GMSK AX25 link to TCP with N200

Hello,

We are currently trying to setup a ground station for GMSK modulated
AX25
packets at 9600 and 19200.

Our current setup that works is the USRP N200 through GNU radio audio
sink
and to a Kantronics 9612+ TNC (gmsk_demod.png, quadrature demod → audio
sink). What we want to do is replace the hardware TNC with GNU radio
packet
decoding blocks and send the packets over TCP.

We first tested the loopback through all the gmsk mod/demod and packet
encoder/decoder blocks, (gmsk_loopback.png) and this works fine.

However, when we send our received AX.25 packets through GMSK demod →
packet decoder → TCP sink (gmsk_demod.png) we do not see anything on
the
TCP server side, which tested fine in loopback.

We are assuming the hangup is in the packet decoder block as it is not
recognizing AX.25 packets or possibly at the end of the GMSK demod
block.

Do we need to look into writing our own packet decoder block? If we need
to
do this, is there a good guide on how to edit existing blocks, or look
at
their work() functions?

Do we need to also implement a derandomizer block before we even can
start
making sense of the bit stream?

If we were able to get a byte stream with the framed AX.25 packets as an
output from GNU radio to our TCP server we can deframe them there
instead
of in GNU radio, but we cannot get any output with this current setup.

We have also looked at trying to get the GMSK Spacecraft Groundstation
project to work but the versions must be out to date because the files
located on https://www.cgran.org/wiki/GMSKSpacecraftGroundstation won’t
make.

Thanks

-Adam


Adam K. Gunderson
Space Science and Engineering Lab
Montana State University
[email protected]

[email protected]

Adam,

I apologize if I’m misunderstanding your question. But the packet
decoder
in GNU Radio is not an AX.25, just a general purpose decode. I’ve
hacked
together an AX.25 receiver with the N200. I can send the files to you
tomorrow.

-John

On Wed, Jun 6, 2012 at 3:52 PM, Adam Gunderson

PS - The decoder I built was tested with live recordings of a stensat
beacon from the phonesat group, so I think it will certainly work in
your
case - if only with minor modifications.

-John

Me too, please.

Darren

Yeah I figured that was the main issue, an AX.25 encoder/decoder would
be
awesome!

Thanks

-Adam

Hi,
I am new in GnuRadio and i like to test your experiment. Could you
please post a picture of your final working flow.
Thanks

Alex