[aprssig] Error checking within APRS packets

Ian Wade G3NRW g3nrw-radio at ntlworld.com
Tue Jun 21 18:09:27 EDT 2011

From: Guido Trentalancia <iz6rdb at trentalancia.com>
Date: Tue, 21 Jun 2011   Time: 22:40:17

>So, at this point my question is why did APRS forbid to use that 
>bit-pattern (you like it that way, I call it that way) if the bottom 
>layer could accommodate in some way through a trick (called 
>bit-stuffing) ?
>First hypothesis: APRS names that "TNC Stream Switch".


This hypothesis is correct.

>From this I might suspect that the same bit-pattern is also used 
>elsewhere (that is not as clever) than in AX.25 start/end flag.

Even though the bit patterns look the same, don't confuse the tilde 
character with the AX.25 start/end flag. Because of bit stuffing, the 
tilde is always converted to something else *before transmission* (so 
that it can never look like a flag in the transmitted signal), then *on 
reception* the bit stuffing is reversed, so you get the original tilde 
back again.

Ian, G3NRW

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