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scanners and "cell blocked" ?

Discussion in 'Electronic Design' started by Rodo, Apr 24, 2005.

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  1. Rodo

    Rodo Guest

    Hi all,

    I've been looking for a scanner to do some experiments and I got confused by
    the "Cell blocked" fine print. I don't really care to listen to cell phone
    conversations of any kind. I thought most cell modulations are either
    digital or code hoping or both. Only old cell phones could potentially be
    heard on a scanner. A professor in college said that cell phones frequency
    hop every 16 ms ( or so ). This is old info (hopefully reliable). So, I
    imagine the new stuff is really protected from a simple scanner ? Yet, all
    scanners sold in the USA are "cell blocked". Could someone shed some info on
    the subject ?

  2. Ken Taylor

    Ken Taylor Guest

    "Cell blocking" dates back to the old analog systems, which are rare now but
    still used in places. Modern digital systems are not able to be eavesdropped
    on by the casual listener. AFAIK only the US mandates blocking of the
    frequency range commonly used by cell systems, so any coming from overseas
    could tune up to listen to white noise or other undecipherable garbage.

    A CDMA system may hop frequency as often as you say, but others (GSM,
    D-AMPS, etc) don't. Their encoding algorithms provide enough content
    protection for scanners to be useless against them. CDMA doesn't do the
    hopping just to make them 'invisible' to scanners, of course, but that's a
    nice side effect.

  3. GSM may or may not frequency-hop, depending on the network settings. GSM
    uses a narrow-band modulation scheme (GMSK) but if frequency hopping is
    enabled then it will use a different centre frequency for each burst. Error
    correcting codes are used to spread the information in a given block over
    multiple bursts, so that if a single frequency happens to be faded then you
    can still recreate the block.

    For GSM at least, there is even some encryption on top of that, so that the
    restriction is particularly stupid. See the FCC Part 15 regulations section
    15.121 though.

  4. mike

    mike Guest

    This was a legal solution to a technical problem. If you can't fix the
    hardware, pass a law. Can you spell DMCA? Never mind that anybody can
    build/buy a simple converter that allows listening. Keeps the harmless
    people out of your business. Only people who could listen to your
    phone conversation were criminals, by definition.

    Older, unblocked receivers are available. There are a bunch of early
    ham-radio handheld transceivers that are unblocked.

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