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.
Sunday, December 9, 2007
VSAT NETWORK EARTH STATIONS
VSAT station
Figure 1.20 illustrates the architecture of aVSATstation. As shown in
the figure, a VSAT station is made of two separate sets of equipment:
VSAT station equipment
the outdoor unit (ODU) and the indoor unit (IDU). The outdoor unit
is the VSAT interface to the satellite, while the IDU is the interface
to the customer’s terminals or local area network (LAN).
1.7.1.1 The outdoor unit (ODU)
Figure 1.21 shows a photograph of an outdoor unit, with its antenna
and the electronics package containing the transmitting amplifier,
the low-noise receiver, the up- and down-converters and the frequency
synthesiser. The photograph in Figure 1.22 provides a closer
look at the electronics container.
For a proper specification of the ODU, as an interface to the
satellite, the following parameters are of importance:
– the transmit and receive frequency bands;
– the transmit and receive step size for adjusting the frequency
of the transmitted carrier or for tuning to the received carrier
frequency;
– the equivalent isotropic radiated power (EIRP), which determines
the performance of the radio frequency uplink. The EIRP
depends on the value of the antenna gain, and hence its size and
transmit frequency, and on the transmitting amplifier output
power (see Chapter 5, section 5.2);
– the figure of merit G/T,which determines the performance of the
radio frequency downlink. The G/T ratio depends on the value
of the antenna gain, and hence its size and receive frequency,
and on the noise temperature of the receiver (see Chapter 5,
section 5.3);
– the antenna sidelobe gain variation with off-axis angle which
controls the off-axis EIRP and G/T, hence determining the levels
of produced and received interference.
Operating temperature range, wind loading under operational
and survival conditions, rain and humidity are also to be considered.
Table 1.6 displays typical values for the ODU of a VSAT. LNA
typical noise temperature of today’s VSAT receiver is 50 K at Cband
and 120 K at Ku-band. Advances in HEMT FET technology
now make possible uncooled LNAs having noise temperatures of
35 K at C-band and 80 K at Ku-band.
1.7.1.2 The indoor unit (IDU)
The indoor unit installed at the user’s facility is shown in Figure 1.23.
In order to connect his terminals to the VSAT, the user must access
the ports installed on the rear panel of the outdoor unit, shown in
the photograph in Figure 1.24.
For a proper specification of the IDU, as an interface to the user’s
terminals or to a local area network (LAN), the following parameters
are of importance:
– number of ports;
– type of ports: mechanical, electrical, functional and procedural
interface. This is often specified by reference to a standard, such
as those mentioned in section 1.6.2 and in Appendix 3;
– port speed: this is the maximum bit rate at which data can be
exchanged between the user terminal and the VSAT indoor unit
on a given port. The actual data rate can be lower.
Coherent modulation schemes such as biphase shift keying (BPSK)
or quadrature phase shift keying (QPSK) are used. For acceptable
Table 1.6 Typical values for the ODU parts of a VSAT station
Transmit frequency band 14.0–14.5GHz (Ku-band)
5.925–6.425GHz (C-band)
Receive frequency band 10.7–12.75GHz (Ku-band)
3.625–4.2GHz (C-band)
Antenna
Type of antenna Offset, single reflector, fixed mount
Diameter 1.8–3.5m at C-band
1.2–1.8m at Ku-band
TX/RX isolation 35 dB
Voltage Standing Wave
Ratio (VSWR)
1.3:1
Polarisation Linear orthogonal at Ku-band
Circular orthogonal at C-band
Polarisation adjustment ±90◦ for linear polarised antenna
Cross polarisation isolation 30 dB on axis, 22 dB within 1 dB beamwidth
17 dB from 1◦ to 10◦ off axis
Sidelobe envelope 29 − 25 log θ
Azimuth adjustment 160 degrees continuous, with fine adjustment
Elevation travel 3 to 90 degrees
Positioning Automatic positioning optional
Tracking None
Wind speed:
operation 75 to 100km/h
survival 210km/h
Deicing Electric (optional) or passive (hydrophobic
coating)
Power amplifier
Output power 0.5W to 5W SSPA at Ku-band
3–30W SSPA at C-band
Frequency steps 100 kHz
Low noise receiver
Noise temperature 80–120K at Ku-band
35–55K at C-band
General characteristics
Effective Isotropic Radiated
Power (EIRP)
44 to 55dBW at C-band
43 to 53dBW at Ku-band
Figure of merit G/T 13 to 14 dBK−1 at C-band
19 to 23 dBK−1 at Ku-band (clear sky)
14 to 18 dBK−1 at Ku-band (99.99% of time)
Operating temperature −30◦C to +55◦C
performance, transmission rate on the carrier should be higher than
2.4 kbs−1, otherwise phase noise becomes a problem. For lower
data transmission rate values, phase shift keying is avoided and
frequency shift keying (FSK) is used instead.
