[aprssig] setting up digipeaters & I-gates
Stephen H. Smith
wa8lmf2 at aol.com
Tue Jul 20 16:54:10 CDT 2010
Jaye Inabnit ke6sls wrote:
> I am most interested
> in finding low power I-gate hardware that I can install either here at my QTH
> or at our low level repeater site.
1) Don't confuse digipeaters (RF-->RF relay function) with igates
(RF<-->Internet connections). Digipeaters are located at places with
good radio coverage; i.e. ideally on hilltops or tall buildings. Igate
setups are located where you have always-on Internet access.
2) Unfortunately, you will NOT typically have Internet access at
remote hill top repeater sites.
3) Igates are normally located at home or office locations where you
DO have always-on Internet access. As long as you can hear the
digipeater(s) decently at these home or office locations with Internet
access , the elevation/antenna height of the station doing igate duty is
4) An igate setup will consist of a radio, TNC, and a computer with
an Internet connection running a suitable program. It could be a
dedicated igate-only program, --or-- a general-purpose APRS client such
as UIview, APRSplus or the Linux-based Xastir. These three programs
can all do triple duty as an end-user client producing map displays and
messaging, act as an igate, and act as a digipeater at the same time.
Unless you live in the dream high-rent district on the hilltop
overlooking town, the digi will probably be configured as a low-level
WIDE-1-1 "fill-in" digi (if you enable it at all).
You do not need bleeding-edge multi-core gamers machines with HD
displays to run APRS software. Even old Pentium II or III "clunkers"
will do the job decently running Win98, Win2K or a Linux distro. This
is an ideal use for elderly computers. For minimal hardware and lowest
power consumption, consider older laptops. For unattended igate
operation, you can live with small low-resolution screens that would be
considered unusable for most of today's software. A virtue of older
machines is that, unlike any of today's current models, they usually
have REAL SERIAL PORTS without the hassle of USB<-->serial adapter
"dongles". [Serial ports are essential for connecting TNCs, or the
Kenwood radios with built-in TNCs, to computers.]
5) Note that more digipeaters can contribute to on-air congestion
and actually REDUCE the channel capacity, if there is already adequate
digi coverage in the area. This is because, unlike voice repeaters,
APRS digipeating is not a simultaneous process. A station makes a
transmission, the digipeater hears it and stores it in memory briefly,
and then retransmits it AFTER the original station's transmission.
Even a single digipeat DOUBLES the air time each user occupies, cutting
the channel throughput (number of packets per minute) in half. An
added digipeater can potentially cause each transmission to occupy a
THIRD time slot, cutting the channel capacity to ONE-THIRD of the
no-digipeaters scenario (assuming that the original transmission and
both digipeats are heard over the same area; i.e. not just at a distant
6) Igates normally DO NOT reduce over-the-air channel capacity. The
bandwidth of the Internet is essentially unlimited, compared to
over-the-air operation at 1200 baud. In general, the more igates the
better. The sooner (in terms of hops) you get packets off the air and
into the Internet, the less on-air congestion and chances for RF
"APRS 101" Explanation of APRS Path Selection & Digipeating
Stephen H. Smith wa8lmf (at) aol.com
EchoLink Node: WA8LMF or 14400 [Think bottom of the 2M band]
Home Page: http://wa8lmf.net
NEW! *** HF APRS over PSK63 ***
Universal HF/VHF/UHF Antenna Mounting System
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