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

[Mark Haun]

Thanks everyone for your responses.  The funny thing is, I already concluded
the way to go was to hook up an RTL-SDR dongle and start poking around. 
Should be here this week.

I know the frequencies (based on FCC license search) and the hardware
manufacturer (IPMN).  AFAICT there are a variety of technologies available
for AVL, so any given transit agency is likely using something different.

I see no insurmountable barriers getting to the point of successful Viterbi
decodes.  After that, it seems quite difficult.  First I have to guess the
whitening polynomial and its initialization, then figure out packet framing,
and possible source coding.  And all of this assumes nothing is
intentionally encrypted...

Mark

Andrew Clegg address@hidden wrote:
> Sounds like an interesting project. I'd like to know more about the spectrum 
> aspect -- do you know which band segments in 700 MHz are used for this in the 
> U.S.? Me and my spectrum analyzer want to know :)
> Andy
> Date: Tue, 26 May 2015 06:28:44 -0700
> From: address@hidden
> To: address@hidden
> CC: address@hidden
> Subject: Re: [Discuss-gnuradio] Question about reverse-engineering a new mode
> 
> 
> 
> On 26 May 2015 03:28, "Robert McGwier" <address@hidden> wrote:
> 
> >
> 
> > [...] 
> 
> > That said, hackers (the good definition) live for this, and I encourage it.
> Just wanted to emphasise this. Go for it! Worst case, you learn a lot of 
> interesting things. 
> Cheers, 
> 
> M
> >
> 
> > Bob
> 
> >
> 
> >
> 
> > 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.
> 
> >>
> 
> >> Thanks,
> 
> >>
> 
> >> Mark
> 
> >>
> 
> >> _______________________________________________
> 
> >> Discuss-gnuradio mailing list
> 
> >> address@hidden
> 
> >> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
> 
> >
> 
> >
> 
> >
> 
> >
> 
> > -- 
> 
> > 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)
> 
> >
> 
> > _______________________________________________
> 
> > Discuss-gnuradio mailing list
> 
> > address@hidden
> 
> > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
> 
> >
> 
> 
> 
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