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[aprssig] The APRS AX.25 Frame

Lynn W. Deffenbaugh (Mr) ldeffenb at homeside.to
Tue Sep 20 19:28:25 UTC 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.

Yes, in the TNC2 humanly-readable format, the source is listed before 
the destination with a > separating them.  In the actual on-the-air 
AX.25 packet, the destination comes before the source.

>     I'm also
> not sure what defines the : between the path and position.

The colon between the path and the payload is simply a separator, again 
for human readability just like the commas between the path components.  
The character after the colon (first character of the payload) is the 
APRS datatype character that determines the format of the remainder of 
the payload.

> 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

It's not the destination because APRS uses exclusively UI packet (kind 
of like UDP) which are outside of the connected mode packet protocol.  
Connected mode uses this as a destination, UI packets don't use it for 
anything in particular so APRS puts one of several things there.  a) 
Application Identifier as you mentioned (see 
http://www.aprs.org/aprs11/tocalls.txt)  b) Alternate Network ID, or c) 
Bits of the position for Mic-E encoded packets.

> "WIDE1-1,WIDE2-1" = the packet path, 3 hops in this case, once
> dilapidated looks like N3KTX-2*,WIDE1*,KV3B-1*,WIDE2*

I think you mean digipeated, not dilapidated? (although there are some 
that might equate that with antiquated in a comparison of the APRS 

> ":" = not sure exactly what this is, found between the path and the
> rest of the APRS info field

Just a visual separator is all.

> "=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

On quick glance, it looks good.  Remember that the table identifier (/ 
in this case) can also be an overlay character for alternate table symbols.

> I have to keep it simple, but I want it to be informative as well.
> I'll probably add a few links, like the symbols definition.

Looks good to my eyes.  aprs.fi's raw packet display is an excellent 
source of actual packets if you need any.

Lynn (D) - KJ4ERJ - Author of APRSISCE for Windows Mobile and Win32

> Thanks&  73 de Joseph Durnal NE3R

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