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README.DX
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1994-06-24
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README.DX USING APRS FOR MONITORING DX CLUSTERS
Since APRS was designed to monitor a packet channel and to capture
selected packets for display, it is an ideal tool for the DX enthusiast. The
position reporting and operator-to-operator message capability of APRS using
UI frames performs the same functions as the DX cluster, but at a significant
improvement in channel efficiency. In addition, the DX spots appear on a map
of the world instead of in text form! The efficiency improvement of APRS is
due to the elimination of the need for a separate transmission and ACK from
every DX cluster user for every spot report. APRS on the other hand, uses
its decaying BEACON algorithm to transmit the spot quickly but redundantly
enough to assure delivery. If there are 20 users logged on to a DX cluster,
then under ideal conditions with NO collisions, then there are a minimum of
40 packets involved. APRS under IDEAL conditions only needs ONE packet.
Even if APRS repeats the packet 4 times to assure that every station gets it,
then there is still a ten-fold reduction in QRM by using APRS.
APRS MONITORING: Due to the entrenched nature of most DX clusters, however,
it is doubtful that APRS will ever replace the DX cluster concept. Paul Evans,
G4BKI, at PACCOMM, suggested using APRS to monitor existing DX cluster
operations. In response, I added a DX cluster mode in APRS version 5.05 that
facilitates the monitoring of existing DX clusters. In this DX mode, APRS
captures and maintains a list of all recent DX spots. The user can review
these spots at will. Secondly, if the spot contains a gridsquare as the first
four letters of the comment field, then APRS puts the DX station on the map.
If there is no grid square, then APRS does a lookup of the callsign prefix
and places the station on the map in the country listed. Thirdly, since APRS
observes all of the redundant spot transmissions to all users, it can
accumulate a list of DX cluster users after hearing only one spot report!
RELIABILITY: Although the APRS user will probably capture the DX spot on the
very first transmission, it can be easily shown that he has an equal, if not
better, probabliily of GUARANTEED delivery as the connected users. If there
are 10 users logged onto the DX cluster, then an APRS user monitoring the
channel sees 10 duplicate transmissions. Similarly, a connected station
running typical TNC defaults only gets 10 attempts before he times out, so
he has no better chance than the APRS user to see the packet. In fact, if
there are two connected stations having RF problems, then the APRS user could
see as many as 28 duplicate transmissions of the spot giving him an almost 3
to one advantage over any connected station!
APRS DX OPERATIONS: To operate APRS in the DX mode, simply toggle on the DX
mode using the dX (X) command under the CONTROLS menu. All APRS functions
continue to operate in the usual manner, but with the following four
modifications. First, DX mode turns off the normal APRS filter so that it
will monitor all OTHER packets on frequency and not just APRS packets (see
warning below). You will see the *DX* flag on the control panel where you
usually see BCNS. Secondly, it saves all DX SPOTS on the ALL BEACONS list
which can be displayed at any time using the A key. The ALL BCNS list will
still get saved to the LOGS directory everytime the list gets greater than 50
entries. Third, the normal use of the ALL list is disabled so you will NOT
accumulate a log of APRS beacons. Fourth, APRS will attempt to parse a grid
square out of the DX spot comment field. (We should encourage DXers to always
type the grid square as the first four characters!) If the grid square is
absent or in the wrong format, APRS will use a table lookup of the callsign
prefix to find the location of the station. Fifth, APRS captures the
callsigns that the DX cluster is transmitting TO and adds them to the LATEST
list. It also saves the latest packet from each station that it hears on the
LATEST list, so that you can see what was the latest DX command used by each
connected user (that it can hear). To exit DX mode and return to normal APRS
BEACON or OTHER monitoring, simply select one of these other modes under the
CONTROLS menu. The DX mode can be saved in a CONFIG file, so you can always
start up in the DX mode if desired.
WARNING: In order for APRS to keep up with the deluge of packets from a DX
cluster, it is running wide-open with minimum filters and context checking.
This is so that it can try to parse all packets for Position reports, grid
squares, etc. The problem is, that it will make mistakes whenever a character
string looks like a grid square report in just the right places; and worse,
there are some fields that can cause CRASHABLE errors. I could have addaed
a lot more filters to eliminate false parsing, but this would slow processing
down significantly and probably make slower machines CHOKE on all the packets
flying out of a DX cluster. So take all plotted positions with a grain of
salt... IE: do a sanity check... In the APRS portocol, only grid squares
surrounded in brackets are used, but on DX clusters, I had to assume that the
brackets would not be there.
IMPLEMENTATION: APRS users can immediately begin to use APRS to monitor DX
cluster activity. For each conventional cluster user that drops his
connection to the cluster and begins to use APRS in the monitor mode, there
is a proportional reduction in the burden on the DX cluster. All users
therefore see an overall improvement in channel capacity, while the cluster
is still serving the same number of users! Of course, this improvement has
a limit. If every single DX cluster user shifted to the monitor mode, then
there would be no one still connected to assure that spots still got
transmitted! The mimimum user number would probably be around 3.
FUTURE CONSIDERATIONS FOR THE DX CLUSTER PROGRAM: It would greatly improve
the effeciency of many DX cluster operations, if DX clusters would encourage
stations to drop into the APRS monitor mode. Only a few minor modifications
to the DX cluster code could greatly facilitate this concept:
1) To assure that there are always enough spot transmissions to assure
guaranteed delivery, the DX cluster code should simply detect when the number
of users dropped below 3 and should simply transmit the balance as UI frames.
So with one user logged on, his packet would be transmitted, along with 2
other UI copies (hopefully spread out over a minute or so).
2) The DX cluster should look for and accept incomming DX spots from users
transmitted as UI frames. Prefereably the APRS single line message format
would be used so that the user would get an ACK from the cluster to confirm
it was entered into the system.
3) The DX cluster could easily send and receive APRS one line messages to
its monitoring APRS users, since they are nearly identical to the existing DX
cluster TALK message format.
CONCLUSION: If some of the casual DX cluster user switched to APRS monitoring
instead of reamining connected to the DX cluster, the burden on the DX cluster
would be greatly reduced to the benefit of everyone in the net. If your DX
cluster is serving more than a dozen users, then you should consider shifting
most casual users over to APRS monitoring. This could result in a ten fold
increase in the effeciency of distributing DX spots. Of course, the DX cluster
offers a lot more capability than just DX spots, so APRS will not ever replace
the database capability of the DX cluster...
CALLSIGN LOCATOR: I was so excited about using the grid square plotter built
into APRS for this application, and my first evening monitoring, most of the
DX spots contained Grid numbers. Two days later when I was ready to release
5.05 I noticed that all evening long, there were NO GS reports! Seems that
only VHF'ers use GS, and I just happened to be listening that first day,
during some sporadic E... Therefre, I had to quickly make a callsign prefix
table so that geographic location could be derived from callsign alone. This
file is named DXCALLS.DAT and is just a list of CALL prefix, LAT, and LONG.
It was just a start, and only contains 120 entries. If anyone wants to fill
out this file completely for all COUNTY prefixes, please do, and send it to
me. Thanks. If the list will be longer than 150 entries, tell me so that
I can increase the array dimension size in the program...
DEFAULT MAP: Notice that when you select DX cluster mode, APRS automatically
sets the default map to the middle of the Atlantic to favor the East Coast
guys. To customize the map to your favorite center, simply move the cursor
and hit HOME key. Then save a CONFIG file which will remember that setting.