APRS PRIORITY BIT (obsolete. See PRECEDENCE BIT) 16 Jan 2013 -------------------------------------------------------------------- Jan 2013: The name of the Priority bit was changed to Precedence Bit to avoid any confusion with the Mic-E PRIORITY status or position comment bits. Dec 2012: The Priority bit is better defined using the RR bits. See http://aprs.org/aprs12/RR-bits.txt Aug 2009: (now Obsolete!) Retained for historical and backwards compatibility purposes. 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 PRIORITY bit. The operator present bit has been defined since the early 2000's and is already covered in the APRS Addendum 1.2. Basically, an overlay "O" on any symbol indicates an operator is present. This page now defines the priority bit. The priority 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 priority packets, and may reduce the non-priority packets to fewer hops. BACKGROUND: APRS only has 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, an a priority 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 are proceeding to use that bit. PRIORITY BIT: If the overlay byte in any APRS packet is in lower case, then the packet is considered to be a priority packet. If the symbol is in the primary table or has an uppercase overlay byte, then the packet is considered to be ROUTINE. APPLICATION: Future digipeater code should 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: Priority packets are digipeated normally. Routine packets are digipeated once and the has-been-digipeated bit is set. MINIMIZE MAXIMUM: Priority 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 priority bit. DISPLAY: There should be an indicator on all APRS symbols as to whether the OPERATOR-PRESENT or the PRIORITY 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 PRIORITY should be considered as fundamental symbol attributes as well. It is a major fialing of many APRS clones that they 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 PRIORITY 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 softeware gives the operator the ability to selectively display these distinctions. LIMITATIONS: Pete Lovell notes that this priority can only be set on Alternate symbols and only those with an ALPHA overlay. Further this priority 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