1.7.2 Hub station
Figure 1.25 shows a photograph of a hub station and Figure 1.26
displays the architecture of the hub station with its equipment.
Apart from the size and the number of subsystems, there is little
functional difference between a hub and a VSAT, so that most of the
content of the above section applies here. The major difference is that
the indoor unit of a hub station interfaces to either a host computer
or to a public switched network or private lines, depending on
whether the hub is a dedicated or a shared one (see above section on
VSAT network options). Typical ODU hub station parameters are
indicated in Table 1.7.
One can note in Figure 1.26 that the hub station is equipped with a
network management system (NMS). The NMS is a mini-computer
or a work station, equipped with its an dedicated software and
displays, and used for operational and administrative functions.
This mini-computer is connected to each VSAT in the network
by means of permanent virtual circuits. Management messages
Table 1.7 Typical values for the ODU parts of a hub station
Transmit frequency band 14.0–14.5GHz (Ku-band)
5.925–6.425GHz (C-band)
Receive frequency band 10.7–12.75GHz (Ku-band)
3.625–4.2GHz (C-band)
Antenna
Type of antenna Axisymmetric dual reflector (Cassegrain)
Diameter 2 to 5m (compact hub)
5 to 8m (mediumhub)
8 to 10m (large hub)
TX/RX isolation 30 dB
Voltage Standing Wave
Ratio (VSWR)
1.25:1
Polarisation Linear orthogonal at Ku-band
Circular orthogonal at C-band
Polarisation adjustment ±90◦ for linear polarised antenna
Cross polarisation isolation 35 dB on axis
Sidelobe envelope 29 − 25 log θ
Azimuth travel 120 degrees
Elevation travel 3 to 90 degrees
Positioning 0.01◦/s
Tracking Steptrack at Ku-band if antenna larger than 4m
Wind speed:
operation 50 to 70km/h
survival 180km/h
Deicing Electric
Power amplifier
Output power 3–15W SSPA at Ku-band
5–20W SSPA at C-band
50–100 TWT at Ku-band
100–200 TWT at C-band
Power setting 0.5 dB steps
Frequency steps 100 kHz to 500 kHz
Low noise receiver
Noise temperature 80–120K at Ku-band
35–55K at C-band
Operating temperature −30◦C to +55◦C
are constantly exchanged between the NMS and the VSATs and
compete with the normal traffic for network resources.
1.7.2.1 Operational functions
Operational functions relate to the network management and provide
the capability to reconfigure the network dynamically by
adding, or deleting, VSAT stations, carriers and network interfaces.
Operational functions also include monitoring and controlling the
performance and status of the hub and each VSAT station, and all
associated data ports of the network. This entails operational management
tools which provide real-time assignment and connectivity
of VSATs, and management and control of new installations and
configurations.
The network control software allows automatic dynamic allocation
of capacity to VSATs with bursty interactive traffic and
to VSATs that will occasionally be used for stream traffic (see
Chapter 4, section 4.3). No operator intervention is required to effect
this temporary capacity reallocation.
The NMS notifies the operator in the case of capacity saturation,
which prevents more VSAT users from entering the service. The
NMS also handles all aspects related to alarm and failure diagnosis.
In particular, in case of any power interruptions at the VSAT
stations, the NMS downloads all the relevant software and system
parameters for operation restart.
1.7.2.2 Administrative functions
Administrative functions deal with inventory of equipment, records
of network usage, security and billing.
The NMS keeps an account of the VSAT stations installed and
operated, the equipment configuration within the hub and each
VSAT station, and the port configuration of each network interface.
This information is available on request by the operator, along with
statistical information on traffic, number of failures, average time
of data transmission delay, etc. The information can be analysed
and printed on a daily, weekly or monthly basis as well as being
stored on magnetic tape for future reference. It forms the basis
for traffic and trend performance analysis, cost distribution based
upon usage, etc.
