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    Is this the reason........?

    This is a discussion on Is this the reason........? within the Sky Broadband help forums, part of the Sky Broadband help and support category; So, I like many others have been experiencing constant slow downs and disconnects like many other people who are on ...

    1. #1
      FragglePete's Avatar
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      Is this the reason........?

      So, I like many others have been experiencing constant slow downs and disconnects like many other people who are on Sky Broadband.

      I live a long way a way from the exchange, only when BT lifted all the initial limits a few years ago could I actually get a basic 512kbps service. At the time I was happy with that, but the Max ADSL service I've had in the past with previous ISP's I've always managed to sync at around 1.8Mbps to 2.2Mbps.

      Since going over to Sky BB last week, I've been somewhat frustrated with the constants 'events' that have been occuring on my line. So I've been in constant contact with Tech Support via E-Mail, giving them clear indications of all my problems, what I've had in the past, etc, etc. I've followed all their instructions (Test Socket, etc, etc), had my profile changed, downgraded for testing. It's taken about a day for a reply to comeback, but they've been thorough and on most occasions taken into account all that I've written.

      But..... this evening I get my last installment from Sky Tech Support, and with the rather interesting information.....

      We have come to the conclusion that when you were connected to BT's equipment, they had implemented a Dynamic Line Management protocol on your service , which would correct any errors and reduce/increase your line speed to ensure a stable connection at all times, which you would be unaware of
      So. This would imply that Sky do not have this Dynamic Line Management Protocol. How pants is this! I'm going to ask the question 'Why Not?' and see what comes back. Why would they not do this?

      I've been capped to 1Mb, but the service seems ok. A lot 'snappier' than before, but time will tell. I guess a working 1Mb service is better that a intermittant 2Mb service.

      Hope this info is of help to some people.

      Pete


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    3. #2
      Isitme's Avatar
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      Re: Is this the reason........?

      I am not sure why Sky TS should use this as an excuse. I think DLM is used on BT's ADSLMax to constantly retrain troublesome lines. It is not required on LLU lines as the connection is constantly trying for the best connection it can. The problem is that it does not always work too well, especially on long noisy lines. This is maybe oversimplifying it, but one of the more experienced members can probably explain it better.

      TomD


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    4. #3
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      Re: Is this the reason........?

      DLM isn't part of the ADSL specs. It's a BT term for all the mechanisms (some of which are part of the ADSL specs, some which aren't) by which the settings for each line are altered according to how well the line has been performing. There are some things that BT do that Sky don't, such as setting interleave depth to a level appropriate for the line, but Sky take a conservative approach on that and I should imagine that pretty much everyone will have a higher interleave depth on Sky than they would have on a non-LLU'd line.

      But as far as I can tell, the main thing that BT's DLM system does is set your BRAS profile. This doesn't affect your sync speed, but sets the maximum speed that the connection between the exchange and your ISP's network can operate at. So if you've got a line that syncs at different times between speeds or about 2.5Mbit/sec and 4Mbit/sec, DLM will set your BRAS profile to 2Mbit/sec and that's the fastest you'll ever be able to download at, even if your router actually syncs faster from time to time. Clearly this feature offers no benefit or advantage to the end user - it's purely for the BT's benefit in terms of managing their network.

      With a Sky LLU'd line, Sky's equipment is actually in the exchange, so there's no BRAS profiling because you're already on Sky's network as soon as you're at the exchange.

      There's nothing about the configuration of Sky's exchange equipment which should make a connection less reliable than if it were connected to BT's equipment. But I can't imagine Sky saying, "You know that router we force you to use? Well, the software on it doesn't work very well and the problem is particularly bad on noisy, difficult lines like yours."

      I am left wondering if, when the new software is rolled out, Sky will automatically remove all the speed caps that they've been imposing on many of their customers.

    5. #4
      IWasNotTheEnemy's Avatar
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      Re: Is this the reason........?

      can....worms....lol
      -------------------------------------------



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    6. #5
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      Re: Is this the reason........?

      Quite a difficult choice for Sky really. On the one hand, as BT start rolling out ADSL2+, if people go onto a competitor's site and see that they can get faster broadband that what they currently getting with Sky, Sky risk having a higher customer "churn" rate if they choose to keep all the existing caps, only removing them on request after the firmware update.

      On the other hand, removing all the existing caps would certainly be opening up one seriously worm-filled can all right! lol

      We shall have to wait and see, I suppose. But if Sky are feeling confident with the new software (and they should be after all the testing it's been getting), then maybe lifting the caps would be the better option.

    7. #6
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      Re: Is this the reason........?

      in an ideal world i totally agree, and perhaps it will happen once the router is on 'general release' - im sure they'd want to keep an eye on things initially before taking such a large step
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    8. #7
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      Re: Is this the reason........?

      Yeah, thinking about it, that does make sense. As new customers sign up, Sky could monitor the situation for a few months and see how many (or hopefully, how few) of them need to have their lines capped, and make a decision on the basis of that.

    9. #8
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      Re: Is this the reason........?

