[aprssig] The APRS AX.25 Frame

Stephen H. Smith wa8lmf2 at aol.com
Tue Sep 20 14:35:12 CDT 2011

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

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"APRS 101"  Explanation of APRS Path Selection & Digipeating

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