The above long and diverse list of functions to be performed by
the NMS shows its important role for the network. Actually, the
adequacy of the NMS’s response to the user’s needs makes the
Figure 1.20 illustrates the architecture of aVSATstation. As shown in
the figure, a VSAT station is made of two separate sets of equipment:
VSAT station equipment
the outdoor unit (ODU) and the indoor unit (IDU). The outdoor unit
is the VSAT interface to the satellite, while the IDU is the interface
to the customer’s terminals or local area network (LAN).
1.7.1.1 The outdoor unit (ODU)
Figure 1.21 shows a photograph of an outdoor unit, with its antenna
and the electronics package containing the transmitting amplifier,
the low-noise receiver, the up- and down-converters and the frequency
synthesiser. The photograph in Figure 1.22 provides a closer
look at the electronics container.
For a proper specification of the ODU, as an interface to the
satellite, the following parameters are of importance:
– the transmit and receive frequency bands;
– the transmit and receive step size for adjusting the frequency
of the transmitted carrier or for tuning to the received carrier
frequency;
– the equivalent isotropic radiated power (EIRP), which determines
the performance of the radio frequency uplink. The EIRP
depends on the value of the antenna gain, and hence its size and
transmit frequency, and on the transmitting amplifier output
power (see Chapter 5, section 5.2);
– the figure of merit G/T,which determines the performance of the
radio frequency downlink. The G/T ratio depends on the value
of the antenna gain, and hence its size and receive frequency,
and on the noise temperature of the receiver (see Chapter 5,
section 5.3);
– the antenna sidelobe gain variation with off-axis angle which
controls the off-axis EIRP and G/T, hence determining the levels
of produced and received interference.
Operating temperature range, wind loading under operational
and survival conditions, rain and humidity are also to be considered.
Table 1.6 displays typical values for the ODU of a VSAT. LNA
typical noise temperature of today’s VSAT receiver is 50 K at Cband
and 120 K at Ku-band. Advances in HEMT FET technology
now make possible uncooled LNAs having noise temperatures of
35 K at C-band and 80 K at Ku-band.
1.7.1.2 The indoor unit (IDU)
The indoor unit installed at the user’s facility is shown in Figure 1.23.
In order to connect his terminals to the VSAT, the user must access
the ports installed on the rear panel of the outdoor unit, shown in
the photograph in Figure 1.24.
For a proper specification of the IDU, as an interface to the user’s
terminals or to a local area network (LAN), the following parameters
are of importance:
– number of ports;
– type of ports: mechanical, electrical, functional and procedural
interface. This is often specified by reference to a standard, such
as those mentioned in section 1.6.2 and in Appendix 3;
– port speed: this is the maximum bit rate at which data can be
exchanged between the user terminal and the VSAT indoor unit
on a given port. The actual data rate can be lower.
Coherent modulation schemes such as biphase shift keying (BPSK)
or quadrature phase shift keying (QPSK) are used. For acceptable
Table 1.6 Typical values for the ODU parts of a VSAT station
Transmit frequency band 14.0–14.5GHz (Ku-band)
5.925–6.425GHz (C-band)
Receive frequency band 10.7–12.75GHz (Ku-band)
3.625–4.2GHz (C-band)
Antenna
Type of antenna Offset, single reflector, fixed mount
Diameter 1.8–3.5m at C-band
1.2–1.8m at Ku-band
TX/RX isolation 35 dB
Voltage Standing Wave
Ratio (VSWR)
1.3:1
Polarisation Linear orthogonal at Ku-band
Circular orthogonal at C-band
Polarisation adjustment ±90◦ for linear polarised antenna
Cross polarisation isolation 30 dB on axis, 22 dB within 1 dB beamwidth
17 dB from 1◦ to 10◦ off axis
Sidelobe envelope 29 − 25 log θ
Azimuth adjustment 160 degrees continuous, with fine adjustment
Elevation travel 3 to 90 degrees
Positioning Automatic positioning optional
Tracking None
Wind speed:
operation 75 to 100km/h
survival 210km/h
Deicing Electric (optional) or passive (hydrophobic
coating)
Power amplifier
Output power 0.5W to 5W SSPA at Ku-band
3–30W SSPA at C-band
Frequency steps 100 kHz
Low noise receiver
Noise temperature 80–120K at Ku-band
35–55K at C-band
General characteristics
Effective Isotropic Radiated
Power (EIRP)
44 to 55dBW at C-band
43 to 53dBW at Ku-band
Figure of merit G/T 13 to 14 dBK−1 at C-band
19 to 23 dBK−1 at Ku-band (clear sky)
14 to 18 dBK−1 at Ku-band (99.99% of time)
Operating temperature −30◦C to +55◦C
performance, transmission rate on the carrier should be higher than
2.4 kbs−1, otherwise phase noise becomes a problem. For lower
data transmission rate values, phase shift keying is avoided and
frequency shift keying (FSK) is used instead.
