Can someone explain contention?

Discussion in 'Broadband' started by Cullen Skink, Sep 28, 2005.

  1. Cullen Skink

    Cullen Skink Guest

    When a broadband service says they have a contention ratio of say 50:1
    what is it that the 50 customers are sharing? If all 50 customers are on
    the ISP's 512kbps service they can't be sharing a single 512kbps
    connection as you would notice slow down all the time. I'm guessing it
    is a much larger "pipe?"

    tia
     
    Cullen Skink, Sep 28, 2005
    #1
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  2. forget "50 customers"

    50:1 is a ratio. Lets call it 25600 : 512 instead. Or 450:9, etc.

    Contention is defined as the amount of bandwidth sold to customers
    divided by the amount available at the point in question.

    So if 400 people with 1M lines are contended 50:1 there will be a
    connection capacity of 400 * 1 / 50 = 8 Mbits/s.

    Also note that in BT ADSL the ratio is "up to" 50:1 and generally held
    to be about 40% of that level in practice.

    Phil
     
    Phil Thompson, Sep 28, 2005
    #2
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  3. Cullen Skink

    Tony Raven Guest

    Nope, right first time. It relies on all 50 customers not requiring
    bandwidth simultaneously but if they did the connection speed would slow
    to 10kbps for each of them until demand started to drop again.
     
    Tony Raven, Sep 29, 2005
    #3
  4. Cullen Skink

    Mark Guest

    I presume by that you mean in the second mile. (BT DSLAM to IP core)

    If a reselling ISP has too many users* on a BT Central, I guess that
    can become (or more likely, is) the determining factor. Correct?


    *Around 8000 or more
     
    Mark, Sep 29, 2005
    #4
  5. Cullen Skink

    nicky.ward Guest

    No, the critical factor is the amount of traffic on your network, which
    depends on the types of user and the amounts of bandwidth they are
    consuming. For example, if you had a business park and a housing
    estate sharing the same contended ethernet ring on a Cable TV network,
    you'd expect the (small number of) business users each to be consuming
    lots of bandwidth between 9-12 and 1-5, with little week-end usage, and
    then the (large number of) consumers each to be consuming smaller
    amounts of bandwidth from 4-10 and at week-ends.

    Nick
     
    nicky.ward, Sep 29, 2005
    #5
  6. DSLAM to ATM "cloud" is the stated produce contention as far as BT are
    concerend.
    "reselling" is a misnomer I suspect, but yes if the contention is
    greater on the BT Central then it will be the limiting factor not the
    exchange backhaul contention. Hence Plusnet's "contention on the
    Plusnet network" quoted in addition to the "BT contention".

    In practice we have seen both exchange contention "Red VPs" and ISPs
    with congestion problems (Demon Home)

    With Centrals at £240/month per Mbit/s I suspect some Central
    contentions are running well over 50:1 to make it pay. Doesn't matter
    if the user demands are modest.

    Phil
     
    Phil Thompson, Sep 29, 2005
    #6
  7. Cullen Skink

    Cullen Skink Guest

    Surely not. Even with 4 people using it at the same time you would
    notice a big drop off in performance and that doesn't seem to happen. If
    one person was connected 24/7 doing downloads how could anyone else use
    it? I think I'm missing something. :)
     
    Cullen Skink, Sep 29, 2005
    #7
  8. the 50 people thing is the error. The pipe might be 10 Mbits/s so it
    needs 20 512k users at maximum download to fill it. At 50:1
    contention there will be *up to* 1000 users able to use it, so 20
    assholes can make life painful for a lot of people.

    The rather feeble 2M backhauls used by datastream operators in the
    early days were often completely utilised by a single 2M user so
    nobody ever say their rated speed unless they happened to be the only
    one using it.

    Phil
     
    Phil Thompson, Sep 30, 2005
    #8
  9. Cullen Skink

    Cullen Skink Guest

    Thanks, the fog is lifting, I think. :) I'm guessing they try to keep
    the "up to" 1000 users much lower than that then as at that rate only 10%
    online at any time would swamp it and 10% seems a lowish figure.
    Was thinking back to that as well. When BB was first being introduced
    there was talk of it slowing down with only a few users online at a time
    which is what got me thinking about it as it doesn't "appear" to be the
    case now.

    Is the contention ratio per exchange or is it configurable across the
    network? I'm on datastream which probably makes a difference.
     
    Cullen Skink, Sep 30, 2005
    #9
  10. its not 10% online its 10% going flat out, which is different. Its
    built for web surfing and collecting email, which aren't flat out
    applications.
    contention ratio is strictly per virtual path, there may be several of
    them handling IPstream. Business services should be on a different VP
    as they have different contention (20:1 vs 50:1). For datastream there
    is likely to be one VP and its likely to be much smaller due to small
    number of users of one ISP on the exchange.

    Phil
     
    Phil Thompson, Sep 30, 2005
    #10
  11. Cullen Skink

    Tony Raven Guest

    Being on-line and using bandwidth are not the same thing. Except for
    streaming, gaming and file download you are rarely continuously using
    bandwith so it works on the principle that those 4 people will not
    actually being continuously overlapping in their demand for bandwidth.
    You might hit short slow downs during brief contentions and if everyone
    wanted to use bandwidth at once they would notice a slowdown.
     
