The ALOHA Circle (or local reliable APRS network size) 13 Jan 2003 --------------------------------------------------------------------- WB4APR There are three things that are very important to the network and user education and which some software does does *not* show to the user to help him be a good steward of the shared channel. These are the proper display of PHG circles, the ALOHA range and number of hops between users. Software should insure that users are aware of the RF network they are in. See these crude APRSdos examples of what every user needs to know about his network: Simple PHG: http://www.ew.usna.edu/~bruninga/aprs/mapPHGs.gif Aloha Circle: http://www.ew.usna.edu/~bruninga/aprs/alohamap.gif Digi Hops: http://www.ew.usna.edu/~bruninga/aprs/digihopsX.gif Users that are not aware of the network limitations and the number of hops it takes to hit their local area are killing the network, because they have no PHG displays and no way of knowing what paths to use nor how busy the channel is, nor anything about the local RF network. 1) Software should display the PHG circles from all digis to make users aware of their relative magnitude of converage. 2) All digis should be displayed in green and their PHG circles for DIGIS also in GREEN to distinguish the circles from the hundreds of user and other station circles. 3) The ALOHA circle showing the limitations of the local network around own station should be displayed on all maps. THE ALOHA CIRCLE: ----------------- Many users these days have no appreciation of the limits of the 144.39 or other national APRS network even though it is trivial to calculate and display since it is just the definition of a 100% full channel based on the TX rates of surrounding stations. The ALOHA circle is that circle that contains the number of users that equals a 100% full channel. I have called on all software to permanently display the calculated ALOHA circle for *own-station* and always display it on the users map so that the user is always aware of his effective range and SHOULD NOT send his packets beyond that 100% saturated channel range... Here is the spec for the ALOHA circle: --------------------------------------------------------------------- Fundamental APRS channel capacity: There is a limit to APRS throughput on 144.39 or any frequency. That limit has been said to be about N stations. (We used to say that N was about 60. Now we want all software to compute it live based on the immediate live activity and mixture of stations) No matter where you live or how dense or sparse the activity, the 1200 baud channel can only handle a certain maximum number of packets before reliability to others falls off drastically. Draw a circle around your nearest N (was 60) stations, and if you are sending packets beyond that range, you are being inconsiderate and adding QRM to others. In LA, the range of that circle may only be 15 miles. In Wyoming, it might be 150 miles. This is *independent* of topology. The size is only limited by the number of stations and their statistical transmit rates. We MUST educate users to this fundamental 1200 baud limitation. To that end, I am asking ALL authors to draw this ALOHA circle on all maps so that no user can claim ignorance of his realistic APRS communications range. Being good stewards of the channel must be the responsibilily of the software to SHOW the user his limits. Here is the algorithm to exactly determine the ALOHA circle range for own-station. Do this once every 30 minutes or so. (You have to wait at least 30 minutes from power up to make the first computation anyway or use old data)... * Start with the "digi-copies" vairable C equal to 1. * 1200 baud can handle about 60 packets per minute or 1800/30mins * Sort all stations heard on RF by range * Starting with closest, add these number of packets per 30 mins for each station. - Mobile in motion = 15 per 30 mins - Other Mobile = 7 per 30 mins - WX station = 6 per 30 mins - DIGIs = 3 per 30 mins - Home stations = 2 per 30 mins * Multiply each of these by the number of digi-copies (C) when you add it to the accumulating total... * C begins as 1, but as you encounter each digi, add 1 to C * When the total accumulated gets to 1800, STOP * The range of the last stations is the limit of the local network of 100% channel saturation. That is the ALOHA circle and should be plotted on the map (every time the map is rendered). Users should be encouraged to NOT routinely send any packets beyond their ALOHA circle because doing so is JAMMING other users with un-invited QRM. Thats the facts folks. We can argue and fine tune the assumptions above, but once we nail down that algorithm, I want to see it in all APRS programs. We have GOT to get users to understand how limiting the 1200 baud channel is and to CUT BACK their expectations, and use APRS locally, or via the Internet. But sending packets routinely outside the ALOHA circle is inconsiderate and everyone suffers. I have implemented this in APRSdos (the MAX version) and it works beautifully. Interstingly enough, it does work out to be about 50 to 60 stations as we had estimated before. But now it is dynamic to match actual conditions and the mixture of mobiles or non-mobiles on the air at any one time.. I was shocked when I ran it the first time... My QTH is half way between Baltimore and Washingon DC. My ALOHA circle is 50 miles and it contains 43 stations of which 13 are digis, 12 are mobiles, and 6 are WX stations.! It was shocking to me to see the number of digis. Oh, and I can hit EVERYONE of those digis with a 2 hop path. Fortunately 3 of those digis are RELAY only ones and another one is the 144.99 INPUT digi, so it is not quite as bad as it looked.. But seriiously folks. Lets get this ALOHA circle on all MAPS so that people know what they are dealing with. Its real, it can be calculated, and it IS the limit of a fully saturated channel. We must get people to live within it. USING FINDU TO FIGURE YOUR ALOHA CIRCLE: ----------------------------------------- Use FINDU to "measure" your network saturation limit and to help you DESIGN the optimum DIGI coverage for YOUR area anywhere from Maine to LA! Simply do the CLICK FOR NEARBY STATIONS, and lets say the reliable ALOHA channel capacity for APRS for one digi is on the order of 40 home stations and 10 mobiles and 5 digis and 5 WX stations. That adds up to about 60 stations. Since FINDU displays the nearest 50 stations and this information is SORTED by distance, just go down to the bottom of the list, and that is the reliable ALOHA network coverage for your area! From now on, think in terms of your ALOHA circle. If your LOCAL digi covers lots more than your local ALOHA circle, then your channel is beyond saturation and reliability suffers... and your local digi is NOT a local digi! de WB4APR, Bob