[aprssig] Error checking within APRS packets
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
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
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