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Paths for fixed, IGATEs, digipeaters, and mobiles

 
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Bill Diaz  



Joined: 24 Jul 2004
Posts: 21
Location: Lockport,IL

PostPosted: Sun Jul 25, 2004 10:04 am    Post subject: Paths for fixed, IGATEs, digipeaters, and mobiles Reply with quote

Our local APRS network is badly clogged due to inappropriate paths. Packets regularly ping-pong back and forth between RELAYS and WIDES. We also see packets being gated when they can also be heard direct.

We need to work on cleaning up our local network so that it will be available in order to pass emergency traffic should the need arise.

We know that we should use the minimum power necessary to communicate with another station. We should also use the minimum path needed to acheive the intended purpose.

There is no single path suitable for all types of stations. Each station is unique as well.

A Fixed station who can hear multiple wides and multiple relays direct could use a single WIDE to good effect. If he used RELAY,WIDE, he could expect to hear multiple copies of his packet as it ping-pong's thru the network.

A Fixed station who cannot hear WIDES reliably but can hear one or more relays could use RELAY,WIDE or RELAY,WIDE2-2.

You can determine your effect on the network by viewing the data from your TNC. In Ui-View you can use the TERMINAL screen for this purpose. If you are connected to an APRS server via the Internet you can set TERMINAL options, Filter to Exclude Internet Traffic.

The APRS IS servers remove duplicate packets. Therefore, you will never see duplicate packets via the Internet but you will regularly see them on RF.

If you send a packet and you see more than 2 or 3 copies being digipeated on RF, you may want to consider changing your path. YMMV.

Some rather modest digipeaters are setup as WIDE, or TRACE. WIDES are digipeaters that have WIDE area coverage. It is not a good idea to have WIDES in near proximity to one another. The ideal WIDE would be situated at 100' plus and be able to hear a few other WIDES direct.

Northeastern Illinois has a number of high, powerful, WIDES with overlapping coverage. W9AZ-15, and KC9DJM-12 presently can be heard by fixed stations in the Cook and SW counties fairly well. WB9WOZ-15 can be heard from downtown Chicago as well.

Each of these WIDES recognizes RELAY and WIDE. Very oten a fixed station using RELAY,WIDE will ping-pong between these digi's. One will digipeat the packet as a RELAy, the next will DIGI a delayed packet as a WIDE. Very often the same packet from a fixed station will appear on a single DIGI twice, once as RELAY, and again as WIDE. Not unusual for a single packet to appear 4 or 5 times on RF in our local area. If propagation is good, it gets even worse.

Mutiple copies of a single packet consume air time and will likely result in collisions that cause this packet and other packets to be lost.

RELAY's are often intended primarily to enable mobiles or HT's to extend their coverage to WIDES. Modest stations setup as RELAY's can improve the network considerably, whereas the same stations setup as WIDE or TRACE can actually degrade the local network.

My modest home station KC9XG-5 is a RELAY digipeater. I use an UNPROTO path of WIDE only when orgiinating or IGATIng since I can almost always hear at least 3 WIDES at any given time. When I originate a packet, I generally only see one copy, sometimes two. Yet, when mobile I can see my home station throughout WILL, DUPAGE and eastern GRUNDY, KENDALL, southern Cook, Kankakee and Nothern Indiana under normal conditions.

My D700 mobile uses a path of RELAY,WIDE when travelling locally, and RELAY,WIDE2-2 when travelling out of state. This ensures my packets will get to the APRS IS, except in extremely remote areas.

The bottom line is to use the minimum path necessary to acheive the intended purpose.

Bill KC9XG
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ka9vnv  
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Joined: 08 Jun 2004
Posts: 152
Location: Woodstock, IL

PostPosted: Sun Jul 25, 2004 10:45 pm    Post subject: Reply with quote

And the one thing to remember... After the first hop, the alias RELAY should never ever appear in your path!
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Bill Diaz  



Joined: 24 Jul 2004
Posts: 21
Location: Lockport,IL

PostPosted: Mon Jul 26, 2004 6:12 am    Post subject: Reply with quote

ka9vnv wrote:
And the one thing to remember... After the first hop, the alias RELAY should never ever appear in your path!


Guaranteed to cause QRM over a wide area. With each hop the likelyhood of a collision with another packet increases dramatically, particularly with RELAY. This is because there are more RELAY digi's than WIDES.

When two packets are transmitted at approximately the same time (from different Digi's) a receiving station who hears both digi's at about the same level will not decode either of them, thus both packets are lost.

This often happens because of the hidden station problem. Station A can hear Station B, but cannot hear station C. Station B can hear both Station A and C. If Station A transmit's a packet at the same time as C, then Station B cannot decode either of them.

Another cause is related to timing. If DIGI A digipeats the same packet at the same time as DIGI B digipeats the same packet, Station C will decode neither.

For these reasons, messaging reliability decreases with each hop. Adding RELAY after the first VIA will surely result in collisons and lost packets over a wide area. The same will happen with excessively long paths of any type.

APRS was designed for local, realtime tactical communications, not for DX messaging. Use the mininum path necessary to acheive the intended purpose.

Bill KC9XG
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Bill Diaz  



Joined: 24 Jul 2004
Posts: 21
Location: Lockport,IL

PostPosted: Sat Jul 02, 2005 12:06 pm    Post subject: Reply with quote

Steve Smith WA8LMF has published a very good explanation of paths. See http://webs.lanset.com/wa8lmf/DigiPaths/index.htm

There is an error on the page and I have notified Steve. The asterisk does not precedesthe digi'd call as he indicated. I have sent an email to him pointing this out.

He also has a very informative animated graphic at
http://webs.lanset.com/wa8lmf/DigiPaths/NNNN-Digi-Demo.htm

Bill KC9XG
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