xtras FS

Discussion in 'NZ Computing' started by Blue, Oct 28, 2006.

  1. Blue

    Blue Guest

    Gentle people, as I understand it with a 128kb/s upstream limit, all that
    one is going to get down stream is approx 4Mb/s. So it seems to me that
    telecom is getting near to the edge of the fraud envelope saying that they
    offer FS/128, when the line may well be able to handle 8Mb/s.
    Blue, Oct 28, 2006
    #1
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  2. "Blue" <> wrote in message
    news:p...
    > Gentle people, as I understand it with a 128kb/s upstream limit, all that
    > one is going to get down stream is approx 4Mb/s. So it seems to me that
    > telecom is getting near to the edge of the fraud envelope saying that they
    > offer FS/128, when the line may well be able to handle 8Mb/s.


    I don't know where ppl get this "4M limit from" but people can get over 4M
    definately (I know from the test results on my website. Most of the limit is
    the Operating system (people with windows XP will get slower downloads on
    single connections than people on say MacOSX or Linux due to the standard
    windowing on TCP)

    I've seem most people Max out at about 4.5Mbps (XP Machines) on single TCP
    Downloads (how the speedtest works on the website), but 6.5Mbps is the other
    threshold of connections (Probably with FS/FS connections), but with
    interleaving off I(which is not possible from Xtra) 've seen 7.2Mbps tests
    (the Max ppl will get)

    My results are not 100% scientific, but are based on the results I am
    seeing.

    Thanks
    Craig
    http://www.nzdsl.co.nz
    Craig Whitmore, Oct 28, 2006
    #2
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  3. Blue

    David Empson Guest

    Craig Whitmore <> wrote:

    > "Blue" <> wrote in message
    > news:p...
    > > Gentle people, as I understand it with a 128kb/s upstream limit, all that
    > > one is going to get down stream is approx 4Mb/s. So it seems to me that
    > > telecom is getting near to the edge of the fraud envelope saying that they
    > > offer FS/128, when the line may well be able to handle 8Mb/s.

    >
    > I don't know where ppl get this "4M limit from" but people can get over 4M
    > definately (I know from the test results on my website. Most of the limit is
    > the Operating system (people with windows XP will get slower downloads on
    > single connections than people on say MacOSX or Linux due to the standard
    > windowing on TCP)


    There will be a theoretical limit due to the rate at which TCP/IP can
    send acknowledgements. Working with some rough numbers:

    The maximum packet size (MTU) is typically 1500 bytes, so assuming the
    sending machine can saturate the connection at 8 Mbps, that is about 1
    MB per second, or 666 packets per second.

    The receiving machine will attempt to acknowledge every incoming packet,
    so it is therefore trying to send 666 packets per second, each
    acknowledging one of the incoming packets. The size of a TCP/IP
    acknowledgement packet is about 40 bytes, so that means sending about
    26.6 KB per second, which is 213 kbps.

    This means that the acknowledgement packets will be backing up,
    saturating the 128 kbps upstream link. At some point, the sending
    machine's transmit window will fill up as it will have to wait for the
    next acknowledgement, and it will automatically slow down its rate of
    transmission to match the rate at which acknowledgements are arriving.

    Working backwards, if the maximum transmission rate for acknowledgements
    is 128 kbps, that is about 16 KB per second, which is 400
    acknowledgement packets per second; multiplying by 1500 bytes gives
    about 600 KB per second, which is about 4.8 Mbps maximum download speed.
    (Actual file data will be less, as 40 bytes out the 1500 are used in
    TCP/IP headers.)

    If the download comes in any faster, the upstream link will be
    completely saturated with acknowledgements, and the download will slow
    down to match.

    All of the above assumes that the receiver is acknowledging every
    packet. If it chose to send acknowledgements less often then a FS/128
    link would able to support a maximum download speed. I don't know
    offhand whether any TCP/IP stack is smart enough to do this.

    There will also be limitations due to the window size and end-to-end
    latency - if the sender gets 64KB ahead without getting any
    acknowledgements, it will stop and wait.

    --
    David Empson
    David Empson, Oct 28, 2006
    #3
  4. In message <1hnxzas.vq3xue1iosc25N%>, David Empson
    wrote:

    > There will be a theoretical limit due to the rate at which TCP/IP can
    > send acknowledgements. Working with some rough numbers:
    >
    > The maximum packet size (MTU) is typically 1500 bytes, so assuming the
    > sending machine can saturate the connection at 8 Mbps, that is about 1
    > MB per second, or 666 packets per second.
    >
    > The receiving machine will attempt to acknowledge every incoming packet..


