[aprssig] APRS on Android

Stephen H. Smith wa8lmf2 at aol.com
Mon Jan 24 14:12:20 EST 2011

On 1/24/2011 10:23 AM, Eric Lorenz K9LGE wrote:
> Dear Georg:
>   ...just wish more igates and such ported TCP packets back to local 
> radio...I have several friends in areas I travel that cannot see me when I am 
> beaconing as they use radio to monitor local APRS traffic and their local 
> I-gates/digis only port RF to the internet, not the other way.
> Thanks,
> Eric K9LGE

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 



Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      WA8LMF  or 14400    [Think bottom of the 2M band]
Skype:        WA8LMF
Home Page:          http://wa8lmf.net

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