DIGIS.TXT 7.9.0     AUTOMATIC PACKET REPORTING SYSTEM DIGIPEATERS

As of version 787, your maximum reporting period dependent on the length
of your digipath.  This is so that at a special event or local area, using
direct or one hop, then your net-cycle time is 10 minutes.  Two hops is
20 minutes, 3 hops is 30 minutes etc up to the maximum value in your
CFIGxxx.APR file.  Currently the MaxTime defaults to 30 minutes.

Ver 783 added the MAPS-OVERLAY-DIGIS command so that you can see the 
location and range of all APRS digis no matter where you are..  Please 
send me data on any new digis so we can keep this DIGIS.POS file
up to date.   Also the format for the WIDE-RELAY digipeaters changed...

    
BACKGROUND:  The range of any AX.25 packet may be extended by specifying
one or more digipeater callsigns.  The packet will be relayed by each
such digipeater in turn.  After each such digipeat, that callsign is
marked as used up so that at any instant, only the "next" digipeater
in the list has the potential to digipeat the packet.  Normally this
requires users to know a priori the complete intended path for their 
packets.  This a priori knowlege of the network is NOT desired for APRS.

   APRS satisfies its real-time, emergency tactical needs without a priori
knowledge by simply using generic callsigns.  ALL APRS stations are given
the generic digipeater callsign of RELAY.   This way any station can use
any other station as a digipeater by simply addressing the packet VIA 
RELAY.  With this form of generic digipeating (RELAY), a mobile, or new 
station does not have to know anything about the network in advance in 
order to be seen by adjacent nodes.  After 10 minutes and his map begins 
to show the location of all stations and digipeaters on frequency, he can 
then customize his outgoing Unproto path to specific digipeater callsigns 
to cover his intended area without as much QRM.  Similarly all wide area 
digipeaters have the generic digipeater alias of WIDE.

CAUTION:  Since the preferred APRS frequency of 145.79 is adjacent to the 
SATELLITE band at 145.8, all APRS users should run minimum power.  High
and WIDE digipeaters in residential areas or near other satellite ops
should probably operate at 10 watts or less to minimimze QRM.  The purpose 
of APRS digipeaters is to HEAR mobiles, NOT to be an aligator!

ROUTES: It is important that as APRS networks mature with fixed, known 
digipeaters, that users at FIXED stations should avoid using the generic 
RELAY or WIDE addressing.  Although it still makes sense for mobiles to 
use the path of RELAY,WIDE, the path of RELAY should rarely be used after 
the first hop by ANYONE, and never after a WIDE.  Remember, every packet
addressed via RELAY will key up EVERY APRS station that hears it.  In 
any but the sparsest areas, the result is total congestion and collisions
which block anyone from copying the packet.  The D-list command lets 
you see what digipeater paths other stations are using and it also marks 
stations that you can hear direct.  Also under the OPS-DIGI command, users 
can save up to 12 different DIGIpeater paths.  Users can select any given 
path that is optimum for their present application with a single key 
stroke.  The MAPS-PLOTS-POWER command will display a range circle around 
all stations proportional to their power, and antenna.  Users can use 
these plots to estimate what paths, through what stations, might be useful.

     Although digipeating is generally not good practice for AX.25 level 2
connections, it is ideal for APRS operation using UI frames only.  The low 
duty cycle of APRS allows everyone to share the channel without anyone able
to totally hog the frequency.  Even Personal mail boxes are acceptible on 
the APRS frequency, since mail is posted at keyboard rates and is NOT read 
back by radio.  Users are NOT welcome to READ PBBS mail on the frequency.  
Other CONNECTED mode operation of BBS's, mail forwarding, file transfers, 
TCP-IP and DX clusters are discouraged.

APRS DIGIPEATERS:  Wide area APRS digipeaters should be widely separated to
provide long distance coverage with the minimum of hops.  But this does not
preclude the need for many interim RELAY digipeaters to fill in weak signal 
areas or valleys.  These sites provide the first hop (via RELAY) for all 
mobiles, which inturn then relay the packets to the main WIDE digipeater.
These WIDE sites link to other cities and provide a backbone for wide area 
coverage.  

