The APRS ALOHA Circle Concept

Do unto others as you would have them do unto you!

Note: All of this info was on the FIX14439 (New-N Paradigm) web page, but has been captured here for clarity.

APRS CHANNEL FUNDAMENTALS: APRS can ONLY work at the local level on RF. It is only a 1200 baud channel shared by your nearest neighbors. . It can only work if you and your neighbors understand how it works and its limitations and you are not QRM'ed to death from packets from out-of-area. . The following table shows the BASIC ASSUMPTIONS about the goals of an APRS network:

THE ALOHA CONCEPT: The 1200 baud 144.39 channel can only support an average user load of about 60 to 100 or so average APRS stations or objects in an RF domain based on the typical transmission rates and number of digipeaters and number of hops. . See the breakdown and analysis. . This is because any greater load than 100% channel capacity guarantees lost packets due to collisions. . The size of your area that holds the number of users capable of generating 100% channel capacity we call your ALOHA circle. You are responsbile to make sure that your packets get to your surrounding ALOHA network but no further so you do not add QRM to other networks. ALOHA Circle concept.

The ALOHA Circle Display:

Every APRS software should calculate and display the ALOHA circle for its user. . It should be overlayed on every map so that it is never out of mind. . But even without the ALOHA calculator built-in, users can still get a good estimate from FINDU.COM. . Simply EDIT-COPY this URL into your browser with YOUR APRS station callsign on the end: <= your call here

That FINDU page sorts your 60 nearest APRS stations by range and so the range to the last person on that list is the radius (or range) of your ALOHA circle. . This list of stations is your APRS ALOHA Neighborhood. It also includes which digipeaters you may need to hit, to be able to communicate with them. . More importantly, your packets should not go beyond this circle routinely. . In Southern California, your ALOHA circle may only be 15 miles and you may only need ONE hop to hit everyone. . In WYOMING, you may need more hops....

The ALOHA circle is automatic now in all copies of APRSdos (APRSmax after . See Map plot above. . There is also an add-on for UIview, and the next release of WinAPRS should have it too. . It computes and maintains a plot of the yellow ALOHA range circle on all map zooms. Around Baltimore it is about 45 to 55 miles depending on the number of mobiles at any time. . Also displayed below is the MAP-PLOT-HOPS command that shows a plot of the number of hops it took for surrounding digis to reach me. MAPS-PLOTS-USERS shows the same plot of user hops. . Notice that 2 hops will get one EVERYWHERE within this Aloha Circle even though there are 13 digis in that area!

Note, that these PHG circles are the new APRS1.1 half-size APRS standard which more closely represent a MOBILE's range. . The actual RF range of these digis to fixed stations and other digis is TWICE that shown above.)

Accomodating future growth:

To allow APRS to grow, just like the cell-phone system, the cell size must get smaller to keep within the ALOHA limits in any one RF area. . High digis can be good if the number of hops of all users is restricted. . But, in generally flat land, more users may require more, but lower digis, and restricting the number of hops. . If you can reduce what the tall-hear-too-much-mountain-top sites hear to only respond to DIRECT packets (and no surrounding QRM) they can support over 100 users... See the very-tall digi problem in the LA basin in Southern California. . There are several options for the tall-super-site digis depending on your area:

  • Option 1: Simply trap all paths to 1 hop. (It can still be used for directed paths beyond 1 hop.) See the fix for Los Angeles
  • Option 1+ You should also reduce what all surrounding digis are re-transmitting as well.
  • Option 2: Move its RX to 144.99 (if avail) so that it has a clear input channel for low-power trackers
  • Option 3: Move its TX to UHF so it can still hear everything but wont QRM anything
    ........ Users can still listen to it on UHF or an IGate on UHF will still get everything

    WE MUST KEEP N SMALL: Looking at the earlier map above, you can see how a WIDE3-3 path can hit most digipeaters throughout the 5 states of VA, MD, DE and SE-PA and southern NJ. . These days, high density areas such as these can only support WIDE2-2 and no more. . See the chart:

    click to see 3 digis.
    click to see 2 digis.

    The New-N Paradigm significantly reduced the number of hops and greatly simplified settings so that the APRS network is beginning to work everywhere for everyone using consistent settings.

