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APRS Routing

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emd001

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So recently I have been taking more and more interest in APRS. I have to admit that for the last few weeks I have felt overwhelmed by the amount of information on the subject. I have been playing with APRS.fi and I am curious as to some of the routes.

When looking at a mobile station there is a pink line that runs to a digipeater with the green star, then a purple line runs from that machine to another digipeater. Now I'm assuming this is because the first digipeater isn't connected to the internet right? So it just routes the data to that other digipeater that is connect to the internet and then it ends up on APRS.fi. So will that first digipeater rebroadcast everything to the other digipeater, or only certain traffic?
 

W9BU

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APRS depends on digipeaters and I-gates to improve the coverage area of a single APRS station.

Digipeaters simply repeat what they hear based on specific rules for beacon routing. Most of the U.S. has adopted the WIDEn-N paradigm. Let me give you an example:

1. You send a beacon with a WIDE2-2 path. The "WIDE2" is just an alias so you don't have to know the exact callsign of the digi near you.
2. Any WIDE2 digi that hears your beacon repeats that beacon, but changes the path to WIDE2-1.
3. Any WIDE2 digi that hears the beacon that was digipeated by the first digi repeats the beacon, but changes the path to WIDE2 (effectively WIDE2-0).
4. Because there are no more hops left in the path (the -2 has become -1 and then -0), any other digis who hear the beacon ignore it.

The whole idea is to keep beacons from propagating from digi to digi without being killed. If I'm in Central Indiana, I really don't care about APRS beacons coming from stations in the next state. WIDEn-N fixes that.

You can read more (if your mind hasn't already turned to mush) at FIXING DIGIPEATERS and I believe this site APRS Paths And Digipeating 101 has a simple on-screen simulator that shows you how a beacon decays as it moves from digi to digi.

I-gates are more sophisticated in that they have a connection to the Internet, specifically to one of the APRS-IS servers. When an I-gate hears a beacon, it sends that information to the APRS-IS. Tools that access APRS-IS, such as Find-U, aprs.fi, or an APRS client program running on your computer, can then get the data from APRS-IS and show you what's going on even without a connection to the RF world through a radio. There are also reverse I-gates that selectively pick information out of the APRS-IS database and transmit it back out on RF. For instance, I was able to send an APRS message to a friend of mine who was travelling towards Las Vegas. We both were on RF, but the message went through the APRS-IS to get to a reverse I-gate near him. You can read more about APRS-IS at APRS-IS.
 

emd001

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Ah ok I understand. The WIDEn-N keeps the message from clogging up systems that are out of a certain range where most people on RF wouldn't care about that data, the packet has a life span on it when it is sent. Makes sense. Taking a couple of hours and that explanation really helped it sink in I think. The more I look at APRS to more I think that will be my project for the holidays this year is to get it setup in the truck. I think the messaging and maps(objects) would be great to have in there with me.
 

W9BU

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APRS transmits short packets of information using primarily 1200 baud on 2 meters.

D-STAR is a digital voice and digital data system using 4800 baud on 2 meters, 440 MHz, and 1.2 GHz.
 

4720kb0snc

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thanks alot.I'm trying to decide which to go with.Seems like in T.C.metro area D-star is what used
 

W9BU

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APRS and D-Star are completely different technologies with different purposes. D-Star is primarily used as a means of transmitting digital voice. APRS is primarily used as a means of transmitting position data and short messages.

While I understand that the amateur radio emcomm folks in Minnesota are developing a D-Star network, it doesn't replace APRS. For a glimpse of the APRS activity in the Twin Cities, take a look at:

Google Maps APRS

For the most part, the mobile stations you see on the map are APRS positions being transmitted over 2m.

DPRS is a conversion tool that translates position data from a D-Star radio into a form that can be re-transmitted using APRS protocols.
 

KE4NYV

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The same WIDEn-N principal applies to the SSn-N paradigm. The SS stands for a state abbreviation.

For example, in Virginia, we would like all digipeaters within the Virginia border to also support the alias VA. That lets me put VA1-1 (One hop) or VA2-2 (two hops) or whatever I want and in theory the packet stays within Virginia's borders. You can get away with something like VA5-5 (five hops). It's frowned on because you still cause QRM within the state's digipeaters, but the packets are not being digipeated across multiple states.

I am also personally an advocate for having a unique alias supported in each individual digipeater. When I had my digi on the Westin building in downtown Virginia Beach, I had an alias of VBTC (Virginia Beach Town Center). If I wanted to do coverage checks on the digi, I just cut my path simply to VBTC and then I knew any digipeated packet was only through my digi. All others would ignore my packets.

I realize I can do the same thing by putting in the callsign of the digi, but to me, it's a pain to remember the callsign of every digipeater in an area AND the correct SSID. Using some kind of meaningful (descriptive) alias makes more sense, at least in my screwed up head :D
 
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