Thursday, 24 November 2011

20CN not fit for purpose?

Interesting discussions with our favorite telco this week. As I am sure people know - we are somewhat tenacious in getting issues fixed, and we have a customer (in a small village) that is having problems with congestion most evenings.

Now, we do a lot of testing and we confirmed that he gets packet loss and latency on the link from us to him when this happens. Clearly showing the problem is within the wholesale/DSL network. Testing from our router means we can be sure it is not "the Internet" being slow, which is, of course the first thing our suppliers suggested!

So we chase our suppliers and they say the VP (yes, an ATM VP for 20CN) is not congested. They claim it is below 25% of its capacity.

We got the figures from them eventually. It seems they are happy for a single 10Mb/s link to be used by 400 customers even though those customers can get 7.15M each on their links.

Now, there are contention levels - you don't just add up all the links, but this level of contention and such a small aggregate link makes no sense to us. If two of the 7.15M customers try to access the internet at the same time they get badly congestion. If one of the 400 customers torrents they would hog all of their 7.15M leaving under 3M for the remaining 399 customers.

We are, of course, trying to get them to fix this. They can simply increase the capacity on the VP. They are pretty much refusing to, saying their planning rules allow 400 customers on it. In practice we are seeing congestion with only 100 customers on the link, and are not at all surprised.

Very frustrating.


  1. Aren't "consumer" ADSL lines rated at ~50:1 contention, or has that number gone away now?

    By my calculations, this exchange is running at more like 300:1!

  2. As the owner of the line in question it's a total farce, especially when most rural lines struggle due
    to line length for this to happen is just a huge also in the face.

    AA have been brilliant with lots of Updates and really leaning on.

    Second time this has happened now BT really need to sort themselves out.

  3. Richw, plan is to try and fill the line and get BT to admit that the link is full - or explain why a non-full link has loss and latency.

  4. Any chance you could name the small village in question as we might then be able to investigate a little further?

  5. The old 50:1 and 20:1 contention ratings disappeared with ADSL Max.

    Nowadays, the theory is that BT aim not to congest anywhere in the BT network, so that congestion occurs at the link from the ISP to BT Wholesale. In turn, when this link congests, the ISP has to pay BT to get it upgraded.

    Practically, there are exchanges where fixing congestion is "too expensive", and BT try not to fix it. 21CN makes this better, by making big chunks of capacity cheaper than any 20CN capacity, but if you're in a 20CN only area...

  6. Nearly all of our old 20CN customers get abysmally slow speeds during peak hours despite us having oodles of free capacity for them. I personally figured that they stopped upgrading the 20CN network links as soon as they started rolling out 21CN.

  7. Useful, thanks.

    We usually managed to get them to update VPs when we see issues, but maybe you are right. Not really good enough though is it?

  8. Im sad to say that this is a issue where we live too. we are on a 20cn exchange with just under 700 lines on it and have had a huge number of congestion issues and still do to some extent.When a Dslam failed in the exchange (apparently water leaked into it !!!!) we had 5 engineers out on seperate jobs over 3 weeks to work out the problem and fix it and while chatting with one over a cup of tea i mentioned about previous congestion issues before we elevated the line and he said we have a 20mb VP over a 34Mb Atm (im assuming the other ~ 10mb payload is for voice?)! for normal day-day traffic there seems to be no issues but whenever there is some event on or even just a incremental ios download or at random times when two people happen to be downloadig something at line rate the village chatter in the shop turns to ooh broadband's/internet/interwebz/online ect is being slow today ect. We had these issues and within three months had switched to a provider that buys the elevated product with gives us nearly twice the upload speed and priority on the VP so we don't see congestion anymore however i do feel a little mean when downloading at 8mbit as i know there will be the other 690 brodband users clunking along with ~12mb! The problem is though these people arn't hugely technicaly clued up on how btw's network works and will be with a isp that couldn't car less if they see congestion like bt retail,orange ect

  9. Has the sufferer written to his MP and get him to write to Chairman BT for an explanation? Questions in parliament pointing out that the "Not-Spots" are not just line length but infrastructure fails by the national monopoly might he useful.

  10. I have written to Norman Lamb the last time it happened and BT happily trotted out how wonderful they are and how they had a date to fix it which was 5 months later they managed to get it sorted much quicker and how they had everyone's best interests at heart blah blah blah - I was sick in my mouth (just a bit) from what they had written lying feckwits!!

    Well have had downloads running a lot since Thursday at different times to max out the available bandwidth and funnily enough the first evening it was pretty much bang on full speed. Friday was up/down and whenever i've checked all weekend it's been at best 3.5Mbps but mostly between 1.2-2.4Mbps when i've been watching. It'll be interesting to see what pattern emerges tomorrow when AA look at the data.

    Exchange is EACLE for anyone interested...