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22883Re: [aprsisce] Strange path for repeater object.

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  • James Ewen
    May 12, 2013
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      On Fri, May 10, 2013 at 5:00 PM, Robert Bruninga <bruninga@...> wrote:

      > We agree completely in ever digi putting out the info, but every DIGi
      > should source this info, and most digipeaters have BEACON room for 2 or 3 of
      > them. And I do like to see this local info…, but the objects should be
      > “sourced” by those 4 digipeaters with a direct path (no hops) so that the
      > DIGI only transmits them when the channel is clear. Only the digipeaters
      > can hear their “input area” and so only they can add this info to the
      > channel so they do not collide with any user packets at all.
      > Sourcing them at one place and having them bounce around to all 4 digis
      > takes up 5 times as much channel capacity. And none of those packets are
      > guaranteed not to collide with user packets. But if they are –sourced- at
      > the digi, then the collision potential is zero.

      The chances of collision potential are not zero in an APRS network.

      If you have a single digipeater that can hear EVERY user station, the
      digipeater will attempt to send the packet during a time when no other
      stations are transmitting. Even in this scenario there is a remote
      possibility that a user station may key up exactly at the same time
      that the digipeater keys up. Regular user stations are supposed to
      have a random hold off time to keep them from clobbering each other,
      and the digipeaters are supposed to key up right away. But if a user
      station decides to key up and waits it's random amount of time, and as
      that hold off expires, the digipeater decides to send the repeater
      object, a collision can result.

      Now add in a bunch of digipeaters surrounding the original digipeater.
      The digi sourcing the repeater object can not tell if the remote
      digipeaters are hearing incoming packets or have a clear channel, so
      there's the chance of the voice object colliding with a user packet
      trying to get into one of the remote digipeaters.

      I agree that sourcing the packets at the digipeater has a lesser
      chance of creating collisions on the network, and using a zero hop
      path reduces the network load much more than a user trying to send the
      packets around the area with a multi-hop path, but to say that the
      collision potential is zero is misleading.

      > > The path VERMLN,LLOYD,PROVST,ALIANC is for repeater object 145.29-RW
      > Again, that is 5 times the channel QRM compared to the way these objects
      > are supposed to be designed so that they have zero impact on users by having
      > no path and being sourced at each digi.

      4 times the load... there's always a source packet whether from a user
      station, or from a digipeater. Again, trying to imply that a packet
      sourced by a digipeater has zero impact on the users is misleading. If
      the user can hear the packet, it has an impact. That packet has used
      up airtime, and is heard by all stations within range, including all
      surrounding digipeaters.

      > > So basically the packets are travelling in a ring or box around the
      > > repeater.
      > But that is the worst possible method generating 5 times the channel load.

      Again I would disagree... there are far worse methods available. Trust
      me, let the users play and you'll find out! If Tony were to use the
      path WIDE2-2, the packets would travel much further and cause more
      network load than his specified 4 hop path.

      > The total amount of QRM being generated is as if these beacons were every 2
      > minutes if you count the total time slots involved. When sourced at the
      > digis, the time slots count is actually ZERO since the digis will only
      > originate the packet (no hops) when the channel is clear. So although I
      > keep saying it is 5 times larger, it is really infinitely larger QRM because
      > 5 times larger than nothing is infinite.

      Again, a misleading analysis. There is no possible way to send a
      packet out that takes up zero airtime. A packet sourced from a user
      station is no different than a packet sourced from a digipeater. The
      user station waits until it hears a clear channel before transmitting.
      A digipeater waits until it hears a clear channel before transmitting.
      Each station can only ensure clear channel occupancy within the area
      it can hear directly.

      The concern is that a packet sourced by a home station may clobber
      another home or mobile user trying to access the nearby digipeater.
      When a packet is sourced by a digipeater, it too has the same
      potential to clobber packets from other users on surrounding

      The biggest difference is that when sourced from a home station asking
      for a digipeat by the local digipeater, the packet needs to be on the
      air two times to get the same coverage area as when that packet is
      sourced by the digipeater. (Given that the digipeater covers a larger
      area than the home station is capable of covering.)

      >> Also we have a very lightly loaded network out here…
      > True, but it sets a bad example and these things tend to get entrenched
      > and hard to correct later.

      This I agree with wholeheartedly. We should all strive to operate as
      efficiently as possible.

      > Thanks for putting out this info. I wish more people did it. But I worry
      > that this is setting a bad example. My web page that describes all this is
      > on http://aprs.org/localinfo.html

      Accurate observation and explanation of the operations of the packet
      radio network would go a long way towards getting people up to speed.
      Unfortunately many people don't take the time to learn and understand
      the intricacies of all of the inner workings of the APRS network.

      There are also a number of concepts touted as being the panacea of
      APRS network operations that just don't work in the real world.

      The concept of APRS and its use is my favourite aspect of amateur
      radio. I have far more time and money invested into APRS than all
      other aspects of amateur radio combined. I love it, but there are a
      lot of basic misconceptions floating around and being espoused as the
      gospel when they are misleading at best.

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