1.7.2 Hub station
Figure 1.25 shows a photograph of a hub station and Figure 1.26
displays the architecture of the hub station with its equipment.
Apart from the size and the number of subsystems, there is little
functional difference between a hub and a VSAT, so that most of the
content of the above section applies here. The major difference is that
the indoor unit of a hub station interfaces to either a host computer
or to a public switched network or private lines, depending on
whether the hub is a dedicated or a shared one (see above section on
VSAT network options). Typical ODU hub station parameters are
indicated in Table 1.7.
One can note in Figure 1.26 that the hub station is equipped with a
network management system (NMS). The NMS is a mini-computer
or a work station, equipped with its an dedicated software and
displays, and used for operational and administrative functions.
This mini-computer is connected to each VSAT in the network
by means of permanent virtual circuits. Management messages
Table 1.7 Typical values for the ODU parts of a hub station
Transmit frequency band 14.0–14.5GHz (Ku-band)
5.925–6.425GHz (C-band)
Receive frequency band 10.7–12.75GHz (Ku-band)
3.625–4.2GHz (C-band)
Antenna
Type of antenna Axisymmetric dual reflector (Cassegrain)
Diameter 2 to 5m (compact hub)
5 to 8m (mediumhub)
8 to 10m (large hub)
TX/RX isolation 30 dB
Voltage Standing Wave
Ratio (VSWR)
1.25:1
Polarisation Linear orthogonal at Ku-band
Circular orthogonal at C-band
Polarisation adjustment ±90◦ for linear polarised antenna
Cross polarisation isolation 35 dB on axis
Sidelobe envelope 29 − 25 log θ
Azimuth travel 120 degrees
Elevation travel 3 to 90 degrees
Positioning 0.01◦/s
Tracking Steptrack at Ku-band if antenna larger than 4m
Wind speed:
operation 50 to 70km/h
survival 180km/h
Deicing Electric
Power amplifier
Output power 3–15W SSPA at Ku-band
5–20W SSPA at C-band
50–100 TWT at Ku-band
100–200 TWT at C-band
Power setting 0.5 dB steps
Frequency steps 100 kHz to 500 kHz
Low noise receiver
Noise temperature 80–120K at Ku-band
35–55K at C-band
Operating temperature −30◦C to +55◦C
are constantly exchanged between the NMS and the VSATs and
compete with the normal traffic for network resources.
1.7.2.1 Operational functions
Operational functions relate to the network management and provide
the capability to reconfigure the network dynamically by
adding, or deleting, VSAT stations, carriers and network interfaces.
Operational functions also include monitoring and controlling the
performance and status of the hub and each VSAT station, and all
associated data ports of the network. This entails operational management
tools which provide real-time assignment and connectivity
of VSATs, and management and control of new installations and
configurations.
The network control software allows automatic dynamic allocation
of capacity to VSATs with bursty interactive traffic and
to VSATs that will occasionally be used for stream traffic (see
Chapter 4, section 4.3). No operator intervention is required to effect
this temporary capacity reallocation.
The NMS notifies the operator in the case of capacity saturation,
which prevents more VSAT users from entering the service. The
NMS also handles all aspects related to alarm and failure diagnosis.
In particular, in case of any power interruptions at the VSAT
stations, the NMS downloads all the relevant software and system
parameters for operation restart.
1.7.2.2 Administrative functions
Administrative functions deal with inventory of equipment, records
of network usage, security and billing.
The NMS keeps an account of the VSAT stations installed and
operated, the equipment configuration within the hub and each
VSAT station, and the port configuration of each network interface.
