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Thread: Contention?

  1. #1


    Hi All,

    Now we've got the data being collected we can get on to the much more interesting debate of how we are going to use them!

    I have a strong gut feel that when my service drops from 2mb (which is really really good!) to .25 it's down to contention, however if I reboot my router at that time I get 2mb again for a couple of minutes before it drops which makes me think it's the ISP doing some kind of throttling.

    Rob and Graham, as the guys that understand this stuff better, I'd be really interested to know if there is anyway of telling if,when the service degrades, is it due to contention, temperamental service, ISP etc. Do you think we'll have enough/right kind of data to help identify such things?



    P.S. Rob, at the meeting you said that we all have a service capable of 8mb but I'm under the impression that mine is only 3mb, did you mean *all* or most?

  2. #2

    Angry BT Wholesale - ADSL Max and DSLAM/VC contention

    Hi Ben,

    Hmmmm ... yes indeed ... contention. Let's start with the bad news. The back-haul network, and how the DSLAM (Digital Subscriber Line Access Modules) line cards are configured is all proprietary knowledge owned by the guys that BT Wholesale outsource the maintenance to (Fujitsu mostly I think, but it doesn't matter). What they are unlikely to tell us (and that's fair as they are a commercial organisation) is what degree of over-subscription they are using to make the line profitable.

    Your ISP (whoever that may be) should be able to run checks to see if the "virtual circuit" that is created from the DSLAM line card back to your ISPs network is congested. All your traffic has to go over this virtual circuit before it gets routed to the Internet. That's why my IP addresses appear to be in Yorkshire where Zen internet have their core datacentre. I'm not convinced of the accuracy of this congestion measurement, as I'm seeing some strange anomalies on 2 of my 3 ADSL connections. But that's another story.

    The good news ... is if we choose to, we could organise a practical test, by coordinating a "crowd sourced" bandwidth thump with 20 or more of us concurrently downloading a large file directly from our ISP. This may prove sufficient to congest the line card(s) we're all on, but it very much depends on which line card our telephone lines are connected to in which telephone exchange. We'd need a couple of hundred people probably to stress the entire DLSAM and back-haul.

    Only by repeating concurrent tests, and proving that we were having a direct effect could we hope to draw any conclusions on traffic QoS policing that BT Wholesale were doing.

    What we are able to do with this information when we get it is also questionable. BT are under no obligation to provide more than 1200Kbps downstream on an ADSL Max line. No matter what your agreement says, that is it. So, your 2Mbps ticks that box unfortunately. What you could do is shout out to others in this forum in your neighbourhood, to check that they're seeing similar speeds. If they're all getting 6Mbps, then you've got a much better shout at BT/Sky/Whoever, *after* you've checked your own infrastructure e.g. filter, extension cables, modem, PC, firewall. If they also get 2Mbps, then it's the distance you and the others are from the switch that is the common factor, and you get whatever that is.

    What this mess should do, is motivate people to understand the marketing approach of BT and other large providers. They necessarily retain details of how they make profit. I would do in their position also - they have shareholders to keep happy.

    The only way to fix this is to take them out of the loop (excuse the pun), and have the community own the network. This is what I am working on. Providing FTTH which fixes this once, correctly, for the remainder of my lifetime at any rate (Community Fibre - FTTH). Transparent on throughput, bandwidth management, you name it. You can own part of the network so have every right to understand how it operates.

    Hope this helps.
    IM me to chat

  3. #3
    Hi Ben,
    I can only speak from my own experience but I used the white box data to achieved significant improvement. See attachment with my "before and after" SamKnows data in chart form (hope I have added it correctly).

    My ISP is BT. I wrote to BT and told them about my pathetic BB performance in the evenings and that the monitoring was being done using professional Ofcom-approved monitoring equipment. An engineer then contacted me and asked me to confirm the poor performance using the BT Speedtester (it logs the last reading taken and so you must ensure it is a really low reading). The BT minimum threshold appears to be 0.6Mbps on my contract. the engineer said there was congestion at the Langton Exchange and that one of the problems is that I was on a card with 99 other users. I was moved to a new card and during the next few weeks they fiddled around and I lost my BB for a complete week. The service then returned, and I found I was now getting a pretty stable high-end 6Mbps 24/7. I don't know how much spare capacity there is at the exchange for new cards but if there is not much then we shall soon face another contention problem.BB performance improvement.pdf. I live about 300 yards from the Exchange and so would expect a higher speed than you might get where you live. If you are not with BT then you will need to go through your own ISP.


