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[aprssig] Local Event using RELAY?

Robert Bruninga bruninga at usna.edu
Thu Mar 31 14:51:01 UTC 2005

Regarding CSMA and "collisions" and Wes's and Pete's

Yes, I agree with Pete's very good and different perspective 
of how the digis use CSMA quite effectively.  I can also 
add one other observation about the differences between
initial user packet collisions and digi packet collisions:

1) If a DIGI hears a collision between two digis, then it 
still has an almost certainty of getting another chance at
that packet under the WIDEn-N algorithm.  Because
every other digi will be repeating it and we know that most
digis hear multiple digis.  Thus, these digi-to-digi collisions
typically mean no loss of data...  But

2) If my mobile hears a silent period (so does another
mobiles around that same digi) then if they are both ready
to transmit, they will, and we get a USER collision and
both sets of data are lost and there is no probabilty
of another chance at that position report.

So, I  think that at the end user perspective and getting
his position into the network, the situation is mostly 
hidden-transmitter collision driven...  Also even if some 
of the digi-to-digi data is lost, at least the majority of it is 
usually coming in from distant areas and is of somewhat
less interest locally.  (Though this will improve as more
people start cuting back their paths under the New n

Just a thought...

>>> aprs at kd4rdb.com 3/30/05 9:00:50 PM >>>
Ah ha!!!  The light came on for me!  Henk and I are talking apples to
oranges...   Apples and Oranges...

The important (yet subtle) difference is the dispairity between a
coverage area and a digipeater's coverage area.  When packets are
along from one digi to the next, and  the digipeaters have a
portion of their coverage areas overlapping, then CSMA can work better.
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
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
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
method is CSMA.  This leads right to Bob's alt input digipeater idea. 
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
frequency, and properly use CSMA to insert the packet when the band is

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
compete with 7% traffic instead of 100% of the traffic in an area. 
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
digipeater site and made it a simple alt-input RELAY digi, the aloha
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
frequency, but hey, if we all know the rules, and we call that input
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
crap shoot on 144.99, but if your local DX cluster is there, they sure
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
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
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
entry into the network?  All it takes is one digipeater in each town...
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
> 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
> 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
> that might be "hidden" to each other but are NOT hidden to the
> generating over 93% of the packets.
> Bottom line: MOST of APRS is CSMA, not blind transmissions as you
> forth.

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