This information is available on request by the operator, along with
statistical information on traffic, number of failures, average time
of data transmission delay, etc. The information can be analysed
and printed on a daily, weekly or monthly basis as well as being
stored on magnetic tape for future reference. It forms the basis
for traffic and trend performance analysis, cost distribution based
upon usage, etc.
The above long and diverse list of functions to be performed by
the NMS shows its important role for the network. Actually, the
adequacy of the NMS’s response to the user’s needs makes the
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.
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.
VSAT NETWORK APPLICATIONS AND TYPES
VSAT NETWORK APPLICATIONS AND TYPES
OF TRAFFIC
VSAT networks have both civilian and military applications. These
will now be presented.
1.4.1 Civilian VSAT networks
1.4.1.1 Types of service
As mentioned in the previous section, VSAT networks can be configured
as one-way or two-way networks. Table 1.1 gives examples
of services supported by VSAT networks according to these two
classes.
It can be noted that most of the services supported by two-way
VSAT networks deal with interactive data traffic, where the user
terminals are most often personal computers. The most notable
exceptions are voice communications and satellite news gathering.
Voice communications on a VSAT network means telephony with
possibly longer delays than those incurred on terrestrial lines, as
a result of the long satellite path. Telephony services imply full
connectivity, and delays are typically 0.25 s or 0.50 s depending on
the selected network configuration, as mentioned above.
Satellite news gathering (SNG) can be viewed as a temporary
network using transportable VSATs, sometimes called ‘fly-away’
stations, which are transported by car or aircraft and set up at a
location where news reporters can transmit video signals to a hub
Table 1.1 Examples of services supported by VSAT networks
ONE-WAY VSAT NETWORKS
Stock market and other news broadcasting
Training or continuing education from a distance
Distribute financial trends and documents
Introduce new products at geographically dispersed locations
Distribute video or TV programmes
In-store music and advertising
TWO-WAY VSAT NETWORKS
Interactive computer transactions
Low rate video conferencing
Database inquiries
Bank transactions, automatic teller machines, point of sale
Reservation systems
Sales monitoring/Inventory control
Distributed remote process control and telemetry
Medical data/Image transfer
Satellite news gathering
Video teleconferencing
Voice communications
located near the company’s studio. Of course the service could be
considered as inbound only, if it were not for the need to check
the uplink from the remote site, and to be in touch by telephone
with the staff at the studio. As fly-away VSATs are constantly
transported, assembled and disassembled, they must be robust,
lightweight and easy to install. Today they weigh typically 100 kg
and can be installed in less than 20 minutes. Figure 1.11 shows a
picture of a fly-away VSAT station.
1.4.1.2 Types of traffic
Depending on the service, the traffic flow between the hub and the
VSATs may have different characteristics and requirements.
Data transfer or broadcasting, which belongs to the category of oneway
services, typically displays file transfers of one to one hundred
megabytes of data. This kind of service is not delay sensitive, but
requires a high integrity of the data which are transferred. Examples
of applications are computer download and distribution of data to
remote sites.
Interactive data is a two-way service corresponding to several
transactions per minute and per terminal of single packets 50 to
250 bytes long on both inbound and outbound links. The required
response time is typically a few seconds. Examples of applications
are bank transactions and electronic funds transfer at point of sale.
Inquiry/response is a two-way service corresponding to several
transactions per minute and terminal. Inbound packets (typically
30–100 bytes) are shorter than outbound packets (typically
500–2000 bytes). The required response time is typically a few seconds.
Examples of applications are airline or hotel reservations and
database enquiries.
Supervisory control and data acquisition (SCADA) is a two-way
service corresponding to one transaction per second or minute
per terminal. Inbound packets (typically 100 bytes) are longer than
outbound packets (typically 10 bytes). The required response time
ranges from a few seconds to a few minutes. What is most important
is the high data security level and the low power consumption of the
terminal. Examples of applications are control and monitoring of
pipelines, offshore platforms, electric utilities and water resources.
Table 1.2 summarises the above discussion.
Military VSAT networks
VSAT networks have been adopted by many military forces in the
world. Indeed the inherent flexibility in the deployment of VSATs
makes them a valuable means of installing temporary communications
links between small units in the battlefield and headquarters
located near the hub. Moreover, the topology of a star-shaped network
fits well into the natural information flow between field units
and command base. Frequency bands are at X-band, with uplinks
in the 7.9–8.4 GHz band and downlinks in the 7.25–7.75 GHz band.
