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[aprssig] Local Event using RELAY?

Wes Johnston aprs at kd4rdb.com
Wed Mar 30 20:59:56 UTC 2005


I couldn't agree more Henk...

But let's not use the term APRS when we talk about Aloha vs CSMA.  Any simplex
packet network in which multiple stations are competing for access to a single
channel is aloha.  That includes me trying to connect to my local PBBS 
to check
mail.  That includes DXClusters.

Unfortunately this aloha vs CSMA argument will persist.  The only way 
to do true
CDMA is to run a packet REPEATER... for example, take a voice repeater and run
packet thru it.  That is the only way that every station in a RF 
network can be
assured of hearing every other station.  Even then, you'll still have the
occasional "double".  Not perfect.

The next closest thing to CSMA you can have in a SIMPLEX network is DAMA - and
heck, DAMA doesn't have to run simplex!  For those in the USA who have no clue
what DAMA is... your station does not speak until spoken to... and the
digipeater is constantly polling all stations it knows are or should be
connected.  But DAMA requires channel overhead to do the polling.  Not 
perfect.

We would have a simplex CSMA network if we could guarantee that each 
station was
within simplex range of every other station... for example, three stations
equidistant from one another.  Of course in most real world situations, the
network gets strung out over a long area so that stations at one end 
can't hear
stations at the other end... and the stations in the middle invaribly hear the
collisions.

On the case of APRS, we end up with no symmetry between the users and the
digipeaters.  I've done the calculations, and your typical mobile user can at
best hear 3% of the area within a 100' digipeater's coverage area.  So any
station located in the other 97% of the digi's coverage can't possibly 
be heard
by the mobile, and therefore can't be avoided.  It's a crap shoot - err ALOHA
-at best.  Not perfect.

So in summary.... No system is perfect.  CSMA only works when a)all 
stations can
hear each other, or b) when using a packet repeater.  Anything else is ALOHA.
It's the nature of the beast.

The best thing we can do to make the netowrk more aloha friendly is to 
limit the
number of stations I have to statistically compete with.  As an analogy, would
you want to take your chances blindly running across the interstate or a
country road?  If we limit the traffic by making stay in a localized area, we
have effectively turned what used to be busy road into a country road (ie
segment the network in to STATEn-n, or limit WIDEn-n hops).  Or we could put a
toll gate along the interstate to occasionally stop the traffic to allow me to
cross... ie the alt input frequency that gets me into the local digi so 
that my
packet can be better merged in with the rest of the traffic.  Not the best
analogy, but it's all I can come up with at this time....

Wes

Quoting Henk de Groot <henk.de.groot at hetnet.nl>:

> AE5PL Lists schreef:
>> APRS is NOT ALOHA!!!  Read what the ALOHA project was and then we can
>> discuss this.  AX.25 UI packets are transmitted using CSMA with a
>> varying probability (depending on station types, power, location,
>> terrain, etc.) that one station will try to transmit at the same time
>
> Yes, I know. But when you are unable to detect any carriers of the other
> APRS stations because they are too weak or hidden from you then, in those
> cases, CSMA just sees a clear channel and hence transmit whenever there is
> something to tranmsit (with a short delay because of the persistence and
> slottime setting but otherwise pure ALOHA behaviour).







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