[Discuss-gnuradio] Help understanding AX.25 encoding and tagged streams

[Jack White]

Hello, knowledgeable ones,

I need to put together testbed satellite uplink using AX.25 encoding and GMSK modulation. The flowgraph source is an Ettus USRP X310 with a WBX-120 daughterboard.

The flowgraph is a modification of one used with a USRP B210. That radio had handle different sample rates to the X310, so some level of resampling is required - in this case from the proposed 614.4ksps to 400ksps.

I understand the premise of constructing the AX.25 packet, but I'm having real difficulty understand tagged streams and how the tagging and the PDU format is affecting my transmission.


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The disabled low-pass filter and the enabled and encircled rational resampler are my additions. The AX.25 line coding hierarchical block is below.


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​So, when I run the flowgraph and try to set a command from the groundstation, I get a huge load of buffer underrun errors in the GRC log: UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU
UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU
UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU
UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU

If I try sending the command again, I get a tP error, followed by more underruns.

In my head, it looks like the USRP is expecting a combined sample of a certain length, but it doesn't get it. The buffer is more than half full. Then the next packet comes in, of the same length, and you get one buffer filled with half of one packet and half of another.

The USRP is transmitting something, because I checked the transmitting antenna on a spectrum analyser.

Can anyone help me understand the flowgraph, especially the tagged streams and PDU input, and get this working?

Cheers,

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Jack White
address@hidden
07875 813 745