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[aprssig] Local Event using RELAY?

Andre PE1RDW aprs at pe1rdw.demon.nl
Thu Mar 31 17:29:52 UTC 2005

Wes Johnston schreef:

> Ah ha!!!  The light came on for me!  Henk and I are talking apples to 
> Pete's
> oranges...   Apples and Oranges...
> The important (yet subtle) difference is the dispairity between a 
> mobile's
> coverage area and a digipeater's coverage area.  When packets are 
> bouncing
> along from one digi to the next, and  the digipeaters have a significant
> portion of their coverage areas overlapping, then CSMA can work 
> better.  The
> greater the overlap, the better CSMA works.  But since no two 
> digipeaters have
> 100% overlapping coverage area (what would be the point?), CSMA will 
> not work
> 100% of the time... it's simply a function of the percent overlap.  I 
> can see
> from a digipeater to digipeater perspective, that without CSMA, the 
> network
> wouldn't work well.
> But my mobile can't hear another mobile trying to get into the digi at 
> the same
> time as me, and it sure can't hear the next digi in the next town. So 
> for a
> mobile station, where the coverage area is extremely different than 
> the station
> he's trying to contact, we have to use aloha.  I also recognize that an
> undecodable signal that keeps tripping my squelch open will serve to 
> prevent my
> station from transmitting due to the slot time and persist variables 
> in my TNC.
> But if we can agree that upon entry into the network from a mobile 
> station, the
> access method is aloha, and once a packet is "in" the network, the access
> method is CSMA.  This leads right to Bob's alt input digipeater idea.  
> Mobile
> stations compete for access into a digipeater on a quiet frequency 
> using aloha.
>  That first digi is able to listen for a break in the traffic on the APRS
> frequency, and properly use CSMA to insert the packet when the band is 
> clear.
> Let's use Pete's number of 93% of traffic on a digi comes from other
> digipeaters.  That means that 7% is local.  I would rather my mobile 
> station
> compete with 7% traffic instead of 100% of the traffic in an area.  While
> neither CSMA nor aloha access methods are perfect in a radio 
> environment, they
> really don't mix well... so let's keep 'em separate!  If each town 
> took one
> digipeater site and made it a simple alt-input RELAY digi, the aloha 
> mobiles
> would transmit on one frequency (while still listening on 144.39), and 
> all the
> rest of the CSMA traffic would be on the main aprs frequency.  Pete 
> has said in
> the past that you end up with mobiles transmitting blindly on the alt 
> input
> frequency, but hey, if we all know the rules, and we call that input 
> frequency
> part of the "system", then it's not a problem on our consciences.  The 
> logistic
> trick is to make sure that there are no other services on 144.99 in 
> your area
> before you put up an alt input digi.  Other APRS people can/will 
> accept the
> crap shoot on 144.99, but if your local DX cluster is there, they sure 
> won't
> like it.
> In Sumter SC, we are running a mic-e input digipeater on a local 
> repeater... for
> all intents and purposes, it may as well be an altinput digipeater.  
> My packet
> is digipeated from the input of our local 2m repeater to a 70cm link 
> freq over
> to the local digi.  That digi accepts the packet (thanks digi_ned), 
> and inserts
> it into 144.39 seamlessly.  The result is 100% success with my 
> mobile... and I
> do mean 100%!  Using alt input digipeater (the mic-e repeater), I'm 
> getting
> 100% of my packets onto 144.39 even though I'm in a swamp far below 
> most of the
> surrounding area.
> So why don't we offer the aloha (mobile) stations 144.99 as a 
> non-congested
> entry into the network?  All it takes is one digipeater in each 
> town... you'd
> be suprized the range you can get into a digi when the noise floor is 
> nill.  If
> we did this, 200mW trackers would be a realy workable reality.
> Wes
that sounds a lot like the plan I put forward a couple of weeks ago with 
puting the fixed stations on a 9k6 70cm frequentie and the mobiles on 
1k2 2m offcourse this plan is simpler to implement and therefor more 
likely to be accepted.
I like it as a good leap to improving the channel usage and a stepping 
stone to moving the main aprs trafic to 9k6.

73 de Andre PE1RDW

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