MD/VA Digipeater Plan

Automatic Position Reporting System

This Plan was updated on 10 April 2005

This digipeater plan for Maryland and Virginia is inspired by all the new recommendations for all digipeaters in North America in the New n-N Paradigm. This major change will fix the congestion presently overloading the APRS network and making it un-reliable. With the reduced QRM by a factor of 3 to 5, the reliablity and capacity of the network will drastically improve.


  • For National interoperability, WIDEn-N is the new universal path (with N optimized locally)
  • WIDE2-2 is the new universal path in our area from Northern VA to Boston, though we may see some rural visitors using WIDE3-3
  • WIDEn-N will be traceable everywhere
  • Generic paths like RELAY, WIDE, TRACE and SS are the biggest QRM generators in our area and must be phased out.
  • The old Mobile path of RELAY,WIDE is replaced with an equivalent WIDE1-1,WIDE2-1 path.
  • The old PacComm TRACE-only digis can also support these WIDE1-1 and WIDE2-2 universal paths.

    See: The KPC-3+ Late 8.2 and all 8.3 versions Recommended Settings in MD and VA
    See: The KPC-3 Early 8.2 Recommended Settings...
    See: The PacComm Recommended Settings in MD and VA

    USER RECOMMENDATIONS ARE SIMPLIFIED: The best path is simply WIDE2-2. It will work everyhwere. In wide-open areas such as western Maryland and Virginia, WIDE3-3 may be OK. MDn-N and VAn-N will also work to support state wide communications without QRM across borders.. For MDn-N or VAn-N use an N based on your location and who you need to hit, probably 3, but may be higher in the far corners of the state.

  • MOBILES and TRACKERS . . . . . WIDE2-2 . . . . . . . (is most efficient)
  • FIXED STATIONS: . . . . . . . . . . . WIDE2-2 . . . . . . . . . . . (the most efficient)
  • SPECIAL STATE NETS: . . . . . . . MD5-5 or VA5-5. . . . . (not for routine operations)

    ABUSIVE WIDEn-N FILTERING: This New n-N Paradigm became possible when we realized that the ALIAS list in the KPC-3 digipeaters can be used also as a TRAP for large values of N originated in the local area. This is accomplished by setting the UIDIGI parameter to trap WIDE4-4, 5-5, 6-6 and 7-7. They will be digipeated once and then swapped with the DIGI callsign. Thus preventing further propagation.

    WIDEn-N TRACING: A second improvement in the New n-N Paradigm is that all WIDEn-N packets can be traced. This is beacuse WIDEn-N is being moved from the UIFLOOD parameter in the TNC's to the UITRACE parameter.

    STATE SSn-N Networks: State SSn-N paths are now able to be supported in the UIFLOOD parameter and are far less damaging than the original WIDEn-N because the total number of copies is automatically limited by the state borders. MD6-6 may only generate 12 copies, all in Maryland compared to almost 140 copies in 8 states for the very abusive WIDE6-6. Actually, if the user actually uses the path of MD1-1,MD4-4 for example then the path will also arrive as identified by the first and last digipeaters used. For example it may arrive as DIGI1,MD1,DIGI3*,MD4-1.

    During emergencies or exercises, communications personnel involved in passing Maryland or Virginia traffic may use the paths of MD1-1,MD5-5 or VA1-1,VA5-5 which should cover most of the state. (Please use sparingly and not for ROUTINE!) Notice the real advantage of these paths is for stations living in the far corners of a state to have a multi-hop path to communicate back to the rest of his state without QRMing the surrounding states that he may be closer to.

    USER DENSITY. Maryland and the mid-Atlantic states are THE HIGHEST TRAFFIC densities in the world for APRS. See map below. This is why we MUST limit our routine paths to 2 hops to keep from shooting our selves in the foot with too much QRM.

    Fix #2: Alt-Input Channel on 144.99:

    In highly congested areas, it is still very difficult for a low-power APRS station or tracking device to be heard amongst all the other QRM on the 144.39 channel. To solve this problem, alternate-input receivers in some areas can also listen for these low-power users on the very clear channel of 144.99. This is simply 144.39 +600 standard Offset. These packets heard on that channel are simply then cross digipeated back to 144.39 where they are heard on an equal footing with everyone else. This simple SYSOP solution can drastically improve local channel reliability!

    Currently there is one alt-input digi and it is 99ANAP In Annapolis. It supports WIDE1-1 and WIDE2-2 digipeating. Any other 144.99 alt-input digis should also begin their geographic name with "99".

    SUCCESS OF APRS: The success of your local APRS is not how many stations you see on your maps, and how far away, but how reliably your mobile or handheld or portable station can communicate with others in the local area. This fundamental principal should drive everything we do with APRS in our local areas.

    Mason-Dixon APRS Group: N3KTX reports 8 N-N Digipeaters, five under his call in MD, plus two in PA, N3BXO-4,-5 and KF3M-1 belonging to the HAHA group. A possible alias for this group might be MASOND or MDIXON?

    Southern Maryland: K3GJ reports several digis for the Southern Maryland APRS Group: WX3SMD-8,-13,-15, KA3POX, and K3WTF-1 digis plus one in work in south Charles County.

    Eastern Virginia Area: KF4HJX supports many of the digis ov the Eastern Virginia Group including the N4EVA-10, -11, -12, and -13 digipeaters.

    FIXING THE 144.39 NETWORK: The most important thing we need is USER EDUCATION that a local 1200 baud APRS network can only support about 60 or so users in its RF domain, and in most congested areas that only works if everyone uses a path of only 2 hops or less ! This digipeater plan for Maryland and Virginia tries to provide WIDE2-2 (and some WIDE3-3 support in the south and west) so that the large number of duplicate packets is eliminated and the reliability of the network vastly inproves.

    This new WIDE2-2, and SSn-N plan is part of the national effort to upgrade our 144.39 system as detailed on the New n-N Paradigm (FIX 144.39) web page.

    APRS 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 or so APRS stations or objects based on the typical transmission rates and typical number of surrounding digipeaters generating multiple copies. 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 is called 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 areas. ALOHA Circle concept.

    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.

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