[aprssig] APRS message-handling algorithms / libraries

Greg Dolkas ko6th.greg at gmail.com
Sun Apr 8 13:25:42 CDT 2012

Hi John,

Your last comment may be the most important...  validating the source of
messages going from IM->APRS.  I have totally abandoned the Microsoft IM
service because of the constant barage of spam, virus-laden links,
unsolicited buddy requests, etc. that appear when I connect.  Some of the
other services are not much better, and in my opinion, with the exception
of Google's (which is integrated in with their email service and so far
spam-free), nearly all are practically worthless to me these days.

I've used the various APRS->Email gateways a number of times, mostly
successfully.  With phone text messaging commonly used and often more
reliable and accessible than a phone call, and email->text message gateways
provided by all of the major carriers, APRS->Phone Text Messaging can be an
effective means of communicating when I'm out of cellular range.

The only trick to interoperating with a cell phone is to pick the right
ARPS email gateway so that the message comes across formatted well for the
phone.  Some add additional information to guide the person on the other
end, and this isn't tolerated well by some phones.  (I crashed my
daughter's Moto Razr once by doing this...)  Others end up with the message
text as the subject, etc.  It can be quite confusing to the participants.

So, if I might suggest, a better gateway to write would be for a
APRS->Phone Text Messge service.  You'd need to use the carrier's
email-text gateways, but both are inherently short-message based, so this
should be quite effective if done right.

The biggest issue for both my preference and yours is the scarcity of
bi-directional iGates.  Still useful for the "arrived safely" messages, but
not what I would like.  Unfortunately, this is not a software problem...

Greg  KO6TH

On Sun, Apr 8, 2012 at 10:24 AM, John Goerzen <jgoerzen at complete.org> wrote:

> **
> Hi folks,
> I am looking to write some APRS software, and I'm hoping to avoid
> re-inventing the wheel.  I am particularly interested in libraries in any
> cross-platform language (C, Perl, Python, Java, etc.) that can be used to
> receive and generate APRS messages.
> In my research, there are quite a few libraries that receive APRS
> messages, but I have found none that are helpful with generating them.
> This seems to be something with some complexity; such as:
>    - Splitting data up into packets of the appropriate size (APRSLink for
>    Winlink appears to somehow detect the type of device in use and generates
>    shorter messages for TH-D7A.  Anyone know how to do this, and what are the
>    appropriate sizes for the D7A, D72A, 710, and VX-8GR?)
>    - Processing/generating ACKs, REJs, etc.
>    - Proper timing for retransmit of packets that weren't ACKed.
> None of this is hugely complicated, and the APRS spec is fairly clear on
> the ACK, REJ, etc. process.  The maximum message sizes appear to be
> undocumented and I'm hoping someone can help me out there.
> My project, incidentally, is to build a bidirectional APRS-XMPP bridge.
> XMPP is the protocol behind the instant messaging tools such as Jabber and
> Google Talk, and is available for free on every modern platform.  In the
> spirit of both amateur radio and the Free Software communities I'm a part
> of, full source code to this project will be available.  I actually already
> wrote a much simpler piece of related software, letting someone take a
> connected AX.25 session and bridge it to XMPP, [1] so I do have some
> experience under my belt.  This is obviously a more complex project, but
> XMPP looks like an excellent way to bridge APRS to other systems.  I think
> it is a better fit than the email gateways out there, and in fact, XMPP
> already has fields for carrying presence information and location
> information with presence packets.
> Are there any particular notes available to those implementing gateways
> between APRS and non-APRS sources?  I have taken note of what sites such as
> OpenAPRS or Winlink do to validate people for access, and plan to do
> something similar here.
> Thanks,
> -- John
> KR0L
> [1] source code at https://github.com/jgoerzen/ax25xmpp
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