How BT get faster speeds than any other ISP?

Discussion in 'Broadband' started by The Natural Philosopher, Aug 2, 2011.

  1. Seems to be obvious.

    Sine my line was fixed I could sustain a really nice 5Mbps at around
    6- 8dB SNR.. However despite having zero issues at this speed, the SNR
    margin has been progressively raised until its now at 15dB.

    Possibly connected with the fact that I spent some serious time this
    weekend watching the BBC HD F1 stream..and listening to test match
    special over the internet? This consuming lots of bandwidth..

    Arte BT using SNR margins to control traffic? And if they are, how do I
    prove it, and who can do something about it?

    I am fairly sure its outside the ISP's powers to fiddle with this,
    beyond asking if it can be reset..

    Comments welcome...
     
    The Natural Philosopher, Aug 2, 2011
    #1
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  2. The Natural Philosopher

    Invalid Guest

    Possibly - would make a fairly good conspiracy theory, but the
    relationship between BT Retail (the ISP) and Openreach who own the ADSL
    kit that controls the SNR is officially the same as between any other
    ISP and Openreach. I doubt that Openreach give BT any more (or less)
    control than they give to other ISP's.

    A more likely explanation is that your line has high(ish) error rates.
    The things the router reports as things like

    SF (CRC) Errors:
    RS Corrected:
    RS Un-Corrected:
    HEC:
    Errored Seconds:
    Severe ES:
    etc.

    I understand that the DSLAM in the exchange monitors these things and if
    the error rate rises "too high" then it increases the SNR to reduce
    them. The theory being that high error rates mean lots of retransmission
    and hence slow(er) real speeds and inefficient network use.

    [I seem to remember reading somewhere that because of the design of the
    ATM protocol if the ADSL error is uncorrectable by the router then
    packet retransmission takes place at the TCP level from the IP's kit not
    from the exchange - so its the load this places on their backbone
    network they are trying to avoid; as well as trying to give you the
    fastest possible download experience :)]

    If you have got an error prone line, then a very heavy usage weekend
    would have increased the numbers of packets passed and consequently the
    number of errors recorded by the DSLAM. So it reacted.

    If you can monitor the error rates (Routerstats does a good job if you
    have the right router) you might get some idea as to the problem.
     
    Invalid, Aug 2, 2011
    #2
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  3. well further to this I said 'what the hell' and phoned up the ISP.

    Who said something very strange..

    "Your UPSTREAM error rate is very high. That's why it's reset the noise
    margin."

    I was seeing no errors at all reported on the router going upstream though.

    And precious few downstream either. But I suppose that correctable CRC
    errors that the DSLAM detected would not necessarily be reported back to
    the router..only completely damaged packets that needed retransmits..not
    sure how the ADSL works at that level.

    Anyway after mulling that one over, I asked for a noise margin reset and
    currently the line is holding a shade under 6Mbps at a noise margin of
    6dB. And no interleave, which is a first EVER. I doubt it will hold that
    through the night-time MW transmissions, but it should settle out over
    5Mbps anyway.

    The tech droid was insistent - no idea why - that this was something
    injecting noise at my end. He didn't seem to grasp the basic fact that
    the UPSTREAM signals were weakest at the DSLAM end, and therefore more
    likely to be affected by noise further up the line... a lecture on
    communication theory being of no profit I simply asked that the line be
    reset.

    Which happened LESS THAN A MINUTE after I put the phone down.

    I am really puzzled by the whole affair. If what he was saying was
    correct, that could only be LF trash of some sort right up at the
    exchange... some kid with a poorly suppressesd motorbike? someone
    fiddling with wires?


    Mind you, its does suggest a way to bugger up your neighbourhood DSL.
    switch your router OFF and then get a spark transmitter going. When they
    are totally narfed at 24dB margin and crawling, you can reboot your
    router and get a clean almost uncontended connection :)

    Any other thoughts welcome.
     
    The Natural Philosopher, Aug 3, 2011
    #3
  4. The Natural Philosopher

    Invalid Guest

    If your line is clean apart from the overnight drop in SNR caused by MW
    interference, you could try my approach.

    I use an Ex-Sky DG834GT router (£10 off e-bay) re-flashed with DGTeam's
    firmware. This allows you to tweak the SNR target, and the tweak
    survives a router restart.

    My line (3.5km all overhead 40db attenuation ) varies about 2dB from
    best (mornings) to worst (late evening). So a daytime reboot at a 6dB
    target gets me 6500 ish in sync speed. The router will normally hold on
    down to about 3-3.5db, but any spikes significantly below that will
    cause a resync. A restart or line drop in the late evening when MW is
    at its worst gets me 4500-5200 ish.

