[aprssig] Careless AX.25 encoding in APRS messages

Pete Loveall AE5PL Lists hamlists at ametx.com
Fri Oct 2 09:18:02 EDT 2009

Be careful, the currently accepted spec is v2.0 on the TAPR site and close attention should be paid to section " Command/Response Procedure" (TAPR web site).  Specifically, if both C bits are set to zero or both are set to one, it is assumed to be a pre-v2.0 version of AX.25 and acceptable.  Therefore, all of the listings with either both C bits set to zero or both set to one are legitimate and within spec.


Pete Loveall AE5PL
pete at ae5pl dot net

> -----Original Message-----
> From: Matti Aarnio
> Sent: Friday, October 02, 2009 5:58 AM
> I am probably only one interested enough in some deep esoteric
> details of the radio link-level protocol used by the APRS to
> even look into this...
> On AX.25 destination address part the SSID byte should have
> its three top bits with value: 111,  and on source address
> part those same bits should be: 011.  AX.25 v2.2 specification
> chapter 6.1.2  - UI frames are sent as "commands".

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