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I am working on space-ground networking for a Cube Satellite, and one of our candidate Layer 2 protocols is AX.25. We haven't settled on a physical layer, but it's likely to be some form of quadrature-encoded PSK.

If we were using Cubesat Space Protocol (CSP) as a Layer 3/4 protocol, what would go in the PID field for I and UI type frames? Would we just pick a random number out of the YY11YYYY and YY00YYYY values other than those listed, or is that a no-no? Would we treat it as 0xF0 "No layer 3 protocol implemented."?

Or alternatively, does it not matter in the end? Does the PID field for I and UI frames do anything?

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    $\begingroup$ I'm not canonical, but the way I read it, I'd go with 0xF0 (no layer 3.) The spec I read, the AX.25 link layer protocol v.20 doesn't explicitly prohibit using other encodings, But much like choosing a frequency, if you're free to choose something, someone else just may choose the same and cause you (and them) problems. If you say "No Layer 3", that removes any expectation (or promise) of what the payload contains, so you would potentially avoid related problems. $\endgroup$
    – Duston
    Sep 24, 2019 at 20:28

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The AX.25 Amateur Packet Radio Link-Layer Protocol Version 2.0 October 1984 document states:

"All forms of yy11yyyy and yy00yyyy other than those listed above are reserved at this time for future level 3 protocols. The assignment of these formats is up to amateur agreement. It is recommended that the creators of level 3 protocols contact the ARRL Ad Hoc Committee on Digital Communications for suggested encodings."

The encodings listed are:

  • yy01yyyy AX.25 layer 3 implemented
  • yy10yyyy AX.25 layer 3 implemented
  • 11001100 Internet Protocol datagram layer 3 implemented
  • 11001101 Address resolution protocol layer 3 implemented
  • 11110000 No layer 3 implemented
  • 11111111 Escape character. Next octet contains more level 3 protocol information.
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