The military use VSAT must be a small, low weight, low power
station that is easy to operate under battlefield conditions. As
an example, the manpack station developed by the UK Defence
Research Agency (DRA) for its Milpico VSAT military network is
equipped with a 45 cm antenna, weighs less than 17 kg and can be
set up within 90 seconds. It supports data and vocoded voice at
2.4 kbs−1. In order to do so, the hub stations need to be equipped
with antennas as large as 14 m. Another key requirement is low
probability of detection by hostile interceptors. Spread spectrum
techniques are largely used [EVA99, Chapter 23].
1.5 VSAT NETWORKS: INVOLVED PARTIES
The applications of VSAT networks identified in the previous section
clearly indicate that VSAT technology is appropriate to business or
military applications. Reasons for this are the inherent flexibility
of VSAT technology, as mentioned in section 1.1, cost savings and
reliability, as will be discussed in section 3.3.
Which are the involved parties as far as corporate communications
are concerned?
– Theuser is most often a company employee using office communication
terminals such as personal computers, telephone sets or
fax machines. On other occasions the terminal is transportable,
as with satellite news gathering (SNG). Here the user is mostly
interested in transmitting video to the company studio. The
terminal may be fixed but not located in an office, as with
supervisory control and data acquisition (SCADA) applications.
– The VSAT network operator may be the user’s company itself,
if the company owns the network, or it may be a telecom
company (in many countries it is the national public telecom
operator
is then a customer to the network provider and/or the
equipment provider.
– The VSAT network provider has the technical ability to dimension
and install the network. It elaborates the network management
system (NMS) and designs the corresponding software. Its inputs
are the customer’s needs, and its customers are network operators.
The network provider may be a private company or a
national telecom operator.
– The equipment provider sells the VSATs and/or the hub which
it manufactures. It may be the network provider or a different
party.
For the VSAT network to work, some satellite capacity must be
provided. The satellite may be owned by the user’s company but
this is a rare example of ‘vertical integration’, and most often the
satellite is operated by a different party. This party may be a national
or international private satellite operator.
The above parties are those involved in the contractual matters.
Other parties are on the regulatory side and their involvement will
be first presented in section 1.9.
Figure 1.12 summarises the above discussion. The terminology
will be used throughout the book and therefore Figure 1.12 can
serve as a convenient reference.
OF TRAFFIC
VSAT networks have both civilian and military applications. These
will now be presented.
1.4.1 Civilian VSAT networks
1.4.1.1 Types of service
As mentioned in the previous section, VSAT networks can be configured
as one-way or two-way networks. Table 1.1 gives examples
of services supported by VSAT networks according to these two
classes.
It can be noted that most of the services supported by two-way
VSAT networks deal with interactive data traffic, where the user
terminals are most often personal computers. The most notable
exceptions are voice communications and satellite news gathering.
Voice communications on a VSAT network means telephony with
possibly longer delays than those incurred on terrestrial lines, as
a result of the long satellite path. Telephony services imply full
connectivity, and delays are typically 0.25 s or 0.50 s depending on
the selected network configuration, as mentioned above.
Satellite news gathering (SNG) can be viewed as a temporary
network using transportable VSATs, sometimes called ‘fly-away’
stations, which are transported by car or aircraft and set up at a
location where news reporters can transmit video signals to a hub
Table 1.1 Examples of services supported by VSAT networks
ONE-WAY VSAT NETWORKS
Stock market and other news broadcasting
Training or continuing education from a distance
Distribute financial trends and documents
Introduce new products at geographically dispersed locations
Distribute video or TV programmes
In-store music and advertising
TWO-WAY VSAT NETWORKS
Interactive computer transactions
Low rate video conferencing
Database inquiries
Bank transactions, automatic teller machines, point of sale
Reservation systems
Sales monitoring/Inventory control
Distributed remote process control and telemetry
Medical data/Image transfer
Satellite news gathering
Video teleconferencing
Voice communications
located near the company’s studio. Of course the service could be
considered as inbound only, if it were not for the need to check
the uplink from the remote site, and to be in touch by telephone
with the staff at the studio. As fly-away VSATs are constantly
transported, assembled and disassembled, they must be robust,
lightweight and easy to install. Today they weigh typically 100 kg
and can be installed in less than 20 minutes. Figure 1.11 shows a
picture of a fly-away VSAT station.