    Realistic TCP/IP stacks are smarter than this. They will try to use delayed
    ACKs where they can to 1) acknowledge more than one received packet at
    once, and 2) piggyback outgoing data in the same packet as the ACK.
    Lawrence D'Oliveiro, Oct 28, 2006
    #4
  5. Blue

    David Empson Guest

    Lawrence D'Oliveiro <_zealand> wrote:

    > In message <1hnxzas.vq3xue1iosc25N%>, David Empson
    > wrote:
    >
    > > There will be a theoretical limit due to the rate at which TCP/IP can
    > > send acknowledgements. Working with some rough numbers:
    > >
    > > The maximum packet size (MTU) is typically 1500 bytes, so assuming the
    > > sending machine can saturate the connection at 8 Mbps, that is about 1
    > > MB per second, or 666 packets per second.
    > >
    > > The receiving machine will attempt to acknowledge every incoming packet..

    >
    > Realistic TCP/IP stacks are smarter than this.


    I was pointing out the theory behind the "4 MB limit" claim, which I
    agree is a theoretical and unlikely scenario.

    > They will try to use delayed ACKs where they can to 1) acknowledge more
    > than one received packet at once, and 2) piggyback outgoing data in the
    > same packet as the ACK.


    Piggybacking outgoing data is only useful if there is bidirectional
    traffic for the same TCP connection. I wouldn't expect much upstream
    traffic (other than TCP acknokwledgements) are involved in an HTTP or
    FTP download.

    Bittorrent will have more traffic going both ways.
    --
    David Empson
    David Empson, Oct 28, 2006
    #5
  6. Blue

    Mark C Guest

    (David Empson) wrote in
    news:1hnxzas.vq3xue1iosc25N%:

    > There will be a theoretical limit due to the rate at which
    > TCP/IP can send acknowledgements. Working with some rough
    > numbers:
    > ...
    > The receiving machine will attempt to acknowledge every incoming
    > packet...
    > ...
    > All of the above assumes that the receiver is acknowledging
    > every packet. If it chose to send acknowledgements less often
    > then a FS/128 link would able to support a maximum download
    > speed. I don't know offhand whether any TCP/IP stack is smart
    > enough to do this.


    Windows implements RFC1122 Delayed ACKs, so usually, only every
    second segment is acknowledged.

    If a downloaded packet contains 1500 bytes, and an uploaded ACK 40
    bytes, then: 128kbps = 400 ACK/s, which would cover 800 data
    packets/s, or 9600kbps of download.
    But (assuming ADSL) the ACKs have to fit into an ATM frame with a 48
    byte payload, so 8 bytes are wasted, and the ACKs will likely be
    counted as if they are 48 bytes each.
    128kbps = 333 ACKs/s, which would cover 666 data packets/s, or
    7992kbps of download.

    In Windows XP (apparently) you can tweak the ACKs to be only every
    third, or fourth segment. See this link:
    http://support.microsoft.com/kb/328890/
    .... which might allow a lower upload.

    AFAIK, Win9x/ME/NT do NOT do RFC1122 Delayed ACKs, so you are
    stuffed.
    Mark C, Oct 29, 2006
    #6
  7. Blue

    David Empson Guest

    Mark C <> wrote:

    > (David Empson) wrote in
    > news:1hnxzas.vq3xue1iosc25N%:
    >
    > > There will be a theoretical limit due to the rate at which
    > > TCP/IP can send acknowledgements. Working with some rough
    > > numbers:
    > > ...
    > > The receiving machine will attempt to acknowledge every incoming
    > > packet...
    > > ...
    > > All of the above assumes that the receiver is acknowledging
    > > every packet. If it chose to send acknowledgements less often
    > > then a FS/128 link would able to support a maximum download
    > > speed. I don't know offhand whether any TCP/IP stack is smart
    > > enough to do this.

    >
    > Windows implements RFC1122 Delayed ACKs, so usually, only every
    > second segment is acknowledged.


    Ah, so the problem is avoided if you have a new enough version of
    Windows, though a similar one might kick in if Telecom goes as far as
    introducing ADSL2+ with a 128 kbps upload limit (the theoretical limit
    would be about 8 Mbps down if every second packet was acknowledged).

    This explains other claims of operating system differences - Linux and
    other Unixes presumably have a smarter TCP/IP stack which uses
    techniques like delaying acknowledgements, and does a better job than
    older versions of Windows.

    --
    David Empson
    David Empson, Oct 29, 2006
    #7
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