WIDE DIGIPEATING:  Since these WIDE area digipeaters are located at 
excellent locations, they should not only provide the WIDE backbone
function for the long-haul, but should also have the RELAY alias as well
for nearby mobiles and new stations.  Many recent TNCs can be set up to 
digipeat either of these generic RELAY/WIDE packets.  The new PacComm 
TNC's can support up to 4 aliases!.  Set MYAlias to WIDE and set one of 
the other TNC functional callsigns to RELAY (such as MYnode, or MYhost, 
etc).  If you use the KPC-3 and the MYnode call, be sure to set USERS 
to 1 and NUMNODES to 1.   These WIDE-RELAY backbone Digi's should be 
spaced 50 miles or more apart depending on topology so that they are 
as widely separated while still being able to hit each other.  All 
mobiles typically use the RELAY,WIDE path so it does not matter whether 
they are near a DIGI or someone's home station to be digipeated.

    Even if these WIDE/RELAY backbone nodes are 30 to 50 miles apart, as
long as every home station and local RELAY digipeater can hit at least
one WIDE, then the mobile path of RELAY,WIDE can cover as far as 100
miles!  Wider ranging mobiles can use the RELAY,WIDE,WIDE path without  
causing too much QRM because of their low antennas.  BUT CONVERSLY, 
RELAY,WIDE,WIDE should NEVER be used by a home station since he will
undoubtly hit many home RELAYS all at the same time and therefore generate
numerous dupes with every packet.

CAUTION: Fixed stations that can hit 2 or more WIDES should NEVER use three
generic RELAY/WIDE callsigns in a row, and RELAY should NEVER be anywhere 
except the FIRST in the list.   FIXED stations should always avoid any 
GENERIC calls if possible after the first two.   Although generic paths
for mobiles are the normal mode of operation, special consideration must
be given whenever there will be a great convergence of generic mobiles
using RELAY,WIDE paths.  Remember, every APRS station INCLUDING MOBILES
will also have the ALIAS of RELAY, so when they all get within range of
each other, there is quite a conflagration!  To minimize these problems
here are the typical recommended settings for ALIASES and PATHS:

   TYPICAL MOBILE  (1-5 watts):  UN APRS V RELAY,WIDE     MYAlias = RELAY
                   (25-50 w  ):  UN APRS V WIDE,WIDE      MYAlias = RELAY

   STAND ALONE TRACKERS:  MYAlias = NONE

All Mobiles converging to one location should hit CONTROLS-BANDS-HF which
will change their MYAlias from RELAY to ECHO.  This will eliminate everyone
from digipeating everyone else's VIA RELAY packets.  They should also
choose a new UNPROTO path specific to that location, without beginning
with RELAY!  The alias of stand-alone trackers should be typicaly set
to NONE since they cannot be quickly changed in the field.


DEDICATED WIDE AREA APRS DIGIPEATER SET UP

   To set up a WIDE area APRS digi, simply connect a TNC to a radio, and put
it as high as you can get it.  Set the following minimum commands:

cmd:   MY W3XYZ-x                 (the digipeater call and SSID)
       MYA WIDE                   (this makes it digipeat WIDE packets)
       MYN RELAY (or)             (This works on KPC-3s as a second alias
                                  (Also requires NUMNODES and MAXUSERS=1)
       MYG RELAY                   (on PK-12)
       UNPROTO APRS VIA WIDE,WIDE,DIGI3...     (so the whole net sees it)
       B E 60                     (Sets Beacon to once every 10 minutes)
                                  (or B E 10 for Kantronics)

       BT !DDMM.mmN/DDDMM.mmW#PHG5360/W-R.... (identifying comments)
            |      |  |      | | |||| 
           LAT     | LONG    | | ||||________ Omni (Direction of max gain)
                   |         | | |||_________ Ant gain in dB
                   |         | | ||__________ Height = log2(HAAT/10)
                   |         | | |___________ Power = SQR(P)
                   |         | |_____________ Power-Height-Gain PHG indicator
                   |         |_______________ # is symbol for digipeater
                   |_________________________ / for WIDE, \ for WIDE-RELAYS
                                              OR put the characters W-R 
                                              in the comment field

As you can see by the integers in the POWER-HEIGHT-GAIN (PHG) string, there 
are only 9 plus 0 possible values for each of these fields as follows:

  DIGITS   0  1  2   3   4   5   6    7    8    9  as used in the Pwr field
  -------------------------------------------------------------------------
  POWER    0, 1, 4,  9, 16, 25, 36,  49,  64,  81  watts  SQR(P)
  HEIGHT  10,20,40, 80,160,320,640,1280,2560,5120  feet   LOG2(H/10)
  GAIN     0, 1, 2,  3,  4,  5,  6,   7,   8,   9  dB
  DIR      0,45,90,135,180,225,270, 315, 360,   .  deg    D/45  This offsets
           *                                       the range circle in the
           * 0 means OMNI                          indicated direction

HEIGHT ABOVE AVERAGE TERRAIN:  Going out 10 miles in all directions, write
down the elevation every mile or so.  Average all of these points and
compare your elevation to the average.  You may be at 2000 feet above
sea level and have a 150 foot tower, but if the ground around you is at
2200 feet, then your HAAT is -50 feet!!!  Be honest!  Your circle should
go no further than the distance to which you can reliably copy an HT!  
Even though you have an OMNI antenna, if the terrain favors a certain 
dierction, then put that in for your directivity.

