margin - it's in the wishlist ...


    The term "codinggain" might be relevant ...

    ... says that it isn't supported in the chipset.

    If you know of specific instructions can you please post a link?
    Graham J, Oct 22, 2008
  8. Graham J

    PeterC Guest

    Thanks Peter, that's useful to know.
    PeterC, Oct 22, 2008
  9. Graham J

    Graham J Guest

    I know it can't be done on the 2600 range.

    But I can't find anything on the Vigor forum to show me how it can be done
    on the 2800 or 2820. Please can you post a link to suitable instructions
    Graham J, Oct 23, 2008
  10. Graham J

    ianh Guest

    We are 9.6 km from the exchange and are running an ADSLnation X modem
    straight out of the box on one line and it syncs ok. on my 2nd line i
    have the same setup and it just wont sync..

    speed isnt the best... 160 D 352 U 63.9db D 31.5db U snr
    varies between 0.4 and 1 D and 7 U

    but its better than dial up...

    Ian H
    ianh, Oct 23, 2008
  11. Graham J

    Graham J Guest

    I take your point, but in my case the router will stay connected for days at
    a time. Even if the connections drops it re-establishes itself almost

    The issue is the unreasonably high SNR margin exhibited only by Vigor
    routers, and only since BT broke the wires (jamming too many joints in a
    junction box that was too small) in March.
    Graham J, Oct 24, 2008
  12. Graham J

    Graham J Guest

    But I have searched the forum, and all recent posts still show it as a
    wish-list item !!

    Graham J, Oct 24, 2008
  13. Graham J

    Graham J Guest

    This came from the BT technician that I spoke to some days after the event.
    It may of course be lies, or irrelevant.

    The original fault occurred in March, and many (20+) customers (in a village
    of 100+ households) had no phone service. After several days the service
    resumed; and the two lines coming into my property both showed reduced
    performance once the repair was complete.

    This type of fault has happened several times before, it just has not
    affected me personally. Neighbours have complained of no phone service and
    asked to use my phone to ring BT to log the fault. When several have asked
    that favour, I've often looked for the Openreach van and spoken to the
    technician - usually the explanation has been that the faults were created
    by the previously attending technician.

    When the fault affected me, I pressed the reparing technician about it who
    explained about jamming too many joints into the one junction box; and that
    a proper repair would entail the installation of a second junction box.
    These appear to be black pods fitted on a pole. Some months later, I spoke
    to another technician who explained that the new junction box had been
    fitted so there should be no further risk of faults introduced inadvertently
    by technicians attending to carry out other work.

    As to why the repair has affected the ADSL performance is anybody's guess.
    And why the problem particularly affects Vigor routers is even more of a
    mystery. It is known that the Vigor routers perform less well on long noisy
    lines (and prior to March the Long Line firmware on a V2600 did make an
    improvement, whereas now the performance is the same regardless of which
    version of firmware I use).

    I suspect the issue is some sort of crosstalk that the Vigor perceives as
    noise. I've tried V2600, V2800 and V2820. When I use a cheap Edimax router
    in place of the Vigor the circuit initialises with the same SNR margin and
    speed as the Vigor (typically 12dB and 1300kbits/sec for the V2600; 18dB and
    800kbits/sec for the V2820) ; and within seconds reports an improvement to
    6dB and 2400kbits/sec.

    Another possibility is that the re-wiring necessitated the use of different
    DSLAMs at the exchange; and that these perform differently.
    Graham J, Oct 24, 2008
  14. It isn't possible - directly - on the 2800. The chipset doesn't support

    A measure of control over SNR is possible by altering ADSL MAXDNRATE and
    ADSL CODINGGAIN, accessible via the telnet command line. They aren't
    documented well, so you have to experiment.

    John Livingston, Oct 24, 2008
  15. Graham J

    Graham J Guest

    But I need the VPN capability offered by the Vigor.

    However I have a work-around. I now use a Vigor 2910 (i.e. Dual-WAN
    ethernet router) and an Edimax router configured in bridge mode as the
    modem. This works fine; and one can still communicate directly with the
    Edimax router to monitor the line state and statistics. In due course I
    will connect a second edimax router and set up the Vigor to perform load
    Graham J, Oct 24, 2008
  16. Graham J

    Graham J Guest

    Yes, I tried both of them. I can reduce performance, but not increase it.
    Possibly useful for increasing reliability ...
    Graham J, Oct 24, 2008
  17. Beware of tweaking routers too far though.

    A firmware update on my very old one resulted in an undocumented flag
    being set to basically turbocharge the chip to an inch of its life. I
    was getting massively fast speeds and dozens of dropouts.

    As a result the connection was almost unusable, and BT dropped my noise
    margin to 15dB.

    When I found the undocumented thing by trying every parameter that
    telnet revealed, my sync speed dropped by HALF and I had to get my ISP
    to get BT to reset the noise margin to something saner.

    What this shows is that despite what the DSLAM suggests, the router can
    be set to ignore it, but you may well trade speed for reliability.
    The Natural Philosopher, Oct 24, 2008
  18. Graham J

    Guest Guest

    MAXDNRATE just limits the maximum sync rate. CODINGGAIN squeezes some
    (notional) additional signal gain out of the modem which increases the
    reported SNR and so the sync rate for a given SNR. The penalty is more
    susceptibility to noise, probably more and/or bigger errors. When I was
    using a 2700 I had coding gain up a couple of clicks to ensure I synced at
    8128 and got the last BRAS profile step without it it would sometimes sync
    at 8096 or 8064.

    Guest, Oct 24, 2008
  19. MAXDNRATE gives problems when the line is set to interleaved. I found it
    would usually fail to train - repeated unsuccessful attempts. It seems
    to work for others though. I guess it might depend on the DSLAM vendor
    or firmware.

    John Livingston, Oct 24, 2008
  20. Graham J

    Graham J Guest

    Can I manage the 2Wire remotely?

    Will it support multiple LAN to LAN VPN connections? Alternatively can I
    configure it in bridge mode so I can use a Vigor for the VPN connections?
    Graham J, Oct 24, 2008
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