CONCLUSIONS

This conclusion summarises the perceived advantages and drawbacks
of VSAT networks.
1.10.1 Advantages
1.10.1.1 Point-to-multipoint and point-to-point
communications
A VSAT network offers communications between remote terminals.
As a result of the power limitation resulting from the imposed small
size and low cost of the remote station, a VSAT network is most often
star-shaped with remotes linked to a larger station called a hub. This
star configuration often well reflects the structure of information
flow within most large organisations which have a point of central
control where the hub can be installed. The star configuration itself
is not a severe limitation to the effectiveness of a VSAT network
as point-to-point communications, which would conveniently be
supported by a meshed network, can still be achieved via a double
hop, using the hub as a central switch to the network.
1.10.1.2 Asymmetry of data transfer
As a result of its asymmetric configuration, a star-shaped network
displays different capacities on the inbound link and on the outbound
link. This may be an advantage considering the customer
need for asymmetric capacities in most of his applications. Should
he use leased terrestrial lines which are inherently symmetric, i.e.
offering equal capacity in both directions, the customer would have
to pay for unused capacity.
1.10.1.3 Flexibility
A VSAT network inherently provides a quick response time for
network additions and reconfigurations (one or two days) as a
result of the easy displacement and installation of a remote station.
1.10.1.4 Private corporate networks
A VSAT network offers its operator end-to-end control over transmission
quality and reliability. It also protects him from possible
and unexpected tariff fluctuations, by offering price stability and
the possibility to forecast its communication expenses. Therefore it
is an adequate support to private corporate networks.
1.10.1.5 Low bit error rate
The bit error rate usually encountered on VSAT links is typically
10−7.
1.10.1.6 Distance-insensitive cost
The cost of a link in a VSAT network is not sensitive to distance.
Hence, cost savings are expected if the network displays a large
number of sites and a high geographical dispersion.
1.10.2 Drawbacks
1.10.2.1 Interference sensitivity
A radio frequency link in a VSAT network is subject to interference
as a result of the small earth station antenna size.
1.10.2.2 Eavesdropping
As a result of the large coverage of a geostationary satellite, it may
be easy for an eavesdropper to receive a downlink carrier and access
the information content by demodulating the carrier. Therefore,
to prevent unauthorised use of the information conveyed on the
carrier, encryption may be mandatory.
1.10.2.3 Loss of transponder may lead to loss of network
The satellite is a single point failure. Should the transponder that
relays the carrier fail, then the complete VSAT network is out
of order. Communication links can be restored by using a spare
transponder. With a spare colocated on the same satellite, a mere
change in frequency or polarisation puts the network back in
operation. However, should this transponder be located on another
satellite, this may mean intervening on each site to repoint the
antenna, and this may take some time.
1.10.2.4 Propagation delay (double hop = 0.5 s)
The propagation time from remote to remote in a star-shaped
network imposes a double hop with its associated delay of about
half a second. This may prevent the use of voice communications,
at least with commercial standards.