[Xastir] Xastir on the Raspberry Pi

[David Ranch]

Hello Max,

Welcome to Xastir and Linux and I'm confident that you'll love both of them!

> I have also been reading a little about AX.25 and at some point I think I would like to have Xastir and APRX both running using AX.25. It's my understanding that AX.25 makes the connection to the APRS-IS and the two applications would use that connection, correct? Or is AX.25 only for the TNC to be shared and each application still makes its own connection to the
>   APRS-IS?  Do packets also get passed between the two applications ?

AX.25 is the low level communications protocol for packet radio which is 
in turn used used by APRS.  To elaborate, classic TNCs support AX.25 
internally where they take your plain user data and format them into 
AX.25 packets and then it out to the radio.  I call this is "command 
mode" operation .  These same TNCs and specifically your TNC-Pi, 
supports KISS mode which is a very plain serial protocol.  This is where 
your *computer* does all the AX.25 formatting, sends the resulting bits 
over the serial port in KISS format, and the TNC dumbly spews out the 
tones to the radio.  Xastir supports APRS functionality either via 
command mode or KISS but since you want to use a TNCPi (a small TNC-X 
unit), you need to use KISS mode.

On the application side, APRS-IS is a very powerful system and typically 
uses a TCP connection to inject objects into the Internet: 
http://www.aprs-is.net/Connecting.aspx . Each connection from an APRS-IS 
client application into the APRS-IS system is UNIQUE so if you have APRX 
and Xastir connected to the APRS-IS system at the same time, those 
connections are completely independent of each other. On the RF side of 
things, if you configure your Linux machine to use the AX.25 protocol 
stack, both APRX and Xastir can share that common infrastructure.

APRX can do several things including to inject APRS Objects into the 
Internet relm as well can also inject Objects,  digipeat, etc on the RF 
side but doesn't display any of that data on a map.  Xastir can also do 
these same things but it can also support GPSes and can show interactive 
maps of heard APRS posits.  Arguably, Xastir can do everything APRX can 
do and a lot more but it requires a lot more system resources.

> What about when one application sends a packet to
>   the TNC, does it also come back in to the other application? I'm trying to get the big picture here as to how all this works
>   together.

With the use of the Linux's  AX.25 stack, both Xastir and the APRX 
applications can see eachother's packets but each application needs to 
have unique SSIDs.

Do you have a need to have APRX communicate directly with Xastir?


Ps.  It's outside the scope of this list but I encourage you to NOT 
compile things for your Raspberry Pi on it's SD card.  You'll quickly 
wear it out if you do this for long.  Feel free to contact me offline if 
you'd like a few hints on how to setup a build environment that won't 
wear out your SD card.

--David
KI6ZHD