ALOHA NETWORKS DESCRIPTION                                     15 Jan 2003
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                                                                    WB4APR
Someone asked:   "BTW, what is ALOHA?"

So I thought I'd answer...

It is the classic definition of the type of network where each station
randomly that has data to send, just does it.  All the text books refer to
it that way, since it was pioneered in the late 60's at the university of
Hawaii when they used VHF radios to link remote 110 baud ASCII teletypes
on the various islands to the central campus computer.

The techincal papers written about that system and how it shared the
channel access was the beginning of such kinds of Time Divison Multiple
Access schemes that we use today on AX.25, Ethernet, ARPAnet, the
Internet, and some digital phone systems..  The channel throughput maxes
at about 18% of channel capacity for connected mode ACKed systems.  Adding 
more users or packets causes a drastic falloff of througput above that value...

Next above ALOHA is where you use Carrier Sense Multiple Access (CSMA)
where each station listens to the channel first before it sends the data.
This doubles channel throughput to 36% of channel capacity.  But it
assumes that EVERY station can hear EVERY OTHER station.  Although AX.25
TNC's are CSMA, it is practically useless because of the 40 people in
range of a digi, usually each person can only hear maybe 4 others direct.
Thus the advantage of CSMA is only 10% of what it could have been.  Thus,
I'd say that CSMA improves the 18% of pure ALOHA only up to about 20% or
so...

I know it drives people crazy to listen to an APRS channel and hear 6
seconds of dead air between every packet (they think, "what a waste")...
But that is when the DIGI is listening and when everyone hopefully has a
significant opportunity to transmit without colliison with someone else.

An APRS (ALOHA) system cannot perform well (for everyone) unless the
channel is QUIET most of the time!

de WB4APR, Bob