VSAT NETWORK

VSAT NETWORK DEFINITION
VSAT, now a well established acronym for Very Small Aperture
Terminal, was initially a trademark for a small earth station marketed
in the 1980s by Telcom General in the USA. Its success as a
generic name probably comes from the appealing association of its
first letter V, which establishes a ‘victorious’ context, or may be perceived
as a friendly sign of participation, and SAT which definitely
establishes some reference to satellite communications.
In this book, the use of the word ‘terminal’ which appears in the
clarification of the acronym will be replaced by ‘earth station’, or
station for short, which is the more common designation in the field
of satellite communications for the equipment assembly allowing
reception from or transmission to a satellite. The word terminal
will be used to designate the end user equipment (telephone set,
facsimile machine, television set, computer, etc.) which generates
or accepts the traffic that is conveyed within VSAT networks. This
complies with regulatory texts, such as those of the International
Telecommunications Union (ITU), where for instance equipment
generating data traffic, such as computers, are named ‘Data Terminal
Equipment’ (DTE).
VSATs are one of the intermediary steps of the general trend
in earth station size reduction that has been observed in satellite
communications since the launch of the first communication satellites
in the mid 1960s. Indeed, earth stations have evolved from the
large INTELSAT Standard A earth stations equipped with antennas
30 m wide, to today’s receive-only stations with antennas as small
as 60 cm for direct reception of television transmitted by broadcasting
satellites, or hand held terminals for radiolocation such as
the Global Postioning System (GPS) receivers. Present day hand
held satellite phones (IRIDIUM, GLOBALSTAR) are pocket size.
Figure 1.1 illustrates this trend.
Therefore, VSATs are at the lower end of a product line which
offers a large variety of communication services; at the upper end
are large stations (often called trunking stations) which support large
capacity satellite links. They are mainly used within international
switching networks to support trunk telephony services between
countries, possibly on different continents. Figure 1.2 illustrates how
such stations collect traffic from end users via terrestrial links that
are part of the public switched network of a given country. These stations
are quite expensive, with costs in the range of $10 million, and
require important civil works for their installation. Link capacities
are in the range of a few thousand telephone channels, or equivalently
about one hundred Mbs−1. They are owned and operated
by national telecom operators, such as the PTTs, or large private
telecom companies.
diameters less than 2.4 m, hence the name ‘small aperture’ which
refers to the area of the antenna. Such stations cannot support
satellite links with large capacities, but they are cheap, with manufacturing
costs in the range of $1000 to $5000, and easy to install any
where, on the roof of a building or on a parking lot. Installation costs
are usually less than $2000. Therefore, VSATs are within the financial
capabilities of small corporate companies, and can be used to set
up rapidly small capacity satellite links in a flexible way. Capacities
are of the order of a few tens of kbs−1, typically 56 or 64 kbs−1.
The low cost of VSATs has made these very popular, with amarket
growth of the order of 20–25% per year in the nineties. There were
about 50 000 VSATs in operation worldwide in 1990, and more than
600 000 twelve years later. This trend is likely to continue.
Referring to transportation, VSATs are for information transport,
the equivalent of personal cars for human transport, while the large
earth stations mentioned earlier are like public buses or trains.
At this point it is worth noting that VSATs, like personal cars, are
available at one’s premises. This avoids the need for using any public
network links to access the earth station. Indeed, the user can directly
plug into the VSAT equipment his own communication terminals
such as a telephone or video set, personal computer, printer, etc.
Therefore, VSATs appear as natural means to bypass public network
operators by directly accessing satellite capacity. They are flexible
tools for establishing private networks, for instance between the
different sites of a company. Figure 1.3 illustrates this aspect by