FIGURING YOUR 50 USER ALOHA NETWORK WB4APR ------------------------------------------------------------------ At about one second per packet, this means that in a 30 minute period maximum possible throughput on a 1200 baud channel is 1800 packets. But this ignores the fact that a collision anywhere in a packet destroys both packets usually, so the actual relaible figure is usually about 2/3rds of that. But for the optomistic, we will use the 1800 number.. Now, how you use those local 1800 packets per 30 minutes is debatable, but, it is very easy to calculate based on who and what is on the air in the area. The packet rates of most categories of stations is well known. The following table suggests a typical local area usage: TYPE STATION PKTS/30m ea Total PCT of LOAD ------------------- ----------- ----- ----------- 30 Home stations 2 60 19 % 3 LOCAL digis (evy 10m) 3 9 3 % 9 2nd-tier digis (evy 30m) 1 9 3 % 5 area WX stations 6 30 7 % 3 Mobiles evy 5m 6 18 6 % 3 Mobiles evy 3m 10 30 9 % 3 mobiles evy 2m 15 45 14 % 3 mobiles evy 1m 30 90 28 % --- 59 TOTAL USER STATIONS TOTAL PACKETS IN THE 30 MINUTES = 321 But, every one of these packets is usually digipeated by usually every digipeater. In the above example there are 3 digis in range of this example so the 321 packets is really more than 960 local copies. AND this does not count any of the 2 hop or greater packets coming in from the surrounding 9 digipeaters further out! At this point, lets just assume that HALF the traffic comes from them. What should be painfully clear is that with just 2 hop packets this area is totally saturated with only about 60 users. You can trade these numbers anyway you want, but you cannot change the laws of physics that says the optimum reliable throughput for our 1200 baud ALOHA channel is about 60 or so average users. In your local area, you can add several more fixed stations for every 1 minute mobile you eliminate. Or, you have to eliminate a lot of home stations (or suffer much worse reliability) to accept the load of just one more high-rate mobile. It should be very clear that the path to reliability is to cut the number of hops used and hopefully out of area packets will not be so burdensome on the local net. APRSdos and hopefully other APRS programs displays this local reliable range of the nearest N stations based on what it hears so that the user is well informed of his effective communications range and will keep his packets within that area by keeping his path short. de WB4APR