                                       
X1J DIGIPEATERS:  You may set up an X1J node to act as a generic
WIDE area APRS digi, depending on whether the ALIAS callsigns are not 
already in use.  The X1J permits 5 callsigns.  The first two are the NODE 
callsign and the NODE alias which apply to level-4 NODE operation and 
can NEVER be made WIDE or you will LOCK-UP the network.  The next three 
ALIASES are for a BBS, DXCLUSTER or HOST mode operation.  By enabling two 
of these extra aliases to WIDE and RELAY, then the node will digipeat APRS 
UI frames as long as the following commands are also performed:

   PARM / 23 1   Digipeat enable  (info provided by WA4HEI 906 341-5718)
   MODE / 14 1   Extra alias monitoring enable
   MODE / 17 3   Refuse digipeated L2 node uplinks & downlinks


OPERATIONS WITH RELAY AND WIDE:

     Although the GENERIC WIDE/RELAY digipeating works well to get an APRS
net going, once you have more than two WIDES, the generic calls should be
avoided by all fixed stations to minimize unnecessary duplicates and
collisions.  Using SPECIFIC callsigns significantly reduces QRM.  Usually
WIDE,WIDE,DIGI3,DIGI4 will get you out 2 hops in all directions and 4 or 
more hops in the direction of DIGI3 and DIGI4.  If you want to go long
distance in two directions, save another long path in the other direction
using the OPS-DIGi-SAVE comamnd and then activate it 50% of the time
usign the OPS-DIGI-ALT command.

     While building a new network, some well situated home stations may 
need to operate as WIDEs temporarily.  To do this, simply manually 
configure one of your TNC's other calls to WIDE.  MYAlias will not work,
because APRS will always reset that to RELAY.  You should not set yourself
to WIDE unless you have local agreement to do so.  Too many WIDE's, too 
close together causes too much QRM.  If you are operating as a WIDE-RELAY, 
be sure to put the BACKSLASH character between the LAT\LONG so you will
show up as green digi or select the DIGI# on the  symbols menu.  This will 
work for a weather station too, to show since it re-writes the POSIT string 
every few minutes.  Also, every time you switch from HF to VHF or the 
reverse, your ALIAS is re-written to RELAY or ECHO.

SEE README.HF for setting up your UNPROTO path for HF and HF/VHF gateways..



**********     WIDE-N ALL DIRECTION GENERIC DIGIPEATING!     ****************
          THE ULTIMATE SOLUTION FOR MOBILE POSITION REPORTING

    We have been asking for 4 years now for someone to develop the WIDE-N
digipeater code.  The WIDE-N digi simply repeats ANY packet with the VIA 
address of WIDE-N; but ONLY ONCE.  If it keeps a copy of the last 30 
seconds of packets, (or their checksums only) and compares each new packet 
that it hears with these last ones that it digipeated, then it could decide 
NOT TO TRANSMIT it again!  This would completely eliminate the multiple 
round-robin multiplication of packets generated when a mobile station uses 
the present generic path of WIDE,WIDE,WIDE.  As it is now, such a 3 hop 
path launched in the middle of 3 WIDE digipeaters that all hear each other 
can generate as many as  3x3x3 or 27 duplicates in the area.  If each WIDE-N 
only digipeated the packet once, then there would only be 3 local copies 
generated, and the packet would still propagate outward 3 levels in all 
directions in the usual manner!  

NUMBER OF HOPS:  Using the via path of WIDE-N, each DIGI that repeats the
packet decrements the WIDE-SSID by one.  This way, the range of the packet 
is self limiting and is under the control of the originator.   If he sets 
it to WIDE-7, then the packet could be digipeated as many as seven times 
outward.  A local commuter might select WIDE-2 to cover his area while 
limiting DX QRM.


NEW TNC COMMANDS REQUIRED:  To make this work, there are three new TNC
commands needed:

     MYFlood:  This command, similar to MYAlias, sets up the callsign at the
     digipeater to be used as the WIDE-N callsign.  In APRS networks, we
     will use WIDE.  THIS IS FOR UI FRAMES ONLY!  That plus the AGELimit
     below, makes it un-useable for connected frames, and for abuse.

