Figuring Out The Best Digipeater Path For APRS Packets

(Or: Which Way Do They Go, George?)

by Arte Booten, N2ZRC

As with any packet networking, path selection depends on what can be heard by you and those that can actually hear you. You can get the equivalent of an MHEARD command, showing you the paths stations near you are using and which stations you hear direct by pressing the D(igipeat) key. See who these are. Hopefully, one of them will be a WIDE.

Any APRS network relies on the majority of fixed stations having their TNC's DIGIpeat function turned on and their MYAlias set to RELAY. Since APRS is an unconnected protocol, packet nodes (with a few exceptions), which rely on connections and error-checking for data transfer, are counterproductive. APRS utilizes that DIGIpeat function by the use of GENERIC callsigns as a TNC's alias(es), most commonly RELAY and WIDE.

There are various flavors of WIDE, all represented differently on the screens of different APRS versions. But first let me describe the function of RELAY in the APRS network, as this is a useful, but often
misused, generic callsign. RELAY stations generally consist of your average "Joe Aperze" in order to allow low-power, low gain mobile and portable stations into the network. In many areas, most of these RELAYs are able to hit the nearest WIDE directly.

This means that if you can hear and get digi'd by the nearest WIDE from a fixed location with reasonable consistency, you should AVOID THE USE OF RELAY because if a WIDE hears you, others probably can as well..and they're probably also using the alias RELAY. This causes collisions as each of the RELAYs that hear you try to digipeat your packet at about the same instant. And your packet will probably die then and there.

Now back to the various flavors of WIDE. The first is just that. It'll digipeat anything that specifically goes via its callsign as well as to the generic callsign WIDE, shown as a small green star in APRSDos. Sometimes, there's also a weather station associated with this DIGI, in which case APRSDos shows them as a green circle. Next comes WIDE-RELAY. These are TNC's that can have two aliases and digi packets addressed via RELAY, WIDE or its own callsign and show on APRSDos as big green stars.

We also use WIDEs that use an alias of TRACE. Their firmware supports the DIGI-SWAP function. When it DIGIs something addressed via one of its generic callsigns it changes that GENERIC callsign to that of its own in the VIA: field of that packet. This allows people to see how the network is propagating and makes selection of paths a bit easier. Those of us running APRSDos, WinMacAPRS and pocketAPRS see these as that large star with a T in the middle.

The next type uses something the inventor of APRS (The Great Oracle of Glen Burnie) Bob Bruninga, WB4APR, first proposed several years ago to enhance unconnected networking via amateur packet radio and Kantronics recently picked up on. It uses a scheme Bob calls WIDEn-n. "N" is a number between 1 & 6. The first of these would represent the number of "hops" you want to take.

The second of these numbers depends on how many times that packet has been digipeated by the time the last digi retransmitted it. It's set by the firmware in current Kantronics TNC's (but I understand that others *MAY* be coming out with it in the "near" future) and decrements one from that number on the packet it's heard, but hasn't yet DIGI'd. WIDEn-n TNC's can also be set for callsign substitution, as I described above. They have an N in the middle of the green star.

The beauty of WIDEn-n is seen best in a network in which most, if not all, WIDEs in a given area have such firmware. In others, users of callsign substitution might consider shutting that feature off. Unfortunately, not everybody that has a digi is USING Kantronics' stuff. And some of those that are aren't willing or able to change the chip. This pretty much describes the APRS network in my part of the woods.

What's the best path? Look around. Who hears you. Who do they hear. And where do you want the packets to go? I suppose you'd like a nice, stable path from Montauk to Albany and Newark to Binghampton. Good luck. The path you need is there, if you can see anybody else on your screen. If a Green Star can hear you, then that's gonna be your starting point. From there, just follow the bouncing green stars. 73


Arte Booten ( AEC for Digital Services, NYC ARES/RACES| |Riverdale, New York [FN30bu] !4052.71N/07354.06WNPHG5370/A=00240| PGP Key Fingerprint: D73E B889 C630 6F4A F31F 3083 56BD 0AAD 9996 3B03


This article resides on the Technical Information and Operations portion of the NYC-ARECS wesbite.