[aprssig] The APRS AX.25 Frame

Tue Sep 20 15:48:37 EDT 2011

Joe,

I don't know if it will help you much, but last year sometime I wrote a VB.net application that will convert a raw KISS packet to the "human readable" version.  I wrote it more of a see if I could do it project.  I never added in direct serial input, so you have to copy/cut and paste a KISS packet into the program to convert it.  Basically in the process of converting the packet, it breaks down the packet byte by byte in some tables and even parses out the converted packet to show the different elements.

Anyways, if it could be helpful, I have it for free download on my webpage at:

http://www.ke4nyv.com/software.htm

See the bottom program "KISS Packet Analyzer".

Jason Rausch - KE4NYV
RPC Electronics, LLC
www.rpc-electronics.com

--- On Tue, 9/20/11, Stephen H. Smith <wa8lmf2 at aol.com> wrote:

From: Stephen H. Smith <wa8lmf2 at aol.com>
Subject: Re: [aprssig] The APRS AX.25 Frame
To: "TAPR APRS Mailing List" <aprssig at tapr.org>
Date: Tuesday, September 20, 2011, 3:35 PM

  On 9/20/2011 3:09 PM, Joseph M. Durnal wrote:

      I'm trying to explain an APRS packet to many non-APRS users and I want
to get it right, but I've confused myself

Here is a basic packet

NE3R>APX200,WIDE1-1,WIDE2-1:=3925.63N/07726.00WxPHG3000Frederick ARC APRS Demo

The AX.25 frame is defined
Flag(1) Dest(7) Src(7) digi(0-56) ctrl(1) PID(1) info(1-256) FCS(2) flag(1)

I'm assuming that there are bits in the frame that aren't translated
to text on the TNC, what confuses me is the source, I obviously see
the source on the TNC, but it comes before the destination.  I'm also
not sure what defines the : between the path and position.

My explanation will look like this

"NE3R>" = The source of the packet
"APX200," = The destination, in practical application, it isn't really
the destination, but it identifies the APRS software generating the
packet
"WIDE1-1,WIDE2-1" = the packet path, 3 hops in this case, once
dilapidated looks like N3KTX-2*,WIDE1*,KV3B-1*,WIDE2*
":" = not sure exactly what this is, found between the path and the
rest of the APRS info field

    The display you see in a TNC monitor window is already considerably
    "sanitized" relative to the actual raw packet that contains binary
    headers, sync bits, checksums, packet type codes, etc.   Viewing a
    raw packet will show considerable binary "gibberish before and after
    the recognizable text payload.      

    ":" is the delimiter added by the receive TNC, for display purposes,
    to separate the path data (itself already reformatted from the raw
    packet)  from the packet "payload".  This will be the same for any
    kind of AX.25 packet, either APRS or "classic" connected packet.   

    Everything beyond this is "just payload text" as far as AX.25 is
    concerned.     Any further delimiters are purely APRS constructs and
    have nothing to do with the low-level AX.25 "shipping container". 

      "=3925.63N/07726.00WxPHG3000Frederick ARC APRS Demo" = the APRS
information, which I'll break down further below
"=" = APRS datatype, this one represents an APRS position without a
time stamp from a messaging capable station
"3925.63N/07726.00W" = the position data, but the / is important to
the APRS symbol definition
"x" = the second part of the symbol, combined with /, the x displays
an X for XAPRS/Xastir (linux software), symbols for cars />, trucks
/k, houses /-, etc can be used
"PHG3000" = optional power height & gain info, important for
digipeaters and fixed stations (your software should calculate this)
"Frederick ARC APRS Demo" = A free-form comment

    --

    Stephen H. Smith    wa8lmf (at) aol.com 

    === Now relocated from Pasadena, CA back to 8-land (East Lansing,
    MI) ===

    Skype:        WA8LMF

    Home Page:          http://wa8lmf.net

    =====  Vista & Win7 Install Issues for UI-View and Precision
    Mapping =====

        http://wa8lmf.net/aprs/UIview_Notes.htm#VistaWin7

    *** HF APRS over PSK63 ***

       http://wa8lmf.net/APRS_PSK63/index.htm

    "APRS 101"  Explanation of APRS Path Selection & Digipeating 

      http://wa8lmf.net/DigiPaths 

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