[aprssig] APRS Callsign Extensions

[Robert Bruninga]

APRS developers.

This topic comes up every few years, but I just got a new data
point, and so thought I would bring it up again.  That is the
use of mixed case letters for extening the Callsigns and other
address fields of APRS.

The new data point is that the KPC-3+ which seems to be a 
large part of the APRS infrastructure (digipeaters) in the USA,
can apparently receive callsigns that are not only mixed case,
but can actually include other characters as well.  This gives
the possibility of using callsigns like Wb4aPr-1 which actually
can carry 6 additional bits of information in the "case" of the
existing 6 bytes for example.  This could be used to uniquely
encode 7 character callsigns, etc..

This has always been a loophole in the AX.25 spec, in that
the spec says that only UPPERCASE letters and digits will
be *transmitted* in the AX.25 hearder, but it does not
say anything about *receiving*.  In the past it has often
been observed that many TNC's will actually decode these
extended range callsigns just fine.

But before we could make any use of this extended address
range, we would need to know what can and cannot decode
these packets.  Problem, is, no one can TX these callsigns
because all TNC's enforce the UPPERCASE only for transmitting.
But someone could easily hack out a few packets using KISS
and see what OTHER TNC's do with it.

Since we now konw that the KPC-3 family can decode them,
I'd like to see what other TNC's decode them.  There are 3
possiblities:

1) TNC Ignores the packet
2) Decodes it just fine, but forces the display to uppercase
3) Decodes and displays the mixed case call.

If the majority of TNC's do either 2 or 3, then special applications
might find a way to use these bits.   For many years I have always
been interested in adding a few additional bits of  informatioon to 
APRS packets (and still be backwards compatible).... For example:

1) Two Priority bits to give 4 levels of priority
2) Human Presence bit
3) others?

Bob, Wb4APR