[Discuss-gnuradio] GMSK AX25 link to TCP with N200

[Adam Gunderson]

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

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Adam K. Gunderson
Space Science and Engineering Lab
Montana State University
address@hidden

gmsk_demod.png
Description: PNG image

gmsk_loopback.png
Description: PNG image