[aprssig] The APRS AX.25 Frame

Steve Noskowicz noskosteve at yahoo.com
Tue Sep 20 18:43:00 CDT 2011


I don't think anyone commented specificly on these, but I have explained these parts as follows:

> "APX200," = The destination, in practical application, it
> isn't really the destination, but it identifies the APRS software
> generating the packet...

It actually *is* a destination in the general sense.  The "AP" in front signifies that it is TO: ANY AND ALL AP(RS) stations within earshot.  This way it isn't such a change for anyone familiar with Packet.  I also consider it to be sort of an APRS 'message' "Super Group" that makes a bit more sense when you compare it to sending bulletins, messages, and group messages.  This way, it can also be seen how it could be compatible with any ordinary Packet Digi, since only APRS stations on freq interpret it as being TO: them.

  Then, the X200" part is where the original Packet has had things stuffed (very cleverly) into places that aren't needed when used for APRS (as there are others), and is used to indicate the client, as you know.

Then this:

> "WIDE1-1,WIDE2-1" = the packet path, 3 hops in this case,...

Nobody else appeared to catch this. This is a TWO hop, but it's either one fill and one Wide, or two Wides.

  Some of the Gurus here may have issues with the detail of these explanations, but this seems to fit with understanding as well as those I presented to, some of whom were old hands on Packet.

It is a balance between being 100% accurate *and* 100% complete  - and - simplifying too much for the sake of brevity.

Then, try explaining Mic-E !!

 73, Steve, K9DCI 

--- 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
> 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
> "=3925.63N/07726.00WxPHG3000Frederick ARC APRS Demo" = the
> 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
> 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.
> Thanks & 73 de Joseph Durnal NE3R
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