[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?
> 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
Stephen H. Smith wa8lmf (at) aol.com
Home Page: http://wa8lmf.net
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