Most satellites are known for their video
transmissions and many of these transmissions can be received with
simple, readily obtainable equipment. On a global basis, the
principal application supported by satellites is indeed video
transmission. However, satellites also carry large amounts of
non-video products that range from telephony to high-speed data
systems. Once in orbit and operational, the quality of service
provided by satellites is exceptionally high and far more reliable
than terrestrial links.
The
main distinction between the satellite solution and its terrestrial
counterparts is the inherent broadcast capability satellite
provides. The large footprints of today?s satellites allow
multicasting to be provided securely and cost-effectively. Another
satellite advantage is the speed at which data can be transferred.
By bypassing today?s relatively low-speed modems, data can be
delivered via satellite at higher speeds than most users can receive
over telephone or even ADSL lines.
VSAT stands for Very Small Aperture Terminal ?
it?s a acronym and as such it?s been adopted by all and every type
of satellite product from small components of a system to complete
systems. Generally, these systems operate in the Ku-band and C-band
frequencies. As a rule of thumb C-band (which suffers less from rain
attenuation, but requires larger antennas) is used in Asia, Africa
and Latin America whilst Ku-band (which can use smaller antennas,
but suffers from rain fade in a monsoon-like downpour) is used in
Europe and North America.
Typically, interactive Ku-band antenna sizes range from 75
centimeters to 2.4 meters and C-band from 1.8 meters to 3.8 meters.
One way systems can use antennas as small as 45 centimeters.
VSAT in general provide point-to-point or
point-to-multipoint connectivity between points across the globe and
offers a number of advantages over terrestrial alternatives. For
private applications, companies can have total control over their
own communication system without dependence on other companies.
Unlike traditional landline services, VSAT services are unaffected
by physical barriers, multiple service suppliers and problems
associated with the "last mile" connection. New sites can be added
and existing connections reconfigured usually within a matter of
days.
The key advantages of a VSAT network are:
-
Single network for voice, data, video, e-mail, fax etc.
-
Unaffected by geographic barriers or natural disasters
-
Quick installation (for a 2.4 meter antenna in general
not more than 3 days - quick deployable systems even a few
hours)
-
Add
new sites cost-effectively so it reduces international expansion
and operating costs
-
High
network availability (99.5% - 99.9%)
-
Customized bandwidth
One-way or
broadcast systems rely on a transmitting station which transmits one
or more carriers to the satellite which re-broadcasts the signal
over its coverage area. All receive-only VSATs under the satellite
footprint can then receive the signal or the user/operator is able
to define groups of VSATs from one to all on the network. Broadcast
systems are typically used for video, audio and data. The most
popular application for data is the transmission of financial feeds
and - Reuters, Telerate
are good examples of companies with large data broadcasting networks
- however, there are many other uses, such as software downloads,
file transfers, transmission of press agency news items (with
pictures) etc.
Interactive or two-way VSAT systems come in
three main network topologies ? point-to-point (aka SCPC), star and
mesh. Star tends to be based either on a shared access scheme
(TDM/TDMA). Mesh usually uses links which are set-up and torn-down
on request to establish a direct link between two sites on a demand
assigned basis. SCPC is a dedicated link (the satellite equivalent
to a leased line).
Satellite systems make an excellent choice for
communications between widely separated points on the earth. They
require no user investment in terrestrial "infrastructure" except at
the termination points. This reduced entry cost makes them
particularly attractive for applications in (rural) areas with poor
or no service at all, especially when the terrain is problematic and
projected traffic volumes may not support a terrestrial network.
VSATs are ideal for international businesses requiring worldwide
connectivity for mission critical applications, disaster recovery,
local access extensions, trunk redundancy, and host-to-host
communications. Companies using VSAT are for example car
dealerships, gas stations, lottery systems, banks, insurance
companies, drug stores, general stores, supermarkets, healthcare
companies, manufacturers, couriers, hotel chains, car rental
businesses, food manufacturers, heavy industries, mines, electrical
utilities, oil and gas pipelines, energy production and exploration,
timber companies, plantations, various government departments and
agencies.
A total network solution is often a synergy
with cable (fiber), fixed wireless, and satellite (VSAT). VSAT networks come
in various shapes and sizes ranging from star data system users with
one site connected to an operator's shared hub to many thousands
based on a dedicated facility located at their own site. Mesh
systems have traditionally been somewhat smaller in size than star
systems - 5 to 30 sites used to be a good rule of thumb - but the
average size of orders has risen as prices have come down and some
rural telephony networks now comprise as many as several hundred or
even thousands of sites.
The data rate, or speed,
at which each of these services is provided, is affected by several
factors, including the user?s requirements, the amount of satellite
capacity available, and the protocols used for each service. In
general, satellite services provide 45 to 70 Mbps capability for Internet/Intranet backbone
connectivity (although they can go as high as 155 Mbps, 2 to 3 Mbps
for multi-casting and 400 kbps for delivery of Internet information
to consumers.
While cost is an important point for comparing
satellite versus terrestrial systems, the question of whether they
compete directly or indirectly depends always on the application
involved. Customers tend to look for the be-all and end-all solution
however it?s not always easy to understand where what is and what is
not available.
All solutions offer high reliability for the
most critical connections and they frequently work well together as
a hybrid solution. The tendency is usually very application-specific
and every case is different in terms of terrain, line of sight,
location, license, etc. Organizations should find a solution that
provides the needed bandwidth for a justifiable cost. Cost includes
installation, on-going charges, upkeep, losses due to downtime and
so on.