CONCEPTS IN TRANSMISSION TRANSPORT
There are three underlying reasons for the use of VSAT networks:
1. An economic alternative to establish a data network, particularly if traffic flow is
to/from a central facility, usually a corporate headquarters to/from outlying remotes.
2. To bypass telephone companies with a completely private network.
3. To provide quality telecommunication connectivity where other means are substan-
dard or nonexistent.
Regarding reason 3, the author is aware of one emerging nation where 124 bank
branches had no electrical communication whatsoever with the headquarters institution in
the capital city.
Characteristics of Typical VSAT Networks
. On conventional VSAT net-
works, the hub is designed to compensate for the VSAT handicap (i.e., its small size). For
example, a hub antenna aperture is 5 m to 11 m (16 ft to 50 ft) (Ref. 12). High-power
amplifiers (HPAs) run from 100 W to 600 W of output power. Low-noise amplifiers
(LNAs), typically at 12 GHz, display (a) noise figures from 0.5 dB to 1.0 dB and (b) low-
noise downconverters in the range of 1.5-dB noise figure. Hub G/T values range from
+29 dB/K to +34 dB/K.
VSAT terminals have transmitter output powers ranging from 1 W to 50 W, depending
on service characteristics. Receiver noise performance using a low-noise downconverter
is about 1.5 dB; otherwise 1 dB with an LNA. G/T values for 12.5-GHz downlinks are
+14 dB/K and +22 dB/K, depending greatly on antenna aperture. The idea is to
make a VSAT terminal as inexpensive as possible. Figure 9.21 illustrates the conventional
hub/VSAT concept of a star network. The hub is at the center.
Typical VSAT network topology. Note the star network configuration. The outlying VSAT
terminals can number in the thousands.