[aprssig] Error checking within APRS packets

[Bill V WA7NWP]

>> 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.

The stream switch characters are configurable and can probably be
totally turned off.   This most likely comes from the early days of
APRSDos when Bob was doing all his work with real time control of the
TNC in converse mode.   Saying we can't use a character in our text
field because somebody has a 20 year old  poorly configured TNC is not
something that should limit what little forward progress we have.

Bill - WA7NWP