            AUTOMATIC PACKET REPORTING SYSTEM DIGIPEATERS

     Although digipeaters work poorly for AX.25 level 2 connections, they are
ideal for APRS operation using UI frames only.  In the Washington DC area and
Chesapeake Bay area, we are establishing a network of wide area DIGI's on the
simplex packet frequency of 145.79.  This frequency is for Keyboard QSO's and
all UI frame applications.  Operation of BBS's, forwarding, file transfers,
TCP-IP and DX clusters are discouraged. 

    All wide area digipeaters have the same alias of WIDE in addition to
their normal HAM callsign.  These wide area Digi's are spaced several tens of
miles apart so that they are not too close, but that they can hit their
adjacent other WIDE digi's.  Similarly all APRS stations are initialized to
have an alias of RELAY and to send all UI frames via the path of RELAY.  THis
way, a mobile, or new station on the air does not have to know anything about
the network in advance, but to simply turn on his computer 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 customize his outgoing
Unproto path to specific digipeaters to cover his intended area.

     Assuming WIDE are digipeaters are about 30 to 50 miles apart it is very
easy to select an UNPROTO path prior to a road trip which will assure that
your location packets will always get back to your home area.  In the
following examples the HAM calls of EAST and WEST are used for clarity.


        WEST-3    WEST-2    WEST-1    HOME    EAST-1   EAST-2   EAST-3
         WIDE      WIDE      WIDE    RELAY     WIDE     WIDE     WIDE


    If the mobile is going east for the day, and will be operating in the
vicinity of EAST-3 digipeater, the operator can preset his UNPROTO path to be
via WIDE,EAST-2,WIDE.  Notice that not only will his packets make it back to
home from the area of EAST-3, but also from the area of EAST-1 since EAST-1
will also respond to the first WIDE in the list.  Similarly, stations in the
vicinity of EAST-3 are alerted to his movements as he leaves home, since the
WIDE,EAST-2,WIDE specification is symetrical.  If he set the UNPROTO path to
EAST-3,EAST-2,EAST-1 in the usual manner, he would not be tracked at his home
until he actually arrived at his destination.  As you can see, having the
flexibility to alternate the generic alias's of RELAY or WIDE with other
known sites gives a good degree of flexibility without having to change the
UNPROTO path while on the road.  Using the three digipeater string, he can
wander up to 150 miles in his planned direction and still be tracked by the
XYL.  If he has no idea where he is going, he can always use the path of
WIDE,WIDE or even WIDE,WIDE,WIDE and go anywhere, but with greater QRM on the
channel.  Yes there are multiple collisions, and repeats, but the packet does
get out to the third tier!

   The ultimate APRS digipeater configuration is to have modified TNC-2
digipeater code so that any digipeater hearing a packet with its callsign
anywhere in the UNPROTO path will digipeate the packet as long as it was not
previously digipeated by any stations earlier in the list.  This way, to
always report your movements back home, you always place digipeaters in your
UNPROTO command in the reverse order of your travels.  Your packets will be
digipeated back to your home area as you enter each new digipeater in your
direction of travel.  For example:


     HOME  DIGI-1    DIGI-2    DIGI-3    etc

     If we can get TAPR to modify the code, the mobile could specify the
UNPROTO path of VIA DIGI-3,DIGI-2,DIGI-1 in order to be tracked anywhere all
the way out to the area of DIGI-3.  If the mobile is in the DIGI-1/DIGI-2
area, DIGI-1 will repeat the packet.  DIGI-2 will not digi the DIGI-1 packet
because it is to the right in the list.  If DIGI-2 also hears the mobile
packet, it will pause and if it does not hear the DIGI-1 repeat, then it
will.  When DIGI-1 hears the DIGI-2 packet, it will further digipeate it,
since DIGI-2 is to its left in the field.  Similarly as the mobile enters the
2/3 area, DIGI-2 will digipeate back in the direction of DIGI-1 and DIGI-3
will pause and not transmit.  If DIGI-3 does not hear DIGI-2, then it will
digi the packet and it will go back via 2 to 1 as desired.

     This algorithm works perfectly well in reverse.  If a mobile desires to
announce his progress forward in the direction of his travel he can specify
the digipeaters in the forward direction.  Then using this algorithm, all of
his packets will be repeated in the forward direction, but not in the
backward direction.

     Until we get new UI forwarding algorithms in standard TNC's, however,
the general aliases of WIDE and RELAY will do nicely.  If fixed, known
digipeaters are available, even with the generic alias of WIDE, it is best
for fixed APRS stations to use the digipeaters unique callsign instead of
alias to avoid any ambiguity.
