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?