[aprssig] Error checking within APRS packets

Lynn W. Deffenbaugh (Mr) ldeffenb at homeside.to
Tue Jun 21 15:11:25 EDT 2011

I think you all need to read carefully and completely the AX.25 
specification paying particular attention to the framing characters and 
their relationship to bit stuffing.  I suspect you'll find that a tilde 
in the middle of an AX.25 packet actually does NOT look like a framing 
character because the framing character really isn't a "character" per 
se, but is actually a bit pattern.  And with bit stuffing, there are 
defined bit sequences that are NOT allowed to appear in the bit stream 
representation of a packet, but characters that may cause confusion 
within a packet have extra bits stuffed (and removed) on the air, but 
you'll never see such things in any byte-wise representation of the packet.

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

On 6/21/2011 3:04 PM, Guido Trentalancia wrote:
> Actually I have to correct myself here and take the opportunity to 
> suggest a minor improvement to the AX.25 protocol itself (say for a 
> whole-new second generation of AX.25 and APRS because it won't be 
> backwards compatible):
> tilde is widely used for example to represent Unix home directories 
> and therefore it is also widely used in URLs (when they are hosted on 
> Unix-like platforms) as in very popular ones:
> http://www.baycom.org/~tom/ham/soundmodem/
> So one ham that needs to send to another ham the HTTP link to Tom's 
> Soundmodem software (supposedly under his own Unix-like home directory 
> on Unix-like host www.baycom.org) would not be able to do that...
> It's not that common nowadays, but it still happens often enough to 
> potentially cause troubles.
> One of the first 32 unprintable ASCII values or something beyond 127 
> would perhaps do a better job (ideally something which changes more 
> often to bring other advantages as well, as 0x7e is 7 consecutive ones 
> and only one zero at the end).
> Guido, IZ6RDB
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