VSAT NETWORK OPTIONS

1.6.1 Star or mesh?
Section 1.2 introduced the two main architectures of a VSAT network:
star and mesh. The question now is: is one architecture more
appropriate than the other?
The answer depends on three factors:
– the structure of information flow within the network;
– the requested link quality and capacity;
– the transmission delay.
These three aspects will now be discussed.
1.6.1.1 Structure of information flow
VSAT networks can support different types of application, and each
has an optimum network configuration:
– Broadcasting: a central site distributes information to many
remote sites with no back flow of information. Hence a starshaped
one-way network supports the service at the lowest cost.
– Corporate network: most often companies have a centralised
structure with administration and management performed at
a central site, and manufacturing or sales performed at sites
scattered over a geographical area. Information from the remote
sites needs to be gathered at the central site for decision making,
and information from the central site (for example, relating to
task sharing) has to be distributed to the remote ones. Such an
informationflowcan be supported partially by a star-shaped oneway
VSAT network, for instance for information distribution, or
supported totally by a two-way star-shaped VSAT network.
In the first case, VSATs need to be receive-only and are less
expensive than in the latter case where interactivity is required,
as this implies VSATs equipped with both transmit and receive
equipment. Typically the cost of the transmitting equipment is
two-thirds that of an interactive VSAT.
– Interactivity between distributed sites: other companies or organisations
with a decentralised structure are more likely to comprise
many sites interacting with one another. A meshed VSAT
network using direct single hop connections from VSAT to
VSAT is hence most desirable. The other option is a two-way
star-shaped network with double hop connections from VSAT
to VSAT via the hub.
Table 1.3 summarises the above discussion.
Regulatoryaspects are also tobetakenintoaccount(see section 1.9).
1.6.1.2 Link quality and capacity
The link considered here is the link from the transmitting station
to the receiving one. Such a link may comprise several parts. For
instance a single hop link would comprise an uplink and a downlink
(Figure 1.4), a double hop link would comprise two single hop links,
one being inbound and the other outbound (Figure 1.10).
When dealing with link quality, one must refer to the quality
of a given signal. Actually, two types of signal are involved: the
modulated carrier at the input to the receiver and the baseband
signals delivered to the user terminal once the carrier has been
demodulated (Figure 1.13). The input to the receiver terminates
the overall radio frequency link from the transmitting station to the
receiving one, with its two link components, the uplink and the
downlink. The earth station interface to the user terminal terminates
the user-to-user baseband link from the output of the device generating
bits (message source) to the input of the device to which those bits
are transmitted (message sink).
The link quality of the radio frequency link is measured by the
(C/N0)T ratio at the station receiver input, where C is the received
carrier power and N0 the power spectral density of noise [MAR02
Chapter 5].
The baseband link quality is measured by the information bit error
rate (BER). It is conditioned by the Eb/N0 value at the receiver input,
where Eb (J) is the energy per information bit and N0 (WHz−1) is the
noise power spectral density. As indicated in Chapter 5, section 5.7,
the Eb/N0 ratio depends on the overall radio frequency link quality
(C/N0)T and the capacity of the link, measured by its information bit
Figure 1.14 EIRP versus G/T in a VSAT network. Curve 1: single hop from
VSAT to VSAT in a meshed network; Curve 2: double hop from VSAT to VSAT
via the hub. Increased Rb means increased link capacity
rate Rb (bs−1):
Eb
N0 =
(C/N0)T
Rb
(1.1)
Figure 1.14 indicates the general trend which relates EIRP to G/T
in a VSAT network, considering a given baseband signal quality in
terms of constant BER. EIRP designates the effective isotropic radiated
power of the transmitting equipment and G/T is the figure of merit
of the receiving equipment (see Chapter 5 for definition of the EIRP
and of the figure of merit).
As can be seen from Figure 1.14, the double hop from VSAT
to VSAT via the hub, when compared to a single hop, allows an
increased link capacity without modifying the size of the VSATs.
This option also involves a larger transmission delay.
1.6.1.3 Transmission delay
With a single hop link from VSAT to VSAT in a meshed network,
the propagation delay is about 0.25 s. With a double hop from VSAT
to VSAT via the hub, the propagation delay is twice as much, i.e.
about 0.5 s.
Double hop may be a problem for voice communications. However
it is not a severe problem for video or data transmission.
Table 1.4 summarises the above discussion. Given the EIRP and
G/T values for a VSAT, the designer can decide upon either a large
delay from VSAT to VSAT and a larger capacity or a small delay and
a lower capacity, by implementing either a star-shaped network, or
a meshed one.