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What is a satellite
network ?
What types of network are available?
Who controls the satellite networks
?
What is a satellite network?
A satellite network is the infrastructure for
sending and receiving information via satellite. The network for broadcasting
a television programme, for example, would differ greatly, from that
used to transmit film from a publishing house to a printer in another
part of the world. Inevitably, end-users require different forms of
communications depending on the nature of their business. They may need
to send data, audio, video, voice or a combination of digital information.
They may require one- or two-way communications capability. Their information
systems may be channelled through a central hub, such as a head office,
or simply passed relay-style from one site to another. Scale, and therefore
volume, of data will vary considerably. A one-way satellite link comprises
several parts:
1 The uplink (transmit) Earth Station
2 The uplink path to the satellite from the Earth Station
3 The satellite itself
4 The downlink path from the satellite to the Earth station
5 The down link (receive) Earth Station
In the case of a two-way link, each Earth station both transmits and
receives. The one-way or two-way satellite terminals installed at customers’
premises are called VSATs, an acronym for Very Small Aperture Terminals.
These allow the transmission of data signals using small dish antennas.
They are reliable and long-lasting. Equipment for one-way and two-way
networks is more or less the same, comprising an indoor unit (IDU) and
an outdoor unit (ODU). The two units need to be positioned as closely
together as possible and take just a few hours to install. Basic receiver
kit comprises a demodulator and decoder, baseband/signal processor,
control circuitry, power supply and interface ports to connect with
the user’s computer set-up. The outdoor unit or antenna is usually
mounted on a roof or building wall with a clear view of the Southern
skies in line with the satellite beam. The indoor equipment is simply
a small box of electronics, which plugs into a mains electricity supply.
Some one-way systems are small enough to fit on a PC expansion card,
larger two-way systems can be the size of a small television set.
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What types of network are available?
Currently, there are four basic forms of satellite
network appropriate to different applications and market sectors:
Information broadcast
Information broadcast is a one-way form of communication where video,
audio, voice, data, or a mixture of these, are transmitted simultaneously
and in real time to one or more sites. Examples include direct to home
PC (e.g. Sky, in-store radio systems for background music, advertising
or customer information and information delivery for news, weather and
travel services.
Point-to-point/multi-point
As the name suggests, this is a two-way form of communication, between
satellite terminals at two or more points. It provides an ideal means
of linking sites especially in isolated parts of the world where local
terrestrial communications are poor or non-existent. Point-to-point/multi-point
can be used for any kind of information transfer, from email, to telephone,
to videoconferencing.
Hub or star network
These networks can provide two-way communication
between any number of sites. All information is directed through a main
control or "hub" site, such as a head office, and is then transferred
via satellite to branch offices or remote sites. Existing examples include
the retail sector, where stores, warehouses and administrative HQ are
dynamically linked, and banking where branches, cash machines and head
office are interconnected.
Mesh networks
This type of network allows multiple terminals to
communicate directly, without the need for a "hub". They are most suitable
for voice applications, but can also handle data and video. In instances
where there are several company offices processing similar amounts of
information, mesh networks tend to be the preferred option.
Commonly used satellite access techniques
TDMA (Time Division
Multiple Access)
In a TDMA network all VSATs share satellite resource
on a time slot basis. Generally, systems with a star topology use a
TDMA transmission technique.
DAMA (Demand Assigned Multiple
Multiple Access)
A managed network with a shared pool of capacity which
is allocated on demand (similar to the telephone network. DAMA is ideally
suited to voice, fax and data requirements and also supports videoconferencing.
PAMA (Pre-Assigned Multiple
Access)
The VSATs are pre-allocated a designated frequency.
Equivalent of the terrestrial leased line, PAMA solutions use the satellite
resources constantly, making them ideal for interactive data applications
or high traffic volumes.
CDMA (Code Division Multiple
Access)
A central network monitoring system that allocates
a unique code to each of the VSATs enabling multiple VSATs to transmit
simultaneously and share a common frequency band. The star topology
is the best type of network and CDMA is suited to large networks with
low data requirements.
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Who controls the satellite
networks?
INTELSAT
Intelsat was a true pioneer, launching Early Bird, the first ever commercial
satellite, in 1965. An international non-profit making cooperative,
INTELSAT today represents 140 member nations and claims to be the communications
provider with the broadest reach and most comprehensive range of services.
The consortium currently has a fleet of 19 satellites in orbit and is
currently developing a new generation of highly advanced satellites.
Major international telecommunications operators, broadcasters, airlines,
banks, and disaster relief organisations all make use of Intelsat’s
services.
EUTELSAT
Eutelsat was created in 1977, to operate satellites for fixed and mobile
communications in Europe. Since then, the organisation’s initial
membership of 17 countries has increased to 47, fronted by government
representatives. Signatories are entitled to use the organisation’s
satellites for domestic and international networks and lease excess
capacity to other users. In 1996, EUTELSAT relaxed its strict membership
criteria, allowing more than one signatory from each member state, which
ushered the way for telecommunications companies and broadcasters to
invest in the system. With 14 satellites in orbit, Eutelsat offers a
range of services including multimedia services, telephony, analogue
and digital entertainment and business communications.
INMARSAT
Inmarsat was established in 1979 to serve the maritime industry, developing
breakthroughs in satellite communications for ship management, safety
and distress applications. More recently, its remit has broadened, and
the consortium is now active in land, mobile and aeronautical communications.
There are over 100,000 terminals throughout the world using the Inmarsat
system, which offers direct dial telephone, fax, data, e-mail, audio,
compressed video and still video pictures, videoconferencing and telemedicine.
The Inmarsat partnership is made up of more than 1,200 companies worldwide
including manufacturers and service providers.
ICO
ICO Global Communications represents 60 investors from the telecommunications
and technology industries. These include the world’s top 20 telecommunications
companies which, it is estimated, serve 25 per cent of the world’s
cellular services market. ICO is currently building a satellite system,
which will link cellular and fixed telecommunications systems across
the world, overcoming local incompatibilities. The organisation is gearing
up to the beginning of the year 2000, when it will offer digital voice,
data, fax and messaging services using a basic hand-held terminal the
size of a portable phone.
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