[aprssig] Error checking within APRS packets

Guido Trentalancia iz6rdb at trentalancia.com
Thu Jun 23 20:13:42 EDT 2011

On 23/06/2011 23:26, Lynn W. Deffenbaugh (Mr) wrote:
> On 6/22/2011 12:43 PM, Bill V WA7NWP wrote:
>> The APRS spec says to never use a tilde in the text field?   If it's
>> true, that's silly.   It's just another ASCI character.  The
>> bit-stuffing magic happens at the hardware packet level and is totally
>> independent of what we put in the packet data -- for APRS or any other
>> packet application.
> The issue isn't with AX.25, but I believe there are some TNCs out 
> there that use the tilde character as a way to switch sessions or 
> streams or something.  Given that, if the tilde character appears 
> anywhere in a packet that needed to be transmitted by that TNC, most 
> likely in some sort of CONVerse mode, not KISS, then really bad things 
> would happen.

It is used also by HDLC/SDLC (along with the same bit-stuffing 
technique) as the start/end flag similarly to AX.25 (which actually took 
it from the former).

However in the original context (it was called "TNC stream switch"), it 
should mean a control character used in TNCs to switch between (data) 
streams of different connections (some other times 0x7c which is the 
pipe "|" is used instead).

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

Guido, IZ6RDB

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