[aprssig] APRS on Android

Andre v Schaijk aprs at pe1rdw.demon.nl
Mon Jan 24 14:58:23 EST 2011

Op 24-1-2011 20:12, Stephen H. Smith schreef:
> It's not practical or realistic to blanket gate packets from IP to 
> RF.    The RF channel is only 1200 baud.   For you Internet types,  
> that's a blistering 1.2 KB/sec!      In turn, collision avoidance and 
> reliable operation on RF requires that the channel not be loaded to 
> more than about 25-30% or so of capacity -- the so-called "Aloha" 
> threshold for a random access network.  This lowers the EFFECTIVE 
> throughput to about 300-400 baud.
>  If an igate retransmitted everything it hears on the APRS-IS to RF, 
> it would transmit nonstop,  monopolizing the RF channel 100% of the 
> time.  And still not be able to remotely keep up with the Internet 
> traffic.  (The full APRS Internet stream is now a non-stop 
> 150-200K/sec -- this is why one uses port 14580 filters to limit the 
> firehose effect when connecting to the APRS-IS.)
> Because of the huge difference in capacity between Internet and RF,  
> reverse igating ( Internet-to-RF) is normally limited to messages,  , 
> directed to or from specific stations, but not transmissions like 
> position reports that are "broadcast" to no one in particular.   And 
> then only if the addressed station has been heard on RF recently 
> (usually within 1 hour)  and "nearby" (heard direct or via only 1 digi 
> hop).
> Some igates CAN reverse-gate other traffic on a very limited basis, 
> but you have to make arrangements to "whitelist" your call with the 
> igate operator in advance.
I did a test not to long ago reverse Igating.
on a dedicated 9k6 channel it is very realistic to stream a 500 km range 
in europe and still only use about 50% or less of channel time, because 
it was a dedicated frequentie there was no aloha effect or slottime and 
txdelay problem, this improved efficency a great deal.

Andre PE1RDW

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