  • Eliminated the multiple dupes of the old RELAY and WIDE (and TRACE and SS) systems.
  • Encouraged the use of WIDEn-N
  • Encouraged lower values of N to reduce flooding QRM from out of area. (typically N = 2)
  • Changed UIFLOOD from WIDEn-N flooding to the local State SSn-N or section communications
  • Moved WIDEn-N to the UITRACE parameter so that all WIDEn-N paths are traceable.
  • Used the UIDIGI ALIAS list to Trap WIDEX-X paths for large values of X but allow at least one hop.
  • Both of these put sysops in control of their networks and drastically reduced DX QRM independently of user settings!.

    By eliminating all the old legacy paths, the New-N Paradigm also simplified user education down to only the receommended path of WIDEn-N with N usually being 2. Though there are still some options for optimizing the 2 hop recommendation as follows:

  • GENERAL MOBILES:........ WIDE2-2
  • LOCAL MOBILES:............. WIDE1-1,WIDE2-1 (if there are FILL-IN digis in the area)
  • HOME STATIONS:............ WIDE2-2 (or 3-3 in eastern USA distant rural areas)
  • STATE OPERATIONS:...... WIDE1-1, SS3-3 as needed

    Collisions and Lost Packets: There is no way to know the reliablity of your network from general statistics of packets heard, because you cannot tell what you DON'T HEAR! . Do not think you can monitor the channel load and make any meaningful conclusions! A channel that is 800% overloaded and only passing 1 out of every 8 possible packets can sound identical to one that is 50% loaded and passing 100% of every packet for everyone... That is why we came up with the ALOHA algorithm, so that you can analyze the network according to the statistics of who is getting through.

    ALOHA NETWORKS: APRS is a random network based on random transmissions of data among many users trying to share a channel. . This technique was well studied back in the 60's and is called an ALOHA network for where it was first developed at the University of Hawaii. It is the basis for most data networks which evolved into what we know of as the ARPANET and now the INTERNET and other protocols. . Click for the original definition of an ALOHA system.

    Basically the channel must be quiet most of the time, so that a sender will have a good chance of a successful transmission. . You can add more and more users, but eventually you get to the point that adding more users only adds more collisions, and the net reliability goes down drastically. . If you add up the statistical load of about 60 or so average APRS users, their packets typically represent a full channel. . Any more and the network suffers due to collisions. So, how many users are there in range of your digi?

    If your digi hears too much then its reliability for local users is poor!
    In this context, LOWER is BETTER (just like CELL phones have grown, by going to smaller and smaller cells!).

    Of course, these numbers are inflated due to a number of non-RF stations that also get counted but it is representative. . Best to look at exactly what is in the footprint by going to and looking at the nearby stations to see what exactly is there. . If there are too many RF users in range, then the APRS community in that area must add limits to n-N traffic in surrounding digis to reduce the local load on the local digi to better serve their local users. . Here are a number of things you can do:

  • First get everyone to use WIDE2-2 or less
  • If there are nearly a hundred users in range of one digi, consider enforcing only 1 hop through that digi
  • Consider switching the input of the BIG DIGI over to 144.99 (if available in your area) to hear low power trackers.
  • Or switch the output over to UHF so that its capacity can double. . (Users monitor the UHF output)
  • Measure your local reliable local network range of your ALOHA-Circle and get everyone in your general area to reduce their paths to not go outside of their ALOHA circles.
  • Add more LOW-LEVEL local digis, or digipeat across town through a neighbor (who doesnt hear the QRM the digi hears)
  • Ignore big N WIDEn-N QRM at your digi with the new State SSn-N and large N ALIAS TRAPS of the New n-N paradigm.
  • Convert high-site digis (that hear too much) to UHF backbone operations. (See below)
  • Make sure there is an IGate within the footprint of most high density digis
  • for the UHF outputting high-site digis, add a UHF receiver at the local IGate
  • Mark all PERMANENT Igates with the "GATE" with an "I" overlay so everyone can see which Igates are permanent

    APRS Channel Throughput:

    The above screen capture of the APRSdos program called SIMULATE.BAS shows how this program can be used to simulate the throughput on any kind of ALOHA channel. . In this example it shows how 85% of the 40 stations all with a brand-new packet of information will be successful in getting at least one successful digipeat of that new information in the first 8 minutes. . This example caused about 280 packets to be generated using the APRSdos decaying algorithm to decay the retry-time after each attempt so that new data is transmitted more often, and old data decays down to much lower rates to preserve channel capacity.

    Many popular APRS systems, unfortunately, use a constant transmission rate of X packets every Y period of time which results in a DOUBLING of QRM (over 480 packets) being generated for only a 2% improvement in throughput over the same 8 minutes.

    Bob, WB4APR