WA8LMF Mirror of WB4APR Website - 21 July 2008
Forward: This page was written in 2005 and was part of the initiative of the New-N Paradigm. It is retained for historical purposes.
SUMMARY: Most of the APRS collision problems in LA can possibly be fixed by limiting ALL packets to ONE HOP in the greater LA basin! Simply take a look at this map and follow this line of reasoning.
ALOHA IS COMPLETELY QRM LIMITED: Notice that ALL of LA and its surrounding areas can theoretically be seen by one tall digi. Of course, due to shadowing, it does take a few more, but the reason we have such a small ALOHA user limit of typically 60 or so is because of the muliti-hops that every packet is taking! With just 2 hops, every packet is cluttering SoCal with potentially 16 copies of itself!!! because of all the digipeaters.
THE SINGLE DIGI CONCEPT: But applying the ALOHA calculation for a SINGLE digi in central LA, and one hop for everyone, the 144.39 channel could handle almost 180 users without problems! By making this digi full duplex, so that the channel is 100% available for uplinks, then a full 360 typical APRS stations could be accomodated with high reliabilty. This is more than everyone within 100 miles of central LA!!!
WHAT ABOUT ALL THOSE DIGIS?: The reason there are a dozen digis in the same area as can be handled by one, is because the channel is totally oversaturated by QRM by all the multi-hops and dupes and copies. Thus it is impossible to get into the high central site because of QRM. Thus, users had to put up more digis in their local areas so that their local signal was stronger than everything else the digi was hearing, and then they had to add another hop (quadrupeling the QRM) to get into the BIG digi! This process just snowballs to where the only way for success is more and more digis with more and more QRM and less and less range. IE, MELT-DOWN!
THE SOLUTION: Although we spent along time trying to solve the problems above with the SUPER-SITE concept and going to 9600 baud, it is immediately obvious that the SUPER-DIGI solution does not take advantage of the fantastic geography available in LA! That is, turning off ALL incoming packets from out of area and using a single BIG digi to cover the basin.
The problems to this simplistic KISS approach, however, are varried and real:
THE REAL WORLD: Probably what would work well would be just a few digis that are optimally placed to cover the entire area. These BIG DIGIs would ONLY respond to ONE hop and ONE HOP ONLY. This would immediately eliminate all out of area QRM and also all ping-pongs in the basin and with a the ONE-HOP only system, success would return to APRS in LA!!! Here would be the phase in plan leading up to DIGI-DAY or "D" Day in LA:
PHASE-1:
PHASE-2:
PHASE-3, "D"-Day: At this point, everything in LA is operating at 1 hop using either "WIDE2-2" or NEWLA but there are still four copies of every packet flying around because every station not only hits his local digi, but also hits one or more of the big 3?. But this is much better than it used to be and so the BIG 3? are starting to hear mobiles further out because of less QRM. Now is the big day.
PHASE-4: But this 180 stations is still about half of the potential full load within 100 miles of LA. To get to the ultimate 360, it is necessary to go to a full -duplex system so that the big 3? digipeaters themselves are not QRMing each other, but are providing a full 100% channel feed. To do this, each of the Big DIGIS should switch to a UHF transmitter and operate their TNC's in full duplex mode. Now each of the big 3 digis can EACH support a throughput of 360 users of every single station they can hear! Users still transmit on 144.39 but can tune in on UHF any of the BIG 3? they want to watch.
WHAT ABOUT 144.39 OUTPUT? When the big digis stop half-duplex digipeating and only digipeat on UHF, doesnt this then prevent anyone running normal APRS on 144.39 from hearing all the activity?... YES and NO!!!
INTERNET and IGATES: Here is where some of the almost-abandoned low-tier digis make their best contribution. These digis also add an individual UHF full duplex AUDIO relay of their 144.39 RECEIVER. On this UHF channel, now, the local IGate has the full EARS on 144.39 for everone in range of that digi and can build the LOCAL LIST for that low-tier digi. This IGate then, with one-hop-only, and ONLY using the SPECIFIC digi callsign of that digi, can send any Internet traffic back to RF for any station in its LOCAL area with minimal QRM to the rest of LA. Since the IGate is using a directed path, there is just one and only 1 copy of this traffic. This is an ideal situation for IGates. Like the Cell phone system, they concentrate only on their small area and do not QRM
LOW-POWER TRACKERS: Another application for the almost-abandoned low-tier digis is to QSY their input to the alternate APRS input channel for low-power trackers (144.99 if available in LA) These digis now only listen for low power trackers using a 2 hop path and digipeat them over onto 144.39. These digis should drop back to an output power of say, 5 watts because all they have to do is hit one of the BIG 3? on the second hop. The only alias they support is RELAY for the low-power trackers that are using the universal APRS path of RELAY,WIDE.
WHAT ABOUT ABUSERS USING MULTI-HOP PATHS? Actually, 1 hop is ENFORCED in LA even thought the BIG 3? did not even have to upgrade to UIDIG ROMS! This is because, when they switched to CROSS-BAND full duplex digipeats, the BIG 3? no longer hear each other! Thus even an abusive path of NEWLA, NEWLA, NEWLA goes still only 1 hop via any one of the BIG 3?, but no farther.
RESULT: APRS works as a LOCAL system throughout LA with maximum efficiency and can support about 300 stations without much QRM. APRS at its finest. Possible only because of the high density of population and the presence of very high digipeater sites. AND of course the use of ONE-HOP only FULL DUPLEX digipeating.
THE FINAL PUNCH LINE:
And saving the best for last, go back to step one, and instead
of using NEWLA as the new special one-hop generic path for the LA BASIN, make it
be RELAY in the first place!!!
This is possible since RELAY has been banished completely in the past from LA so not many people
are using it. Also then it makes LA 100% compatible with the rest of the world that
is using RELAY,WIDE a the universal path. Also once you get out of range of the big
LA digis, the next tier outward can be allowed to support RELAY and WIDE for up to 2 hops
only. Also RELAY,WIDEn-N can even be supported with smart digis that truncate N to
only N within N hops of LA or other city...
The only down side will be any home stations that are still acting as RELAY and
these will be impossible to track down since most of them do not do callsign substitution.
But even this can be solved. Here is the plan:
Once all RELAYS have been exterminated, and not until then, add the RELAY ALIAS alongside the NEWLA in the BIG 3? DIGIS.
Then you can keep NEWLA as the LA Basin home path, but then now LA fully supports the universal path of RELAY
but at no expense to existing operations!!!
Enjoy.
You are visitor number:
Since 6 Jan 2004.. The Naval Academy is a registered user of APRS
and WinAPRS. The purpose of this web page is to show several applications
currently in use at this site and should not be considered as an advertisement
or an endorsement of any commercial product.
WA8LMF Mirror of WB4APR Website - 21 July 2008
de WB4APR, Bob