     HOPLimit:  This is a SYSOP parameter that can be used to limit the
     maximum number of HOPS permitted in a network.  Already the maximum
     number of hops is limited to 7, since the upper bit of the SSID is
     reserved for future use.  However, some SYSOPS may feel empowered 
     to limit the maximum number of hops to some smaller number.

     AGELimit:  The WIDE-N digi has to keep a copy of all digipeated packets
     for a brief period for comparison with new packets heard.  This is the
     key to the WIDE-N algorithm.  Every digi that hears a UI packet will
     digipeat it, BUT ONLY ONCE.  To do this, it compares each new packet 
     with all packets digipeated in the last X seconds.  This X age-limit 
     needs to be a variable depending on the speed of the network.  If the 
     time is too long, then the list is big, If it is too short, then there 
     is the chance that a packet may propogate in a circle and get repeated 
     again.  My guess is that 30 seconds is a good default value for 1200 
     baud channels.  Actually only a checksum of each packet needs to be 
     saved instead of the whole packet.

     If ALL APRS WIDE area digipeaters changed over to the WIDE-N code, we
would HAVE THE ULTIMATE GENERIC MOBILE GPS NETWORK!  Metropolitan area
commuters could set their digi paths to WIDE-2 and Mobiles on extended trips
within a 200 mile radius of home could be tracked with a path of WIDE-5,
without fear of clogging the network!  This WIDE-N algorithm is so powerful
that it could even be added to ALL TNC's as a FLOOD-N mode.  It would only
work on UI frames and would default to the generic callsign of WIDE.
That way, on ANY packet frequency, a UI frame launched into the air, could
be seen by EVERYONE!


LEVEL FOUR and TheNET CONSIDERATIONS:

     Since NODES are so much smarter than digipeating, the ultimate solution
is to have the NODES do all UI frame routing.  The APRS station simply sends
his UI frame TO APRS VIA HOME;  Any NODE hearing that transmission that has
knowledge of the route to HOME, will send the single packet via the NODE
network (internode, level 4) to the HOME node!  When it arrives at the HOME
node, it is transmitted once as a UI frame.  With this arrangement, a mobile
only has to specify his one intended destination, no matter where he travels!

    The G8BPQ TheNET code for the DataEngine includes a DIGIon command that
can restrict Digipeating to UI frames only.  This is very useful, in that it
allows SYSOPS to leave DIGI ON without inviting LEVEL-3 connects to use it
which is detrimental to normal level-4 operation.  Back in 1995 you could
not make the original 2 TheNET aliases RELAY or WIDE since they were for
level 4 only.  Rumor has it that now G8BPQ code permits 2 other digi only
aliases.


DIGI/NODE COMPATIBILITY:  Since the user should not have to change his digi
path as he drives from one area to another, he should be able to specify a
path that is compatible with both nodes and digipeaters.  This is easily
accomplished by assuming that the LAST field in an UNPROTO digi list is the
HOME NODE and should be the ultimate destination for the UI frame through any
level 4 network.  Any and all preceeding fields are assumed to be DIGI's only.
With this arrangement, the user could use an UNPROTO path of APRS VIA WIDE,
HOME so that any generic WIDE digipeaters would repeat his position to their
local area as would any WIDE NODES in the usual digi fashion.  Only the
node that hears the direct packet would also forward it through the network
at level 4 to the HOME NODE.  If only one field is included in the digipeater
string, it would be interpreted as both a digi and a HOME destination without
any difficulty.  Digi's and NODEs would digipeat it, and nodes (hearing it
direct) would forward it at level-4.  It is important that NODES hearing
digipeated UI frames from other digis do NOT enter the packet into the network,
to eliminate duplication.  Only the ones hearing the direct signal should
be repsonsible for doing the level 4 routing..

EXAMPLE:  A typical mobile just wanting to keep his spouse informed of his
whereabouts might want to just use the UNPROTO path of APRS VIA HOME.  Then
his UI frames will be digipeated by the local HOME node or digi and will
also be routed back to HOME by all NET-NODES along his travels.  If he also
wants to be seen by most HAMS in the areas of his travels, then he sets
his path to APRS VIA WIDE,HOME.  If he travels through a region that has
both DIGIs and NODES, he might choose APRS VIA WIDE,WIDE,HOME.  This way any
areas with digis would digipeat via WIDE,WIDE and if he gets to an area with
nodes which are aware of the path to HOME, then they will forward his packet
there.


