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Re: [Discuss-gnuradio] Question about reverse-engineering a new mode

From: Robert McGwier
Subject: Re: [Discuss-gnuradio] Question about reverse-engineering a new mode
Date: Tue, 26 May 2015 03:27:06 -0700

FIPS compliant security, device security, network security, access controls, and application level security are all integral parts of Public Safety Network design and operation and AVL in particular.  It is just not intended to be "super duper"  APRS.  I would not spend a lot money on equipment if this is your only goal and the amount of money I would spend would cover a RTL-SDR dongle and not much more until such time as I was certain that these serious impediments were surmountable.  That said, hackers (the good definition) live for this, and I encourage it.


On Tue, May 19, 2015 at 3:04 PM, Mark Haun <address@hidden> wrote:
This is a bit of an idle question, but I'm hoping some knowledgable folks on
here can offer advice.  Mostly I'm trying to understand better what I
don't know, and the size of the challenge, before jumping in to a project:

I'd like to try decoding some AVL traffic in the 700-MHz band (GPS locations
broadcast by transit vehicles to a central collector, where predictors are
used to generate the ETAs displayed on electronic bus-stop signs).  The
modulation is 4-FSK, similar to P25 except wider with a higher symbol rate,
emission designator 20K0F1D.  The particular frequency(s) should be easy
enough to discover.  Transmissions are short packets on shared channels with
some kind of slotted aloha or CSMA MAC.  A rate-3/4 convolutional code is
used.  The preceding is public information gleaned from the web.  I haven't
captured any signals yet.

The known unknowns:  preambles and framing stuff, symbol mapping,
the particular rate-3/4 code used (only a couple of candidates though), and,
the scrambler (whitener) and its initialization.  AFAIK there is no
encryption per se.  The payload is supposed to be TCP/IP, so there could be
some sort of header compression.

My question, then, is given this information, are there reasonable odds of
success?  I have some digital comms background from grad school but little
to no practical experience.  Wondering if this might be an excuse to pick up
a HackRF etc. and learn GNU Radio, or if it's likely to be a dead end.



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Bob McGwier
Co-Founder and Technical Director, Federated Wireless, LLC
Research Professor Virginia Tech
Senior Member IEEE, Facebook: N4HYBob, ARS: N4HY
Faculty Advisor Virginia Tech Amateur Radio Assn. (K4KDJ)

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