[aprssig] Hops and iGating?

Stephen H. Smith wa8lmf2 at aol.com
Fri Apr 10 12:36:08 EDT 2020

On 4/10/2020 11:33 AM, carcarx at gmail.com wrote:
> Hawaii has several Kantronics KPC 3 full-time APRS stations.
> One on Oahu rarely iGates (and intermittently, at that), but always digipeats.
> Another is "rock solid" in iGating and digipeating. A third is on Maui, about a 
> mile above sea level, with a Wide3 digipeat. In that manner Hawaii Island, 
> Maui, and Oahu can
> exchange APRS data.
> We'd like to "fill in" coverage on Oahu, given all our valleys, and modest 
> mountains.
> The questions - what is the default algorithm about what gets iGated? Does a 
> station only get iGated if there are no hops in the APRS packet, for example?

NO!   The decision has nothing to do with the TNC.  It's the APRS software on 
the computer with the Internet connection that makes the decision to igate.

Note that a KPC3 or similar TNC can function standalone as a digipeater-only, 
using it's own internal firmware.  To igate, it would have to be connected to a 
computer with an Internet connection.
    Once the TNC is under control of a computer,  playing the role of both a 
digipeater and an igate,  the TNC's internal settings are overridden by the 
computer program settings.  In the required "KISS" mode of software control 
takes over, the TNC is reduced to being a "dumb modem".

Normally, ANYTHING heard by the radio/TNC/computer setup will be igated, 
regardless of number of digipeater hops. In turn, if more than one radio/igate 
heard the same transmission, regardless of number of hops, and passed it to the 
APRS Internet System,  The APRS-IS automatically rejects the duplicate reports. 
   I.e. "first man in wins".

Normally, a transmission heard directly by one igate will get to the Internet 
slightly sooner than a version delayed by one or more digipeats.  As a result, 
the "transmission of record" seen on the APRS-IS is most likely to be the 
direct version.
    However, this is NOT always true, depending on the latency of Internet 
connections. if the station that heard the transmission direct is crippled by 
slow or overloaded routers on the Internet, a digipeated version of the 
transmission might reach the APRS-IS first.

> Our "fill in" stations may not be able to have internet access, so it may be 
> two hops just to get to an iGate.

That's perfectly normal -- no big deal....

> I've been through the KPC 3 and 3+ manual online, but see no overt mention of 
> configuring iGating and hop counts.

Again, this is because igateing depends on software on an attached computer. 
The KPC3 becomes basically just a dumb modem.  Note that the KPC3 firmware 
dates from an era BEFORE the APRS Internet System even existed.  It's totally 
focused on radio-to-radio communications.  As further evidence of this, note 
that a KPC3 does not have an IP address or a RJ-45 Ethernet jack!

> Any information welcomed!

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

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