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On 9/20/2011 3:09 PM, Joseph M. Durnal wrote:
<blockquote
cite="mid:CAAbqqEPYcoDZ5-AexWeP5_c50buvO5x_OTiCpmZ8wJmGCtnigA@mail.gmail.com"
type="cite">
<pre wrap="">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</pre>
</blockquote>
<br>
<br>
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. <br>
<br>
":" 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.
<br>
<br>
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". <br>
<br>
<br>
<blockquote
cite="mid:CAAbqqEPYcoDZ5-AexWeP5_c50buvO5x_OTiCpmZ8wJmGCtnigA@mail.gmail.com"
type="cite">
<pre wrap="">
"=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
</pre>
</blockquote>
<br>
<br>
<hr size="2" width="100%"><br>
--<br>
<br>
Stephen H. Smith wa8lmf (at) aol.com <br>
=== Now relocated from Pasadena, CA back to 8-land (East Lansing,
MI) ===<br>
Skype: WA8LMF<br>
Home Page: <a class="moz-txt-link-freetext" href="http://wa8lmf.net">http://wa8lmf.net</a><br>
<br>
===== Vista & Win7 Install Issues for UI-View and Precision
Mapping =====<br>
<a class="moz-txt-link-freetext" href="http://wa8lmf.net/aprs/UIview_Notes.htm#VistaWin7">http://wa8lmf.net/aprs/UIview_Notes.htm#VistaWin7</a><br>
<br>
*** HF APRS over PSK63 ***<br>
<a class="moz-txt-link-freetext" href="http://wa8lmf.net/APRS_PSK63/index.htm">http://wa8lmf.net/APRS_PSK63/index.htm</a><br>
<br>
"APRS 101" Explanation of APRS Path Selection & Digipeating <br>
<a class="moz-txt-link-freetext" href="http://wa8lmf.net/DigiPaths">http://wa8lmf.net/DigiPaths</a> <br>
<br>
<br>
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