[aprssig] Error checking within APRS packets

Guido Trentalancia iz6rdb at trentalancia.com
Tue Jun 21 16:24:54 EDT 2011

Hi Lynn !

On 21/06/2011 21:11, Lynn W. Deffenbaugh (Mr) wrote:
> 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. 

What would change to the eyes of a machine between a character (that 
would be printed off as tilde) and 0x7e or 01111110 ?

> 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.

Yes, I am going to read it immediately, but in the meanwhile you please 
read below (you could even leave the above note)...

> 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/

What I would like you to double-check (takes 30 seconds including 
downloading and opening the file) is:

APRS spec version 1.0.1 page 71 ("Messages", roughly in the middle of 
that page)

And then tell me if you still think that I am wrong about the above...

>> 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

Now I am going to have a look at what you say and we'll update each 
other later on.


Guido, IZ6RDB
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