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[aprssig] BIG-TALL digis that hear FAR too MUCH!

Robert Bruninga bruninga at usna.edu
Tue Dec 14 17:02:48 UTC 2004


Steve is exactly correct about the wide variability of that
ALOHA range of "60". It is not at all a magic number.

But that is also why we have asked ALL authors to put
the ALOHA calculator in their code so that their programs
compute the number LIVE more precisely based on actually 
what they hear.  We typically have 20 mobiles in my area 
and my  ALOHA number drops to about 40 during comute
times.  APRSdos and now UIview (with addon) compute and
always display the local ALOHA range on all maps.

See the algorithm on:
http://web.usna.navy.mil/~bruninga/aprs/ALOHAcir.txt

Thanks
Bob

>>> k4hg at tapr.org 12/14/04 10:20:01 AM >>>
On 12/14/04 at 10:06 AM Robert Bruninga <bruninga at usna.edu> sent:

>Any digi that can hear more than 60 or so
>other stations is going to have more than
>a 100% saturated channel and cannot possibly
>serve locals reliably,...
>
There cannot be a simple line drawn where 59 is good and 61 is bad.
Even after
eliminating non-VHF LAN stations, you must take into account packet
rates of
those stations and the configuration of the local network...if one has
20
mobiles sending a NMEA packet every minute with WIDE7-7, and 10 WIDEn-n
digis in
range, the channel will be massively overloaded with only 30 stations
in range.

At the other extreme, say there is a digi in a place like Key West
(therefore
other activity in only one direction), with only 1 other digi in that
direction,
a local network could easily support 200 home stations sending a posit
every 30
minutes...the channel is not overloaded despite a 201 station count.

Obviously these are extremes, and in many cases the average number of
60 is
probably as good as any, but the point is that each individual case
MUST be
looked at with a human brain, you cannot rely on findU or any other
computer to
answer this question in all cases...

Steve K4HG

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