  4. #4
    Hi Ben,

    It's also worth noting that an ADSL router will always sync the fastest it can when rebooted but will monitor the connection and negotiate down to a more stable level. What you may be seeing is that there is an issue with your line, internal wiring, router or other device plugged into your phone line that means it cannot reliably perform at the 2mb. A couple of things to try are:

    1. Replace your Microfilters - this is the first thing your ISP will ask you to do
    2. Assuming you have the correct type of BT Socket, remove the front faceplate from the BT master socket and plug the filter / router directly into the test socket you'll find underneath. Then give it the same amount of time and check the speed again.

    If after these two tests are done you are still syncing at a much lower speed than those around you then it's likely there's an issue with the line. If no.1 fixes it then great! If no.2 fixes it then this proves there is an internal wiring problem or faulty piece of equipment within your house (not BT or your ISP's responsibility). Let me know if it's no.2 or if you don;t have a BT master socket and I'll see if I can help you out.



  5. #5
    With one month of data now confirming how poor our download speed is in the evenings (0.5mbps is typical and consistent, but 6.8Mbps at 3am no problem....good for insomniacs) I have written to BT today with supporting data from Sam Knows. Will advise how this goes....hopefully an imporvement along the lines seen by Laurence (his helpful post of 28th Feb).

  6. #6

    Lightbulb Why do the speeds vary over time?

    Hi Ben,

    Without going into all the nitty gritty, if you live 20Km away from the Exchange, you'll get zip bandwidth over ADSL. It just won't work.

    The primary factors which count:

    1) Distance from DSLAM/Exchange
    2) Quality of copper line
    3) Congestion with others on DSLAM line card and anywhere after that towards the internet (BIG topic this)
    4) Local hardware/software - your router, configuration, parameters, wireless environment etc.

    The IP Stream product, or whatever BT Wholesale are calling it now, is "capable" of 8Mbps if you can tick all the above boxes in such a way that the technology can deliver 8Mbps to you. Yes, it's a crock of crap, and the phrase "up to" should be removed from all broadband advertising. But it isn't, and BT and others leverage this all the time. The only certainty is that you will NEVER get more than 8Mbps downstream/832Kpbs upstream with the current technology deployed by BT in the NDLAN telephone exchange. They need a forklift upgrade to change that.

    For some more background info you can try here:

    And for more technical detail than you ever wanted, here is BT Wholesale's own product guide explaining what IP Stream is - look at the picture on page 7 to understand the complexity of how an ADSL connection gets to the internet, and back!


    It's heavy going this if you're not a network engineer, so don't worry too much. The pictures are pretty good though.

    Why precisely you see a return to 2Mbps for a few minutes after rebooting your router is strange. That is unlikely to be anything BT or the DSLAM is doing, but more related to your local configuration. If you were seeing a gradual slowdown during typical "busy" periods (schools getting out, early evening watching the news, big Premiership football game, etc.) then that is very likely to be pure congestion on or after the DSLAM at the Exchange. Your ISP should be able to clarify whether they, the virtual circuit they get from BT for their ADSL customers, or indeed BT are to blame. Zen confirmed that Zen were not the bottleneck for me, but I couldn't get repeated speedtester results below 600Kbps downstream so they were unable to escalate within BT.

    Oh - DSLAM = Digital Subscriber Line Access Module - it's the piece of equipment installed into a telephone exchange that provides the ADSL capability over the analogue telephone lines. Not much bigger than a normal PC/server.

    The whitebox will provide you with good stats over time, which should demonstrate repeated congestion at similar times of day. This is good evidence to begin the task of going after your ISP first (you have a contract with them, noone else), then depending on their answer, getting them to go further down the line ('scuse the pun) towards BT.

    Last edited by RobL; 27-03-2012 at 01:13 PM. Reason: Spelling errors
    IM me to chat

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