APRS PRECEDENCE BIT                                      16 Jan 2013
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Jan 2013:  This used to be called the "precedence bit" but the name
           was chaged to Precedence bit to avoid confusion with
           the Mic-E "precedence" status/position comment bits.

Dec 2012:  The precedence bit may use one of the RR bits as abetter
           method of indicating packet precedence.  Please see:
           http://aprs.org/aprs12/RR-bits.txt

Aug 2009:

APRS needs a way of differentiating important packets from routine
packets for network management under stress.  There are two 
parameters that need to be available in every packet.  These are the
OPERATOR PRESENT bit and a precedence bit.  The operator present bit 
has been defined since the early 2000's and is already covered in 
the APRS Addendum 1.2.  See "operator-bit.txt".  Basically, an 
overlay "O" on any symbol indicates an operator is present.  This 
page now defines the precedence bit.

The precedence bit and Operator Present bit can be used to
differentiate how packets are handled and displayed during times of 
stress.  For example, Operators can select OPERATOR-ONLY symbols 
on their display to quickly be able to see what live-assets are
available on the map without all the clutter of unattended stations.
Or, when the local Emergency coordinator declares an emergency, 
digipeaters can be put into "minimize" state and the digipeaters 
then will be able to pass all precedence packets, and may reduce the 
non-precedence packets to fewer hops.

BACKGROUND:  Prior to discovering the RR bits, APRS only had 3 
available bits in the standard position packet.  These are the 
CASE/case of the N/S, the E/W and the symbol overlay bytes.  For 
many years we have wanted to use these bits to 
indicate if a live operator is present, and a precedence bit.  The 
problem is that UI-View is not backward compatible with the use of 
lower case in the N/S or E/W bytes.  We cannot use those bits until 
UI-view is finally made obsolete.  It is however able to display the 
lower case Overlay byte.  Therefore we proceeded back in 2007 to 
suggest the use of that bit.

PRECEDENCE BIT:  If the overlay byte in any APRS packet was in lower 
case, then the packet is considered to be a PRECEDENCE packet.  If the
symbol is in the primary table or has an uppercase overlay byte (the
default), then the packet is considered to be ROUTINE.

APPLICATION:  Future digipeater code could allow for a SYSOP 
parameter called "minimze".  When this is OFF, digipeater functions
are normal.  If MINIMIZE is enabled, then the digipeater can
implement levels of decimation of packets.  There can be multiple
levels of MINIMIZE depending on the degree required in the immediate
situation.  For example, these conditions may be applied:

MINIMIZE MINIMUM: precedence packets are digipeated normally. Routine
packets are digipeated once and the has-been-digipeated bit is set.

MINIMIZE MAXIMUM: precedence packets are digipeated normally. Routine
packets are ignored.

It is not expected that this higher level processing will applly to
all digipeaters, but at least it will give us the tools to use in
some digipeater code to apply to critical situations in some small
geographic areas or special events.  The burden on the digipeteater
is significant since the digi now has to parse the packet format to
find the precedence bit.

DISPLAY:  There should be an indicator on all APRS symbols as to 
whether the OPERATOR-PRESENT or the precedence BITS are set.  Luckily
this is inherent on most systems, since they information is conveyed
in the existing overlay byte.  But in-keeping with the original 
intent of APRS that used SYMBOL COLOR to convey the 8 basic 
attributes of each symbol, these two new conditions of OPERATOR
present and precedence should be considered as fundamental symbol
attributes as well.  

It is a concern that many APRS clones do not always
display at least the original 8 attributes on every symbol.  The
most critical example is the abliity to differentiate STATIONS from
OBJECTS and MY objects from OTHER objects.  This ownership is very
critical and very important to the tactical situation, because
ownership can change on a packet-by-packet basis.  Ownership and
responsiblity for objects is critical to operations.  This 
paragraph does not necessarily apply to the precedence topic, but it
has been so overlooked by so many authors, that it is revisted here
as a reminder.

SELECTION:  Given that stations and objects can be easily differen-
tiated by prioirty, operator-present, and ownership, the good
client software gives the operator the ability to selectively 
display these distinctions.

LIMITATIONS:  Pete Lovell notes that this precedence can only be set
on Alternate symbols and only those with an ALPHA overlay.  Further
this precedence bit cannot be set on COMPRESSED packet formats as it
will conflict with the use of the letters a-j to translate to the
digits 0-9.

Also it is interesting that the Mic-E formatted packets have another 
means of indicating SPECIAL, PRIORITY and EMERGENCY which can also be 
exploited in the future.

UI-VIEW NOTES: Keith VE7GDH reports that UI-View will still only 
let you enter an overlay in upper-case, you can exit the program 
and edit uiview32.ini and edit the [STATION] section to change the 
TABLE_CHAR to lower case and it will stay put.  UI-View does
display stations with a lower case overlay correctly... but only
on symbols that accept an overlay.  

To make any alternate symbol capable of displaying an OVERLAY, 
simply add "No." as the first four bytes in the description in the 
mysymb.txt file.  This is an abbreviation for "numbered" which is 
what we originally used to refer to overlay symbols.  The reason 
we did this was that we use the NUMBER sign to indicate the overlay 
generically as in "#>" would show that the alternate symbol ">" can 
have an overlay #.

Anyway, this technique can be used to update UI-View to be able
to use the new overlay and symbol concepts shown in:

www.aprs.org/symbols/symbols-new.txt

Bob, WB4APR