[aprssig] ALT input channel - different perspective
wes at johnston.net
Sun Sep 26 08:20:34 CDT 2004
I know this alt input thread is going to turn into a holly war soon..... ;-)
It seems that there are two camps... those who are worried about
widespread deployment of these alt input digipeaters because their
concept involves transmitting in the blind on 144.99. The other camp
says it's OK to TX in the blind on that frequency as long as it's
I want to throw another idea in.... that of the mixing of CSMA and Aloha
type collision avoidance.
In order for CSMA to work all stations in a network have to be able to
hear all other stations. This works very well, but not perfectly, on a
wired network. I still see collisions on my home ethernet, but not as
many as we have in packet. The number of collisions you will see in a
CSMA network becomes a function of the overlap in coverage area that the
network participants have. The greater the overlap, the fewer collisions.
Let's consider two neighboring digipeaters with an effective range of 30
miles. If they are 30 miles apart they share an overlap of just a
little less than 50%.... so there is a little more than 50% chance one
could be hearing something the other can't, and the other might
transmit. That's OK - well fair --- livable.
Now consider a car with an effective range of 5 miles. The car can hear
stations in a 78 square mile area around him. The digipeater can hear
stations in a 2800 square mile area. The car doesn't hear 97.5% of the
digpeater's coverage area. That means that there is an _enormous_
chance that the car will try to TX when the digi is hearing another
station that the car can't. In this case, the car getting into the digi
is a crap shoot... err I mean aloha.
My point here is that we are presently operating two network models at
the same time on 144.39. Some of the participants in the network are
running CSMA while others are running aloha. So let's coordinate some
local frequency (ie 144.99) and let the aloha stations go there. From
there, their packets will enter the CSMA network model properly. Blind
stations like tiny trak and pocket trak AND mobiles who can't hear a
significant portion of the digipeater's coverage area should consider
themselves "aloha" stations and take their chances on 144.99 - where
they compete for access with a very limited number of stations.
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