[aprssig] Error checking within APRS packets

Guido Trentalancia iz6rdb at trentalancia.com
Sat Jun 25 11:42:57 EDT 2011

On 23/06/2011 23:54, 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.
> 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.

I have now cc'ed Bob because there little else I can add (I have 
originated this and somewhat I have already replied to your comments in 
a previous message a few minutes ago).

I can only add in some way that it's still risky to do that. Have you 
checked all possible TNCs being used nowadays ? Is there a standard for 
TNCs ?

Most APRS firmware (and software) would need to be changed then to 
exploit the relaxed protocol (for example, Kenwood radios do not even 
have the tilde character at the moment).

> Bill - WA7NWP

Guido, IZ6RDB

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