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[aprssig] Practical APRS: Using PHG and CSMA

Robert Bruninga bruninga at usna.edu
Sat Dec 4 21:07:51 UTC 2004

Practical APRS: Using PHG and CSMA

Even though we have argued that CSMA is basically useless 
in APRS for any stations except the digis, it is, however, easy 
to take advantage of full Carrier-Sense-Multiple Access (CSMA)
on a local level using PHG circles even with a saturated DIGI
with out-of-state QRM.

CSMA is practically useless for the individual station because 
only the digis hear everyone, and a single user only hears less 
than 5% of all other stations direct.  Thus at any instant he
hears a quiet channel and decides to transmit, he has a 95%
chance of colliding with a packet from a distant digi he cannot
hear.  Now if he is stronger than the distant digi or much closer,
he wins, but more often, his packet is lost.  But none-the-less,
 there *is* a way to still take advantage of CSMA for local users 
for local traffic!

Simply do a PHG display of the RF range of local stations around
you.  If your objective is to communicate with station C and you
need to digipeat, then *dont* use the WIDE path through
the local digi D and try to compete with every packet that the 
local DIGI hears (it hears 100% busy or collisions all the time).  
Instead, if you are station A, use a path via another local station 
B that is between you and station C.  The overlapping PHG
circles will tell you who can see whom.  This kind of ad-hoc
come-as-you-are pathing was the fundamental basis of APRS
in the first place.

But why does using the path via local station B to get from 
your station A to station C work so much better than simply
going through the Digi D that covers everyone?  Simple,

Your station is waiting for silence to transmit.  When that silence 
comes, not only you, but your neighbor B also hears silence.  
Thus, you transmit, he gets it.  Now his station B waits for 
silence to digipeat it onward.  When he hears silence, he transmits, 
and both C and YOU hear the digipeated packet.  Thus, C got the 
packet, and You saw the digipeat with no apparent collisions.  
CSMA works perfectly for you.

But what about the path D through the digi?  Same scenario.
You wait for silence to transmit.  But when you hear silence
the digi D is now listening to the NEXT packet coming over
the hill from 300 other stations and digis.  THus a guaranteed
colision that sometimes you win, sometimes you dont.  Your
packet is lost because CSMA at your station did NOT
work because you cannot hear what the digi hears.

Again, TALL WIDE digis were great back in 1994, but become
less and less valuable locally as the APRS network grew and
grew..  They are GOOD at bringing in lots of DX and out of 
area packets, but this same *high* goodness is what in busy 
areas, is BAD for local use.

Think about it.  Try digi-ing through a neighbor next time
you want to do a 1 hop QSO.  Look at the intersecting, and
overlapping PHG circles around your neighbors and plan your 
2 or 3 comon paths that will get y ou everywhere locally.
Good APRS software lets your assign such local paths
locally on the fly for each message line.

Notice, that this also is what helps local mobiles get out
with RELAY.  This way, a local neighbor's station that does
not hear everything that the digi hears will hear the local
RELAY mobile and will then wait till the channel clears
to re-send the packet.

Good luck.  Think LOCAL and come up with  good local uses 
for APRS.  It is a great radio application, not just a video game.
You are a Radio Ham, know your RF network!

de Wb4APR, Bob

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