DIGIS.TXT 7.8.4     AUTOMATIC PACKET REPORTING SYSTEM DIGIPEATERS

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 that are installed so we can keep this DIGIS.POS file
up to date.   Also the format for the WIDE-RELAY digipeaters changed...

    
BACKGROUND:  To satisfy the objective of an APRS network without dependence
on specific nodes, APRS stations are designed to begin operating without
any prior knowledge of the network.  This is done by having all APRS
digipeaters and home stations initialized to act as a digipeater with 
the alias of RELAY and to send all UI frames via the UNPROTO path of 
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.  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.  If your TNC does not have dual aliases, AND 
yours is the only WIDE around, you can temporarily make its MYcall be 
WIDE and its MYalias be RELAY, but be sure to put the FCC callsign in 
the BText to keep it legal.  You can also have one other DIGI with the 
opposite MYcall RELAY and MYalias WIDE without confusion.  These WIDE/
RELAY backbone Digi's should be spaced several tens of miles 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 picked up by the network. 

    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.  Yes, RELAY,WIDE,WIDE should NEVER be used by a home 
station, but since a mobile can rarely, if ever, hit more than 1 digi the 
first time, the number of repeats is not much worse than a typical home 
station via WIDE,WIDE.

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/WIDE-RELAY...(identifying comments)
            |      |  |      | | ||||  |___|_ makes station show up green
           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

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.  These pertain 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.  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

    This may be the simplest mechanism for making an order of magnitude
improvement in APRS status and position reporting networks.  In a WIDE-N
network, each WIDE 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 at least a copy of the FCS) and compares each new packet that it hears
with the LAST one 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 tier 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!
Since this algorithm has uses on ALL amateur packet networks needing a
FLOODing mechanism for special packets of interest to everyone, I am calling
it the FLOOD-N algorithm.  For APRS, however, I will still refer to it as
the WIDE-N algorithm.

NUMBER OF HOPS:  Using the via path of WIDE-N, each DIGI that repeats the
packet decrements the WIDE-SSID by one.  This way, each packet carries along
with it information on how many times it has bounced through the network.
The user specifies how far it goes, by the initial setting of the SSID!
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 FLOOD-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, on NON-APRS networks, 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.  Howie
     Goldstein, N2WX has improved the effeciency of this idea, by only saving
     and comparing the CHECKSUM of the packets, 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 FLOOD.
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.  The problem, however, is
that the generic APRS RELAY and WIDE aliases cannot be used on a NODE, since
the digipeat ALIAS is the same as the NODE alias, and you cannot operate more
than one NODE with the same ALIAS on the same frequency or even in the same
network!  I got a call yesterday, 22 Nov 95 that G8BPQ has now agreed to add
two more UI digipeater aliases that are not tied to any of the node functions.


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.

     Finally, since most regional area Tracking networks are dedicated to
APRS, the node should also permit the SYSOP to turn off other level 4 
routing.  Without this feature, such a network could be swamped if all of 
the BBS and other CONNECTED protocol users began to use it, and the original 
purpose of the network would be defeated.

   Still, most of these APRS support ideas could be included in all NODES so
that a minimum of APRS tracking could be supported by ALL networks on all
frequencies, especially where there is not yet a dedicated APRS TRACKING
NETWORK.  I think there are other undeveloped applications for shipping UI
frames through ALL networks which have not yet been explored.  The capability
should be there, in any case, so that experimentation can proceed.  Time will
tell.  These few paragraphs were primarily written to the NODE CODE writers
such as John G8BPQ and Mr. Roberts of X1-J.  But are included here in general
distribution for all to read.

