Forward: This page was written in 2005 and was part of the initiative of the New-N Paradigm. It shows how to greatly imrove APRS reliability in heavily congested areas by simplying forcing all packets to one hop only. Simply take a look at this map and follow this line of reasoning.
An ALOHA Channel is QRM LIMITED: Once the channel as heard at the digipeaters is more than say about 30% busy, there is a 30% chance that most users will not be heard and the network is unreliable or worse. We can no longer allow multiple-hops on every packet! With just 2 hops, every packet is cluttering the surrounding area with potentially 16 copies of itself because of all the digipeaters!!!
THE SINGLE HOP CONCEPT: Applying the ALOHA calculation in a large metro area such as in central LA, and one hop for everyone, the 144.39 channel could handle almost 180 users with good reliability! By taking the extra effort of making the digipeaters 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.
NETWORK SATURATION! In areas where the network is saturtated there is a tendency to just add more digis closer to the users 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! and total saturation with miserable reliability.
THE SOLUTION: Although there are some exotic solutions such as with the Super-site concept(no longer recommended) and going to 9600 baud, the fix is much easier. The simple solution is to make sure there is an IGate and then simply trapping all packets at one hop. This eliminates packets from out of area while still fully supporting local users!
Here is how to do it at any KPC-3 DIGI:
FUTURE Expansion: The 1-hop support for 180 stations is still about half of the potential full load within 100 miles of LA. To get to the ultimate support level of 360, it would be necessary to cross band the oputput of the main big 3? digipeaters to UHF so that they can still provide a full 100% channel feed, but not hear each other's output on their input. All they hear now are users direct, and can provide good coverage for up to 360 users! 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 low-tier digis make their best contribution. These digis can 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 low-tier digis is to put 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 WIDE1-1 for the low-power trackers that are using the universal APRS path of WIDE1-1,WIDE1-1. Notice the double hop.
WHAT ABOUT ABUSERS USING MULTI-HOP PATHS? Actually, if they implement the 144.39=>UHF at the big digis, then 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, none of the other digis are listenign to the UHF outputs of any of the other digis. THus, all packets still only go 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.
de WB4APR, Bob