    I have tweaked the router to restart at 66% of SNR (so it aims for about
    4dB). A spike at night that drives SNR below 3db causes a recovery to
    4dB and 6400-6500 which the router can easily hold at night in the
    absence of spikes. (current WAN uptime is approaching 1000 hours)

    The downside is that if I (or the power company!) restart the router
    during the day I get 4dB and see sync speeds in excess of 7000 - knowing
    full well that a dropout will happen when it goes dark.

    But it all conspires to maintain my BRAS profile at 5500 or 6000 ( a
    significant improvement on my neighbours 4000-4500).
     
    Invalid, Aug 3, 2011
    #4
  5. I am doing that sort of thing already. Actually all I want is 4000 BRAS
    so I can watch more F1 in HD. Brilliant!


    Burt if what I think has happened did happen, then its basically a bum
    algo in the BT DSLAM stuff..you shouldn't lose downstream noise margin
    for weeks just because you get a bit of corrupted upstream packets..

    ...which then vanishes..
     
    The Natural Philosopher, Aug 3, 2011
    #5
  6. The Natural Philosopher

    Mark Guest

    FWIW I've seen target margins rise when the line is apparently
    (almost) error free. Many people will scream I am wrong but I have
    observed this too. For years my line was stuck at 9 and 12dB but
    recently has dropped back to 6.

    The algorithms that control the noise margin are secret and I've never
    met anyone who knows what they are.

    I've not had a huge amount of success tweaking the noise margin with
    the dgteam firmware. It just causes instability for me but YMMV.

    You might be better off with interleaving on since forward error
    correction will then be enabled too.

    And I dream of an IP profile of 4M. The best I've got in recent years
    is 2.5M.
    --
    (\__/) M.
    (='.'=) Due to the amount of spam posted via googlegroups and
    (")_(") their inaction to the problem. I am blocking some articles
    posted from there. If you wish your postings to be seen by
    everyone you will need use a different method of posting.
     
    Mark, Aug 9, 2011
    #6
  7. Well Mark, at 5:30am Monday the carrier went COMPLETELY.

    It came back at 1pm today after the usual 'it must be your router, there
    is nothing wrong with the line' shenanigans from the ISP. And Me
    insisting it wasn't ME. Like what sport of changes would I have made at
    5:30 am?


    I got a new router (waste of £30) and it said the same. 'No signal'...

    Now sizzling at 6.52Mbps..which its never ever achieved before..(and
    probably wont hold for long, either)

    I am guessing that something was failing at the exchange, and FINALLY
    they have swapped it out..
     
    The Natural Philosopher, Aug 9, 2011
    #7
  8. The Natural Philosopher

    Phil W Lee Guest

    If you're really really lucky, the something that was failing at the
    exchange will be the same something that was restricting your speed,
    and the 6.52Mbps will stick (or at least only reduce a little).
     
    Phil W Lee, Aug 9, 2011
    #8
  9. It is remarkable that I get that.

    I've had a slew of issues that led to a complete line switch and much
    less attenuation, but all the final resolutions have been 'at the
    exchange' and with no word by Openreach as to what they consisted of.

    I probably won't hold 6.2 Mbps through the night: CRC errors are piling
    up a bit.
     
    The Natural Philosopher, Aug 9, 2011
    #9
  10. The Natural Philosopher

    198 kHz Guest

    I sometimes wonder *if* anyone knows what they are.

    My normal 6dB margin went up to 12 after a line fault a couple of years ago,
    and stayed there for several months. Then I lost service because of a MUX
    fault in the exchange, and whaddya know, on restoration I was suddenly back
    to 6dB. Coincidence - surely not.

    Earlier this year the margin went up to 9dB for no apparent reason. A few
    weeks later, I had occasion to try a mate's router on my line to prove it
    faulty or otherwise, and it connected at 6dB, where it's remained since.
    Again, I can't believe it was coincidence.

    Conclusion: if the DSLAM is playing silly buggers, try surprising it. ;)
     
    198 kHz, Aug 9, 2011
    #10
  11. No, that is not. They seem to always reset everyone whose been affected
    by a line fault or a mux fault.

    What probably happened is that you resynced during a noise burst and
    simply didn't notice when the burst went away, and the margin reset itself.

    Or your mates router has a different coding gain blah blah.

    Its possible to synch at different noise figures on the same noise margin...
     
    The Natural Philosopher, Aug 10, 2011
    #11
  12. The Natural Philosopher

    Gaius Guest

    Those who DO know are the people in BT Labs. ("Adastral Park" aka
    Martlesham). They design and maintain the algorithms.

    A few of them used to lurk on this group, but you're unlikely to provoke
    any comment from them - the internal BT punishments for being "The
    Honest Engineer" are severe.
     
    Gaius, Aug 10, 2011
    #12
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