    Tony Raven, Sep 30, 2005
    #11
  12. Cullen Skink

    7 Guest

    Its not 50:1 but the numerical equivalent of a larger ratios
    i.e. something like 50x40:1x40 i.e. 2000:40 or something like that.
    That means 40 users can go on full 512kpbs speed
    without problem. The 41st user will start taking a little bit
    of bandwidth away from all the other users.

    But let me emphasise there has and never will be a shortage of bandwidth.
    95% of all fibers laid are unlit.
    ADSL contention is complete crap dreamt up by POTS management
    when bandwidth was scarce. Which is no longer true!
    Technology has progressed rapidly to the point that the same
    fiber can deliver 100 times and then 1000 times and then 10,000 times
    or more of the capacities available when they were first laid.
    Thus 95% are unlit with any kind of laser beam.
    Crap POTS management are still trying to milk the public
    with bandwidth scarcity stories, when in fact, they haven't lit
    their own fibers. Incumbant operators like BT$ should be broken
    up and the markets deregulated and the task of
    delivering THz bandwidth to the desktop should be left to
    more qualified people who would rush in with THz equipment
    if the markets were opened up.

    Bring on LLU and we shall see!

    BWAHAHAHAHAHAHAHAHAHAHA!
     
    7, Sep 30, 2005
    #12
  13. Cullen Skink

    Cullen Skink Guest

    Yeah sorry I meant 4 people actually using the connection.
     
    Cullen Skink, Oct 1, 2005
    #13
  14. Cullen Skink

    Muxton Guest

    It's a bit like trying to explain how 5,000,000 cars can use the UK
    road network at the same time without any of them dropping their
    average speed below 40mph ... apart from isolated areas where the
    average speed might drop to 10mph or so.

    Does that help?

    Jake
     
    Muxton, Oct 1, 2005
    #14
  15. Cullen Skink

    Tony Raven Guest

    The M25 can hold approx 20,000 vehicles travelling at 70mph and keeping
    the recommended 2s separation. There are 28 million registered vehicles
    in the UK giving the M25 a contention ratio of 1400:1. Fortunately not
    everyone wants to use the M25 at once although it often seems that way ;-)
     
    Tony Raven, Oct 1, 2005
    #15
  16. Cullen Skink

    stephen Guest

    OK.

    The main complication is that there isnt just 1 backhaul - there can be
    several and each can have its own contention.

    If you are with an ISP using Datastream, then contention happens in a
    dedicated VP for that ISPs customers on that DSLAM - and the max bandwidth
    on on the VP is 10 Mbps or so.

    IPstream ISPs seem to share a bigger common pipe, but i think that different
    IPstream style services such as business vs consumer are kept separate.
    Agreed.

    the expensive thing to do is dig the hole and put the fibre cable in - which
    is why telcos either put in large cables with plenty of spare cores for
    possible future use and / or put down something like blown fibre where extra
    cores can go in easily later.

    but just because all that spare fibre is out there doesnt mean that a) its
    cheap to get at and b) it goes to the right places via the right routes....
    And whenever i plan a new network there always seems to be a bunch of
    awkward holes in the coverage somewhere.

    but the equipment to go on the end of the fibre isnt cheap either - and
    telcos are being careful with money for the last few years.
    No. There is no point taking away the contention at the 1st hop aggregation
    point and just pushing it back further into the network if that adds a lot
    to the systems cost and doesnt increase overall capacity.

    you can certainly argue that the contention ratio you live with isnt good
    enough, but inpractice contention only seems to be an issue in small parts
    of the network at any one time. Not much consolation if that is your
    exchange tho.....

    in practice contention ratios quoted are all worst case, and i think that
    real life rarely gets close to the quoted numbers.
    Again no, or not really. a lot of fibre is fairly new, and this assumes
    fibre went in a long time ago.

    FWIW some new gear at work support 200 * 10G on a pair of fibres - but we
    would only use that on long haul routes. better stuff is around but a lot
    more expensive.

    Bleeding edge systems from 5 to 10 years ago could do 32 or 64 * 10G then,
    so there hasnt been a massive step increase in theoretical capacity for
    practical systems that get deployed. A lot fo that may be due to the dot-com
    troubles over the last few years making high end telco gear much harder to
    sell, and making manufacturers cut back on R+D.

    For short distances it is cheaper to use lower capacity stuff, since the
    hardware needed is more expensive than using metro DWDM or just extra fibre
    cores
    you have to remember that it doesnt do any good to open up the pipe unless
    you uprate all the server farms as well - or you just move the contention
    around.

    the only thing likely to alter that assumption is a change to the current
    use of the Internet where traffic is almost all point to point flows.

    Now if a broadcaster could send high quality streaming video using
    multicast, so that the network replicates the data - then getting rid of
    contention might pay off big time - but that needs ISPs to understand how to
    do it, be willing to pay for the equipment and other complications, and
    figure out how to cover the costs in some way.
    LLU only fixes the last mile loop - contention is about backhaul bandwidth
    into the aggregation point - so how does that help.

    FWIW all the companies doing LLU seem to be Telco types - so if this is a
    "big conspiracy" then LLU wont fix it.
     
    stephen, Oct 1, 2005
    #16
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