1.4.1.2 Types of traffic
Depending on the service, the traffic flow between the hub and the
VSATs may have different characteristics and requirements.
Data transfer or broadcasting, which belongs to the category of oneway
services, typically displays file transfers of one to one hundred
megabytes of data. This kind of service is not delay sensitive, but
requires a high integrity of the data which are transferred. Examples
of applications are computer download and distribution of data to
remote sites.
Interactive data is a two-way service corresponding to several
transactions per minute and per terminal of single packets 50 to
250 bytes long on both inbound and outbound links. The required
response time is typically a few seconds. Examples of applications
are bank transactions and electronic funds transfer at point of sale.
Inquiry/response is a two-way service corresponding to several
transactions per minute and terminal. Inbound packets (typically
30–100 bytes) are shorter than outbound packets (typically
500–2000 bytes). The required response time is typically a few seconds.
Examples of applications are airline or hotel reservations and
database enquiries.
Supervisory control and data acquisition (SCADA) is a two-way
service corresponding to one transaction per second or minute
per terminal. Inbound packets (typically 100 bytes) are longer than
outbound packets (typically 10 bytes). The required response time
ranges from a few seconds to a few minutes. What is most important
is the high data security level and the low power consumption of the
terminal. Examples of applications are control and monitoring of
pipelines, offshore platforms, electric utilities and water resources.
Table 1.2 summarises the above discussion.
Military VSAT networks
VSAT networks have been adopted by many military forces in the
world. Indeed the inherent flexibility in the deployment of VSATs
makes them a valuable means of installing temporary communications
links between small units in the battlefield and headquarters
located near the hub. Moreover, the topology of a star-shaped network
fits well into the natural information flow between field units
and command base. Frequency bands are at X-band, with uplinks
in the 7.9–8.4 GHz band and downlinks in the 7.25–7.75 GHz band.
The military use VSAT must be a small, low weight, low power
station that is easy to operate under battlefield conditions. As
an example, the manpack station developed by the UK Defence
Research Agency (DRA) for its Milpico VSAT military network is
equipped with a 45 cm antenna, weighs less than 17 kg and can be
set up within 90 seconds. It supports data and vocoded voice at
2.4 kbs−1. In order to do so, the hub stations need to be equipped
with antennas as large as 14 m. Another key requirement is low
probability of detection by hostile interceptors. Spread spectrum
techniques are largely used [EVA99, Chapter 23].
1.5 VSAT NETWORKS: INVOLVED PARTIES
The applications of VSAT networks identified in the previous section
clearly indicate that VSAT technology is appropriate to business or
military applications. Reasons for this are the inherent flexibility
of VSAT technology, as mentioned in section 1.1, cost savings and
reliability, as will be discussed in section 3.3.
Which are the involved parties as far as corporate communications
are concerned?
– Theuser is most often a company employee using office communication
terminals such as personal computers, telephone sets or
fax machines. On other occasions the terminal is transportable,
as with satellite news gathering (SNG). Here the user is mostly
interested in transmitting video to the company studio. The
terminal may be fixed but not located in an office, as with
supervisory control and data acquisition (SCADA) applications.
– The VSAT network operator may be the user’s company itself,
if the company owns the network, or it may be a telecom
company (in many countries it is the national public telecom
operator
is then a customer to the network provider and/or the
equipment provider.
– The VSAT network provider has the technical ability to dimension
and install the network. It elaborates the network management
system (NMS) and designs the corresponding software. Its inputs
are the customer’s needs, and its customers are network operators.
The network provider may be a private company or a
national telecom operator.
– The equipment provider sells the VSATs and/or the hub which
it manufactures. It may be the network provider or a different
party.
For the VSAT network to work, some satellite capacity must be
provided. The satellite may be owned by the user’s company but
this is a rare example of ‘vertical integration’, and most often the
satellite is operated by a different party. This party may be a national
or international private satellite operator.
The above parties are those involved in the contractual matters.
Other parties are on the regulatory side and their involvement will
be first presented in section 1.9.
Figure 1.12 summarises the above discussion. The terminology
will be used throughout the book and therefore Figure 1.12 can
serve as a convenient reference.
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
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
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