[aprssig] Error checking within APRS packets
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:
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
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
>> 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.
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