[aprssig] radio aprs vs internet aprs

Stephen H. Smith wa8lmf2 at aol.com
Tue Mar 20 08:56:59 CDT 2012

On 3/20/2012 9:23 AM, Mike Goldweber wrote:
> Hi Everyone,
> So I'm a little confused.  I can imagine a pure internet set of links (and 
> displayed nodes), and I can imagine a set of pure radio linked nodes.  I am a 
> little confused about what seems to be a partial mix of internet or radio 
> nodes.  Could someone please explain the mechanics of this to me?
> Thanks,
> Mike Goldweber

1)    APRS began as a purely RADIO-based system based on AX.25 packet radio 
(data transmission over over radio) some decades ago.

2)    The APRS-IS (APRS Internet System) was added to the radio-based network 
some years later.

3)    Normally, traffic heard on RF is passed to the Internet system of 
dedicated APRS servers, by radio base stations with Internet connections 
(Internet Gateway stations a.k.a. "igates").   Once passed to the Internet,  
traffic can be monitored on connected devices, or by viewing public websites 
such as Findu.com or APRS.fi .    Virtually EVERYTHING heard on RF by an igate 
station will be passed into the APRS-IS.

4)    Because the 1200-baud radio channel has such severely limited capacity 
compared to the Internet, traffic passes the other way (Internet-to-RF a.k.a. 
"reverse gating") only under certain very limited conditions.    Normally, only 
messages directed to a specific station (but not the usual broadcast location 
beacons) will pass Internet-to-RF.

In order for such a message to pass from Internet to RF, the RF station being 
addressed must have been heard recently (typically within the last hour) and 
nearby (either heard directly by an igate station, or within only one 
over-the-air digipeater hop).

5)    This may seem obvious, but many newcomers unwittingly assume the APRS 
network has the same kind of ubiquitous coverage as cellular networks.  It 
DOESN'T!     The APRS network has nowhere near the number of base station igate 
sites as cellular network have cell sites.  The APRS RF network has large 
coverage holes in many parts of the country.     If the RF station being 
addressed has NOT been heard recently, or is not within range of an igate 
station,  then messaging attempts Internet-to-RF will fail.

6)   The APRS Internet System can function, in a manner similar to EchoLink or 
IRLP, as a kind of "wormhole" connecting two radio users too far apart to 
communicate directly with each other by radio-only.   In this case, the 
limitations on what will pass Internet-to-Rf will apply to BOTH parties 
involved.   I.e. BOTH parties need to be within the local coverage area of 
igate stations.



Stephen H. Smith    wa8lmf (at) aol.com
Skype:        WA8LMF
Home Page:          http://wa8lmf.net

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