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[aprssig] THOUGHTS ON RELIEVING APRS CONGESTION ON 144.39

Robert Bruninga bruninga at usna.edu
Wed Nov 17 18:32:37 UTC 2010


> How did you decide which type of traffic to move 
> to the ALT frequency?  It would seem to me that 
> the primary traffic on 144.39 MHz 1200 baud should 
> indeed be mobile traffic. According to WB4APR's 
> plan (I think), ... APRS should be an information 
> mode largely for mobiles who should see other 
> mobile traffic but more importantly objects such 
> as the local voice repeater, ham events, etc.

Agree.  We still want everything (of value) to the mobile
operator to OUTPUT on 144.39.

The number #1 problem with congestion on 144.39 is collisions on
the inputs of the digipeaters.  The outputs of the digipeaters
on 144.39 are not a problem, because they can hear the entire
regions and hold off until there is a quiet instant.  If only
digipeaters are outputting on 144.39, then the channel can carry
FIVE times what it is carrying now.

So the goal is to move as many local stations *TX* to the local
alternate channel.  All the locals will know it and so they will
get immediately improved performance because they are TX'ing on
a channel with no competition from ANY digipeaters or ANY out of
area traffic.  This is a WIN-WIN way to do APRS.  Generally if
144.39 +600 (144.99) is available in an area, it makes a perfect
local input channel for all locals.  They just have to remember
to change to 144.39 simplex when they leave their area. 

> Also, how would mobiles coming into your area 
> know to make the switch to the ALT frequency/
> ALT baud rate?

The local system still LISTENS for 1st-hop WIDE1-1 inputs on
144.39 visitors still get in perfectly and no change from the
rest of the world.  But locals who want better performance for
themselves (and hence, everyone else) will input on the
alternate channel (Maybe 144.99).

Visitors to the area who want to improve their performance with
a less congested input could also shift to the alternate channel
(+600 like the locals).  He will know this is availablle by the
BEACON from everyy digi in the area that is also listening on
144.99 input.  Their beacon will contain something like W2,
MDn-N,alt 144.99 ... (this is for Maryland MDn-N digis)...

Notice, this will display on the visitors screen as:

W2, MDn-N,
alt 144.99 
...

And the digi should use the digi symbol with an "A" overlay
indicating it has an alt-input.

We have peen promoting this for over a decade.  Glad to see the
PNW is making progress.

As for 9600 baud, its input and output can be anywhere, because
it cannot monitor 144.39 information data at the same time, so
it also needs a separate 9600 baud OUTPUT from all its
digipeaters too.  So just about any local frequency can be used
for the 9600 baud local network, as long as all such packets are
aggregated, and converted to 1200 baud and eventually output on
144.39 for everyone else to see.

I believe the PNW does not do this last part, so the visitor
will see less and less  as locals move to the alternate freq.
(Therefore, I think this is not a good approach), also, because
of co-location of most ham repeaters and digipeaters, and using
dual-band D700's as the combbined 1200/9600 baud sites it only
make sense to me to make the 9600 on UHF so that there is no
mutual interference. And the radio can be doing both at the same
time.  (I think you have to interface the external 2m 144.39 TNC
to the speaaker and mic connections, h0owever, because if you
use the back panel DIN connector, then there is a muting issue
when it transmits on one band... I think...

Bob, WB4APR


> 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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