APRS UHF BACKBONES for HIGH-SITE DIGIS 17 April 2002 -------------------------------------------------------------------------- WB4APR To solve the problem of TOO MUCH TRAFFIC on 144.39, the HIGH sites that are hearing too much in dense areas must be silenced or converted to backbones. This is an easy overnight cold-turkey phase-1 approach.... Leave these digis in place, listening on 144.39 but transmit on UHF. That reduces channel load immensly right off the bat. I call these digis as HIGHx sites. SPLIT WIDEn-N TRAFFIC TO UHF: Or you can leave a 144.39 RELAY/WIDE digipeater at this high site to serve local users, but disable WIDEn-N packets so that this digi does not FLOOD the area with DX WIDEn-N traffic. If everyone still wants to see this WIDEn-N traffic, simply add a second WIDEn-N TNC that continues to listen on 144.39, but its transmitter is on UHF. Thus none of this traffic is lost if someone wants to put an IGate on the UHF channel or tune there just to read the mail. THis is a right-now-no-new-hardware approach. INTERLEAVING MULTIPLE RECEIVERS: The next step at these sites, is to install multiple 144.39 receivers and TNC's but with separate BEAMS situated so that packets arriving from different directions do NOT collide, but are each processed simultaneously by the different TNC's. For example, a HIGH3 site has three different beams in 3 directions... HIGH1: 1 RX on 144.39. hears everything but transmits on UHF. HIGH2: 2 RX on 144.39. Beams collect packets from valleys on either side HIGH3: 3 RX on 144.39. 3 directions covered with narrower beams HIGH4: 4 RX on 144.39. 4 directions covered with low tower leg beams They all transmit only on UHF so thats a 50% channel booster already even for a HIGH1 site. A HIGH4 site can see a 8 fold increase in traffic... Notice the INSTANT advantages of these HIGHx site conversions: 1) Factor of 2, 4, 6 or 8 improvement in throughput and reduction in QRM. First a factor of two because each HIGH digi is now operating cross band full duplex. So it is not adding to channel QRM. Additional factors of X because each site has X multiple receivers. 2) Notice that even for the HIGH4 digi, the output can still be 1200 baud! Since the ALOHA saturation rate is on the order of 18%, then that means 5 such channels can all be heard at a HIGH4 site and fed into a single 1200 baud UHF transmitter without saturation. 3) Instant gratification in that this is still just a DUMB digipeater, and no software, firmware or anythign has to be developed. ALthough I think DIGI_NED software on an old PC may be required for the multiple ports. 4) Regional IGates simply listen to the UHF output of their HIGH digi so they can still hear everything, just like what the HIGH site hears. 5) FIxed stations or mobiles desiring to see everything the HIGH site is hearing can also just tune in to the UHF output for their area. They still transmit on 144.39. 6) Coordination (other than to get a UHF freq) is not required. The HIGH sites only listen on 144.39, so they cannot add to QRM on 144.39. 7) Low local digis still listen and transmit on 144.39 and provide full 100% APRS connectivity to everyone in the local area using the simple RELAY and WIDE local paths, but their digipeated packets go no further than the nearest HIGH site where they then are backboned to the internet, or adjacent other HIGH sites or whatever.. USERS: Users may think that they are being cut off. But this should not be the case. What it does is eliminate all out-of-area QRM from their area so that now they CAN hear their "local" low area digi. They can hear the HIGH stuff any time by tuning in the UHF downlink. Now LOCAL areas can gain BACK some control over their 144.39 LAN by installing their OWN low-area 144.39 digis for their area of interest. These digis support the APRS universal RELAY,WIDE and WIDEn-N so that APRS gets back to working like it is supposed to. A few hops through these low digis should cover quite an area. And everything gets into an IGate somewhere for FINDU and all the other magic of APRS... In these digis WIDEn-N digipeating of out-of-area packets can be turned on and off at will remotely to throttle out of area QRM from flooding the local network. This will give us a year or two of breathing room to develop any higherorder backbone logic that would then LINK the HIGH's together. For HIGH2,3, or 4's, remember, that you dont need sensitivity, what you need is DIRECTIVITY. SO you can run 100 feet or RG58 if you need to one beam on one side of the hill (low down) and then another cable to the other side of the hill at the base of the tower so that both independent 144.39 receivers do in fact hear two different areas at the same time. Yes, they will hear opposite side signals too, but they should be 10 dB or more weaker and that is all you need for FM capture effect. These directional receivers are NOT intended to hear mobiles, but only the cluster of LOW digis on that side of the mountain. The multiple low digis are supposed to hear the mobiles and they CAN because they are no longer QRM'ed by bazillions of other cross state traffic on 144.39 coming in from the mountaintops... It would be fun to sit down with a TOPO map of SOCAL and look at each HIGH digi and figure out whether it needs to be a HIGH1,2,3 or 4 site... In summary, GET CONTROL OVER YOUR LAN. 1) Disable WIDEn-N digipeating of long haul traffic entering your busy LAN 1) Shut down the XMTRS of the really high sites, or move them to UHF 2) Connect a second TNC to route WIDEn-N packets to UHF. Keep RELAY/WIDE on 39. 3) IGates listen to the high site UHF outputs 4) Start installing more low site RELAY/WIDE digis that dont do WIDEn-N Just a suggestion. Again, this only applies to HIGH site digis that can see LARGE population densities. It does NOT necessarily apply to HIGH site digis in remote areas. Each area needs to design to TOPOGRAPHY. Every site is different. de WB4APR, Bob