      Hi all,
      I'm confused!
      I thought the caps were put in place to give users, who had unstable lines, a higher SNRM so that they did not suffer from so many errors/dropouts? Can firmware have an effect that is so pronounced as to reduce such errors?

      Also, if the new firmware is good enough to reduce the number of folk who are on caps would'nt it be possible for Sky to offer a service of a higher maximum sync. than they do at present?

      Sorry, as I'm guessing I've got myself all mixed up on this!
      Graham Cutting


    10. #9
      IWasNotTheEnemy's Avatar
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      Re: Is this the reason........?

      what u complaining about? lol!!!
      -------------------------------------------



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    11. #10
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      Re: Is this the reason........?

      Quote Originally Posted by Anomie View Post
      I thought the caps were put in place to give users, who had unstable lines, a higher SNRM so that they did not suffer from so many errors/dropouts? Can firmware have an effect that is so pronounced as to reduce such errors?
      The caps are indeed there as a way of increasing the SNR margin.

      When your router connects to the equipment at the exchange, it tests the signal to noise ratio across the ADSL frequency spectrum. From the SNR value at each frequency, it takes off 6.9db (the SNR margin) then divides the result by 3db (it takes a signal to noise ratio of 3db to carry one bit of data) to give the number of bits of data that can be carried at that frequency at the basic 4.3125kHz signaling rate. If the number of bits is 2 or more, then that frequency will be used to carry data, up to a maximum of 16 bits of data.

      For example, at a frequency of 310.5kHz, my router has determined that the SNR figure is 50db. So take off 6.9db and you get 43.1db, then divide that by 3db and you get 14.367, so the router can use that frequency to transmit 14 bits of data at the basic signalling rate, or 60,375 bits per second.

      Now let's say that I start to get some interference at that frequency - some noise. As the noise goes up, the signal to noise ratio goes down. Let's say it drops to 40db. Well that's dropped by more than the safety margin of 6.9db, so that frequency can no longer carry 14 bits of data, so errors are going to start occurring. A piece of clever mathematics in the router (the Reed-Solomon error correcting algorithm) can work out, for every 233 bits of data transmitted if an error has occurred in anything up to 16 bits out of the 233. Not only that, it can work out which bits were wrong, so the router is able to (a) correct the data and (b) identify on which frequency the error is occurring. What the router is then supposed to do is say to the exchange, "You know I told you to transmit 14 bits of data at 310.5kHz? Well, I'm starting to get errors on that frequency, so I want to move 3 bits of data to a different frequencies where there are no errors, so that only 11 bits of data are being transmitted on 310.5kHz from now on."

      Now you can see the wisdom of leaving that 6.9db safety margin. If there's another frequency where there are no errors, there's a chance that that frequency might be able to hold an extra bit (or maybe even two extra bits) of data. Find three frequencies where no errors are occurring, and you've got candidates for for where to put the 3 bits of data that you're moving from 310.5kHz. Of course, the signal to noise ratio might have dropped at these other frequencies as well, maybe not enough to start causing errors, but enough to stop them taking on an extra bit of data. If that's the case, then that extra bit will have to be moved again to a different frequency. Eventually, the router should find some frequency that is able to take an extra bit, since it did allow a 6.9db safety margin at every frequency across the ADSL spectrum.

      It's this process, called bit swapping, that the Sky router doesn't do very well. As errors start to be detected, the router should start bit swapping, but it doesn't seem to do it, at least, not reliably. But if the signal to noise ratio drops even further, you start to end up with more than 16 error bits out of every 233 bits of data, and at that point the Reed-Solomon algorithm can only detect that there is an error, it can't detect where it is, so in turn the router can't work out on which frequency the errors are occurring and it just has to sit and watch the connection deteriorate, until there's uncorrectable errors in virtually every packet of data. At that point, the only solution is to "retrain" the modem, so it can learn the new signal to noise ratio profile across the ADSL spectrum, which is what you're doing when you reboot the router.

      Anyway, by increasing the SNR margin from 6.9db to a higher figure, the router will be able to cope with a worse decrease in signal to noise ratio before it has to try and do this "bit swapping". But of course, with a higher SNR margin, fewer bits of data can be carried at each frequency, and so the overall sync speed is likely to be lower (unless the total number of bits was well above the maximum sync speed already). The SNR margin can be increased either directly by changing the router's ADSL driver configuration (this is what the Mognuts utility page does), or indirectly, by Sky setting a cap on the maximum speed that the line will sync at.

      Now the hope is that the updated router software will do bit swapping properly, so when dusk comes and the radio signal from Südwestrundfunk Zwei starts interfering with data on my ADSL line being carried on the 1017.75kHz channel (SWF2 broadcasts on 1017kHz), instead of the router just experiencing more and more errors at that frequency, it will start to move, one by one, bits of data which were carried at 1017.75kHz to other frequencies that aren't deteriorating. So the errors are always kept at a level that the Reed-Solomon algorithm can correct for, and I just carry on using my broadband connection, oblivious to what's been happening.
      Last edited by James67; 19-10-07 at 12:06 PM. Reason: Little typo in the figures, but I got the maths right!

     

     
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