[aprssig] APRS spec 101

[Curt, WE7U]

On Wed, 16 Jul 2008, Steve Noskowicz wrote:

> That's the version number ... one I have printed out.  
> Thumbing throiugh it for some light reading, [yea, right]  I can't
> find where / if it describes the Frame Check Sequence.
>
> it is a simple sum type checksum?

Nope.  CRC16.


> Now one may correctly argue that I don't *really* need to know this,
> but being so possessed, I'm curious.   OR should we just
> assume it is covered int the  AX.25 spec?

It _is_ covered in the AX.25 spec, and is _not_ covered in the APRS
spec, as you surmised.  Whether it is covered adequately in the
AX.25 spec is quite another question, but at least we have
open-source implementations of the algorithm in the wild now to
glean from.  In '85 I wrote a CRC16 algorithm in 6803 assembly and
had only an IEEE article and the AX.25 spec to go on.  It took some
bit twiddling and hair pulling to get it to work.  Scott probably
had similar experiences.

-- 
Curt, WE7U.				archer at eskimo dot com
http://www.eskimo.com/~archer
   Lotto:  A tax on people who are bad at math. - unknown
Windows:  Microsoft's tax on computer illiterates. - WE7U.
The world DOES revolve around me:  I picked the coordinate system!"