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[aprssig] The APRS AX.25 Frame

Joseph M. Durnal joseph.durnal at gmail.com
Tue Sep 20 19:40:47 UTC 2011

Thanks for the explanations - I don't want to get too far into the
weeds, so I'm keeping it to the text you'd get on your TNC.

I did a presentation to the radio club last night, but I wasn't happy
with it, so I'm going to follow up with some e-mails that get into a
little more detail, but keeping it basic enough for non-APRS users to


On Tue, Sep 20, 2011 at 3:35 PM, Stephen H. Smith <wa8lmf2 at aol.com> wrote:
> 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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