Got slow broadband? Spare a thought...

Discussion in 'Broadband' started by ureds, Jul 16, 2015.

  1. ureds

    ureds Guest

    ureds, Jul 16, 2015
    #1
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  2. ureds

    Graham J Guest

    So still faster than telex or an ASR33 teletype!
     
    Graham J, Jul 16, 2015
    #2
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  3. ureds

    MB Guest


    I was thinking myself that it is a lot faster than I expected.
     
    MB, Jul 18, 2015
    #3
  4. ureds

    Mark Carver Guest

    There's not a VPN tunnel to the probe and back, I can direct my lad's
    laptop connection via ?
     
    Mark Carver, Jul 18, 2015
    #4
  5. ureds

    me Guest

    Sony playstation
     
    me, Jul 18, 2015
    #5
  6. I had to smile at that. The data rate has had to be dropped the further
    away it gets to ensure reliable reception on Earth. IIRC the current
    data rate is 2Kbps, god knows what it'll be in 16 months.

    Here's hoping we get all the data back and that a piece of space debris
    doesn't get in the way of the probe. Wouldn't go it much good, given
    the speed it's travelling at.
     
    Mike Tomlinson, Jul 19, 2015
    #6
  7. ureds

    Mark Carver Guest

    If only, they were the days ! No, Netflix and similar, eat up 400-500
    GB/mth now !!
     
    Mark Carver, Jul 19, 2015
    #7
  8. ureds

    chris Guest

    According to this, they do it twice...
    http://www.bbc.co.uk/news/science-environment-33570131
     
    chris, Jul 20, 2015
    #8
  9. Interesting problem. With hours of latency, you can't just ask for a retransmission. The mechanism I would use is preventative cyclic retransmission, but that is for satellite links via geostationary satellites (about 1s delay)
     
    R. Mark Clayton, Jul 21, 2015
    #9
  10. Having RTFA - "New Horizons will then send back all of its data - about
    50 gigabits - in compressed form, starting in September, before
    repeating the downlink in an uncompressed form."

    That's the twice. Gives them a pretty good idea of the pictures early
    on, before sending them uncompressed (so no jpeg artefacts) later. As I
    suspected they aren't sending each packet twice.

    But there's no reason they can't ask for a retransmission. "That frame
    you sent yesterday at 01:23:45.67890 - send it again please". You just
    need a _really_ big window.

    Andy
     
    Vir Campestris, Jul 21, 2015
    #10
  11. ureds

    chris Guest

    Except the data is not images as we know them - it is measurement data
    that is then corrected, calibrated and converted into a graphic images.
    What is transmitted is pure data (i.e. numbers). Not sure what kind of
    lossy compression they use...
     
    chris, Jul 22, 2015
    #11
  12. Yes, once with lossy compression (which makes good science analysis
    impossible because of compression artifacts), then the full version,
    presumably with lossless compression.

    It'd be interesting to know if they have prioritised the data so the
    most important/critical to science is sent first.

    The probe uses a MIPS R3000 CPU, same as that used in the original
    PlayStation, so not exactly a wildly powerful one.

    http://www.sciencealert.com/nasa-s-new-horizon-probe-made-it-to-pluto-
    with-a-playstation-cpu-for-a-brain
     
    Mike Tomlinson, Jul 22, 2015
    #12
  13. They could send the data in blocks with parity blocks; that allows the
    original data to be reconstructed if some is missing. That would negate
    the need to ask for a retransmission.

    https://en.wikipedia.org/wiki/Parchive
     
    Mike Tomlinson, Jul 22, 2015
    #13
  14. ureds

    ureds Guest

    Probably was when they started building the thing.
     
    ureds, Jul 23, 2015
    #14
  15. Probably. The main requirement would have been a stable CPU that was
    well-known and efficient to program. The one in New Horizons is a
    radiation-hardened version to prevent stray gamma rays bolloxing it up.

    Some time ago I sold an RCA CPD-1802 CPU on eBay, this was a CPU
    produced in the 70's, the one I sold was the sapphire-based radiation-
    hardened version used in spacecraft. It came in a box of odd bits I
    bought for next to nothing from a scrapper.

    https://en.wikipedia.org/wiki/RCA_1802

    "In addition to standard CMOS technology, the 1802 was also available
    fabricated in Silicon on Sapphire semiconductor process technology,
    which gives it a degree of resistance to radiation and electrostatic
    discharge (ESD). Along with its extreme low-power abilities, this makes
    the chip well-suited in space applications"

    It fetched the huge sum of £2.80 on eBay, but the American buyer was
    thrilled with his purchase. He mailed me some time later to let me know
    he'd tested it and it worked fine.
     
    Mike Tomlinson, Jul 23, 2015
    #15
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