[aprssig] APRS Messaging

[Stephen H. Smith]

  On 7/28/2010 10:29 PM, Brett Friermood wrote:
> On Wed, Jul 28, 2010 at 11:56 PM, Adi Linden<adil at adis.on.ca>  wrote:
>> I've read the docs you've recommended. I suppose I am interested in a lot more detail. In order
>> for a message to make it from IS to RF, does the iGate need to listen to the complete IS data
>> stream? Or is there a mechanism that allows an iGate to forward the "heard" list to upstream
>> server(s) to establish a path for messaging?
> The IS is smart. It knows which stations have been heard by which
> IGates.

Actually the APRS-IS does NOT make these decisions.   The decision to gate or 
not gate   IS-to-RF  is made by each igate.   APRS-IS servers constantly 
exchange data with each other.  It does not matter which APRS-IS server an 
igate connects to. The igate will receive the same data from the IS, as 
determined by the user-defined filters.

Every igate maintains a table of stations heard on RF.   If a given station was 
heard recently (typically within the last half hour or hour) and is considered 
local (meaning typically heard direct or via only a single digi hop), any 
messages bound for that station from the Internet will be grabbed from the IS 
stream and re-transmitted on RF.

When a mobile moves from the local area of an igate (or shuts down), the 
station will be dropped from the "locals heard" list of the igate after a 
half-hour or hour or so.  As a result that particular igate will quit grabbing 
messages for that mobile from the IS, and will stop transmitting them on RF.   
   Hopefully, by that time, the mobile has moved into the local area of another 
igate where the process repeats itself.

In other words,  for a message on the Internet to get to a mobile, the mobile 
must have been heard first on RF recently and nearby by an igate.   Further, 
that igate must be explicitly configured for "reverse-gating".



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--

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

NEW!    *** HF APRS over PSK63 ***
    http://wa8lmf.net/APRS_PSK63/index.htm

Universal HF/VHF/UHF Antenna Mounting System
   http://wa8lmf.net/mobile/UniversalAntMountSystem.htm

"APRS 101"  Explanation of APRS Path Selection & Digipeating
   http://wa8lmf.net/DigiPaths



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