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What determines that my line speed is exactly 3Mb?

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  • 18-04-2009 6:44pm
    #1
    Random
    Closed Accounts Posts: 19,080 ✭✭✭✭


    Every broadband company is telling me that my line is only capable of 3Mb. As a result they won't sell me the 7 or 8Mb package.

    I ask them can I try it out and if I get 4 or 5Mb then I'll be happy and signup to the 7Mb package and not complain.

    (that sounds like a pick of a tongue twister .. hope it makes sense)

    I'm being told my line is 3Mb and that's it. What determines that it's 3Mb and not 3.5Mb? 4Mb?

    Thanks


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    #2
    dub45
    Moderators, Technology & Internet Moderators Posts: 12,448 Mod ✭✭✭✭dub45


    http://www.kitz.co.uk/adsl/lowSNR.htm

    http://www.kitz.co.uk/adsl/linestats.htm


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    #3
    Random
    Closed Accounts Posts: 19,080 ✭✭✭✭Random


    Some useful links there but at the same time it's still a bit confusing. Does my connection either sync at 3Mb or 7.6Mb and never in between?


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    #4
    dub45
    Moderators, Technology & Internet Moderators Posts: 12,448 Mod ✭✭✭✭dub45


    Random wrote: »
    Some useful links there but at the same time it's still a bit confusing. Does my connection either sync at 3Mb or 7.6Mb and never in between?


    If your line is bad and not up to the speed you want you will just have problems with disconnects etc. Someone like Smart will help you get the best out of your line if they can but dsl has its own limitations even with a good line.


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    #5
    watty
    Registered Users Posts: 32,417 ✭✭✭✭watty


    In theory you are correct, Random, that any circuit has a maximum speed based on noise, frequency response, crosstalk or attentuation. Basically the "Shannon limit" is related to SNR x Bandwidth.

    However real world equipment only has certain settings of number of tones/channel OFDM (bandwidth)and QPSK/QAM rate per channel (SNR limited). Additionally eircom may not have provisioned the exchange equipment for all possible settings. Some modems may also be more limited.

    So engineering / Marketing fix on a set of "packages" to sell. If they only sell or support 3Mbps OR 7.6MBps that is all you can get.

    Note that in any case if the line is in theory good enough for 3.9Mbps reliably, it's unlikely that is supported. The actually supported speeds on all systems go in increments based on bandwidth per OFDM channel or change in QAM. In between levels aren't possible.

    Also you are always better to be synced at next lower speed for reliability.


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