[aprssig] Local Event using RELAY?

Wes Johnston aprs at kd4rdb.com
Wed Mar 30 20:00:50 CST 2005

Ah ha!!!  The light came on for me!  Henk and I are talking apples to Pete's
oranges...   Apples and Oranges...

The important (yet subtle) difference is the dispairity between a mobile's
coverage area and a digipeater's coverage area.  When packets are bouncing
along from one digi to the next, and  the digipeaters have a significant
portion of their coverage areas overlapping, then CSMA can work better.  The
greater the overlap, the better CSMA works.  But since no two digipeaters have
100% overlapping coverage area (what would be the point?), CSMA will not work
100% of the time... it's simply a function of the percent overlap.  I can see
from a digipeater to digipeater perspective, that without CSMA, the network
wouldn't work well.

But my mobile can't hear another mobile trying to get into the digi at 
the same
time as me, and it sure can't hear the next digi in the next town. So for a
mobile station, where the coverage area is extremely different than the 
he's trying to contact, we have to use aloha.  I also recognize that an
undecodable signal that keeps tripping my squelch open will serve to 
prevent my
station from transmitting due to the slot time and persist variables in 
my TNC.

But if we can agree that upon entry into the network from a mobile 
station, the
access method is aloha, and once a packet is "in" the network, the access
method is CSMA.  This leads right to Bob's alt input digipeater idea.  Mobile
stations compete for access into a digipeater on a quiet frequency 
using aloha.
  That first digi is able to listen for a break in the traffic on the APRS
frequency, and properly use CSMA to insert the packet when the band is clear.

Let's use Pete's number of 93% of traffic on a digi comes from other
digipeaters.  That means that 7% is local.  I would rather my mobile station
compete with 7% traffic instead of 100% of the traffic in an area.  While
neither CSMA nor aloha access methods are perfect in a radio environment, they
really don't mix well... so let's keep 'em separate!  If each town took one
digipeater site and made it a simple alt-input RELAY digi, the aloha mobiles
would transmit on one frequency (while still listening on 144.39), and all the
rest of the CSMA traffic would be on the main aprs frequency.  Pete has 
said in
the past that you end up with mobiles transmitting blindly on the alt input
frequency, but hey, if we all know the rules, and we call that input frequency
part of the "system", then it's not a problem on our consciences.  The 
trick is to make sure that there are no other services on 144.99 in your area
before you put up an alt input digi.  Other APRS people can/will accept the
crap shoot on 144.99, but if your local DX cluster is there, they sure won't
like it.

In Sumter SC, we are running a mic-e input digipeater on a local 
repeater... for
all intents and purposes, it may as well be an altinput digipeater.  My packet
is digipeated from the input of our local 2m repeater to a 70cm link freq over
to the local digi.  That digi accepts the packet (thanks digi_ned), and 
it into 144.39 seamlessly.  The result is 100% success with my mobile... and I
do mean 100%!  Using alt input digipeater (the mic-e repeater), I'm getting
100% of my packets onto 144.39 even though I'm in a swamp far below 
most of the
surrounding area.

So why don't we offer the aloha (mobile) stations 144.99 as a non-congested
entry into the network?  All it takes is one digipeater in each town... you'd
be suprized the range you can get into a digi when the noise floor is 
nill.  If
we did this, 200mW trackers would be a realy workable reality.


Quoting AE5PL Lists <HamLists at ametx.com>:

> Hmmm.  So if I use your statements of "fact", those stations generating
> 10% of the packets are blind to each other AND to the wide area
> digipeaters.  Folks, this just ain't so.  Of the stations generating 10%
> of the packets, only about 20% are mobile or portable.  The rest are
> homes, weather stations, etc. with reasonable antennas.  So we are
> talking about maybe 7% of the originating packets coming from stations
> that might be "hidden" to each other but are NOT hidden to the stations
> generating over 93% of the packets.
> Bottom line: MOST of APRS is CSMA, not blind transmissions as you put
> forth.

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