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[aprssig] THOUGHTS ON RELIEVING APRS CONGESTION ON 144.39

Wes Johnston, AI4PX wes at ai4px.com
Wed Nov 17 16:11:48 UTC 2010


Not to be a wet blanket, but the tmd700 does not allow for setting txdelay,
and it's horrendous 500ms delay makes 9600 baud useless for trying to
conserve bandwidth.  You end up with 500ms of flags and 12ms of data.  And
those are the radios that the appliance operators don't have to set the
deviation on.  At least on the TMd7 HT's you could set the TXD.  And to be
honest, you can set the txd on the tmd710 and probably on the new data HT
(kenwood and yaesu).  So, maybe you just say you don't want d700's on your
9k6 network.  But really, are you going to get 70ms TXD?  You'll probably
get 110ms txd's at best for 12ms of data.

Why use an alt freq that is so close to 144.39?  If you hook up two TNCS to
one radio on two different frequencies, one goes deaf when the other TX's.
What good did that do you?  I'm in the middle of TXing a 9k6 packet and the
1200baud side transmits and the 9k6 radio doesn't hear the end of my
packet.  I just don't see where this bought you anything.

Why not shoot for the alt input frequency of 144.99?  That frequency was
chosen b/c it's in teh middle of a packet subband and it's 600khz from
144.39 so it's super easy to setup a radio for it.  Or an alt input on
70cm?  I like the alt input because it prioritizes the locals.  Yes, you
*will* hit the digi... and if _not_ setup properly, the digi will squirt out
a packet onto 144.39 blindly - but who cares if the local alt input digi
QRM's a packet from another city?  Did you care about seeing the position of
that car 3 states away?  Will you see the WX from the next town in 10 more
minutes?  APRS is supposed to be a local thing, and sure I don't mind seeing
the dx stuff when local stuff is quiet, but local stations and trackers
should take priority.  http://aprs.org/newN/altchannel.txt .  It can be done
TODAY at 0 cost... just go to your local digipeater and put it on
144.99/-600 .

Another idea that was batted around a few years ago was to bundle packets at
the digipeater site.  Do this by placing a timer on the external CD input
pin of a TNC.  Make it think the channel is busy for 10 or 20 seconds at a
time.  It will continue to accumulate rx'ed packets until the timer allows
teh CD pin to drop... then the digipeater will squirt out all accumulated
packets with only ONE txdelay.

We tried hashing this out about 5 years ago (and there certainly could be
some new ideas since then).... I wanted some sort of self organizing time
slot system like the maritime AIS uses.  But given the legacy trackers, the
best we could come up with was the alt input frequency.

Wes
---
God help those who do not help themselves.


On Tue, Nov 16, 2010 at 22:28, David Dobbins <ddobbins at gmail.com> wrote:

> Hello fellow APRS enthusiasts. I’m curious to know what the rest of the
> world is doing to relieve some of the congestion of APRS traffic on the
> primary freq 144.39? In my wide travels, my experience has been the larger
> metro areas are suffering from APRS indigestion, meaning if you turn the
> volume up on 144.39 and listen, it’s uncommon to hear any break in the
> packets the closer you get to the big cities.
>
>
>
> I’d like to share what we’ve been doing about this oversaturation of the
> primary freq in the Puget Sound area. For the past couple years we’ve been
> experimenting with, and done some implementing of APRS at 9600bd on both UHF
> and VHF. The initial alternative frequency/speed experimentation began on
> UHF 440.800MHz at 9600bd by Bob King, K7OFT. We have several UHF APRS
> digipeaters around the Seattle area. The typical digi setup is an Icom
> IC-207 and Kantronics KPC-9612, using the same configuration settings found
> on the primary frequency. The biggest group of users are mobiles, typically
> those with TM-D700/D710 or the Yaesu FTM-350R and similar handhelds. Those
> tracking on the UHF frequency are gated to the APRS-IS, and those with an
> internet connection at their home station will see both the VHF primary, and
> UHF secondary APRS activity. Our VHF 9600bd effort commenced a little later
> than the UHF interest, but has grown significantly over the past year, with
> several high level digipeaters serving the alternate VHF frequency
> 144.35MHz. Scott Cronk, N7FSP has been leading this effort. We eliminated
> the possibility of interference between the two freqs (144.39 and 144.35) by
> replacing an existing APRS digipeater VHF radio with a Kenwood TM-D700 or
> D710, and retaining the existing KPC-3 (or an Argent TNC). The internal
> radio TNC operates at 9600bd on 144.35, and the other side of the radio is
> set to 144.39 and uses the external 1200bd TNC. No change to the filter
> solution has been necessary, or need to add or change antennas, since the
> two freqs are close together. We’ve had three arguments presented why this
> solution wouldn’t work, and all three have been dismissed. First, folks said
> 9600bd won’t work on VHF. It does, and quite well. Second, folks said the
> two freqs were too close together and would cause interference. Not so, as
> the D700 becomes “deaf” on one side of the radio when the other side is
> transmitting, thus no issue. Third, while one side was transmitting, there
> would be packets not digipeated on the other side. We’ve found this to be
> not much of an issue, and the increase in packet loss at the digipeater is
> insignificant. We have a growing number of mobile users throughout Puget
> Sound using the 144.35 APRS @ 9600bd with very good results. With several
> iGates, including direct IGating from the mountaintop digipeaters with
> internet access, everyone trying to get their data to the APRS-IS is getting
> through. We’re encouraging home and other fixed stations to make the switch
> to the alternative frequency to further relieve some of the congestion on
> the primary frequency. It’s too early to tell just how much relief we’ve
> seen on the primary frequency, and difficult to measure. We've also been
> experimenting with cross-banding of data, feeding packets and messages back
> and forth between the VHF 1200bd, VHF 9600bd, and UHF 9600bd channels.
>
>
>
> We’re also encouraging makers of the VHF trackers to add 9600bd capability
> to their products. For those folks in areas oversaturated by APRS packets, I
> suggest the next time you make the mountaintop digipeater run to replace the
> existing VHF radio with a Kenwood TM-D700/D710 and give dual-APRS ops at
> 1200bd and 9600bd a try.
>
>
> Dave K7GPS
>
> NWAPRS Spokane Area Coordinator
>
> _______________________________________________
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>
>
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