[aprssig] Precedence Bit and IP Encapsulation

[Nick VA3NNW]

Robert Bruninga wrote:
> I'd love to hear if any system in APRS actually implemented the Precedence
> bit?
> http://aprs.org/aprs12/precedence-bit.txt
> Basically it is making the Table-Overlay character a lower case character.

I was about to process all of April's APRS-IS through Ham::APRS::FAP.pm
but was suspicious when there were *NO* lowercase symbols.

Then I found in the code:

        if ($symboltable !~ /^[\/\\A-Z0-9]$/) { _a_err($rethash,
'sym_inv_table'); }

... which basically means it accepts / \ A-Z (upper) or 0-9 (lower) or
considers it to be INVALID   :-D

I believe aprs.fi is based around this library, but possibly a newer
version of it?

So re-parsing myself, and looking at the entire APRS-IS for April, I see

148019309 packets parsed with symbols for the month
147998954 packets parsed with GOOD symbols ("approx ASCII")
 90278050 use table "/" (which obviously can't be upper/lower case
  4944477 use alternate "\" (ditto)
 50955070 use UPPERCASE symbol table overlay
  1717233 use 0-9 as an overlay
    72662 use LOWERCASE symbol table overlay
    28403 use bad (8-bit / control-char / corrupt) symbol tables
    23414 use ASCII symbol table overlays like ~@#$%^&*() (ASCII but non-alphanumeric)
    20355 use bad symbols (not the table, the actual symbol is non-ASCII)

So... lowercase symbols DO happen, but less than 0.05% of the time. Hard
to tell how many of those might be deliberate "precedence" vs "Just
didn't know and thought lower-case was fine" vs "data corruption" but
they are quite a bit higher than (ab)use of 8-bit.

For comparison, the upper/lowercase of the LatLon N/S/E/W chars:

129128571 NS=N (Notable bias toward Northern hemisphere as well)
  7279962 NS=S
 77838643 EW=E (E/W a bit closer but bias to E, yes I was surprised)
 58559906 EW=W
    77842 NS=n
    11534 NS=s (Still a notable N bias, but notably less - interesting?)
    18753 EW=e
    80607 EW=w (... but much more westerly bias in the lowercase - WAT?!?)

I see (from the same doc) there's also a standard to use an overlay of
"O" for "Operator Present".

I've seen that used far more on repeater objects to mean OFFLINE! This
is almost but not quite the opposite of "Operator present"!  :-D

They'll use I0 for idle/IRLP, C0 for Connected, B0 for Busy, O0 for
Offline (not Operator Present)

Nick VA3NNW - self-declared APRS "big data statistician" nosing around
APRS data  ;-)

-- 
"Nosey" Nick Waterman, VA3NNW/G7RZQ, K2 #5209.
use Std::Disclaimer;    sig at noseynick.net
Men still remember the first kiss after women have forgotten the last..

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