Introduction
Low cost business terminals with small antennas
(generally less than 2 metres in diameter) are often termed Very Small
Aperture Terminals (VSATs). These are usually perceived as being two way
data terminals, though strictly speaking many of the systems used for data
broadcast are really one-way VSATs. Taking the USA as an example,
approximately half of all installed VSATs are only used for one way data
links.
ETSI take a different definition for a VSAT as a one
or two-way terminal used in a star, mesh or point to point network. Antenna
size is restricted to being less than or equal to 3.8 m at Ku band and 7.8 m
at C band.
A more general definition is that a network is a VSAT
network if it consists of a large high performance hub earth station (with
an antenna of up to 9 m in diameter) and a large number of smaller, lower
performance terminals. Being completely general, these small terminals can
be receive only, transmit only or transmit/receive. Even this definition is
not universal. Meshed VSAT networks exist in which all terminals have the
same size and performance.
As terminal technology advances, the size of the
antenna required to achieve a particular link quality (bit error rate)
decreases. A class of terminals smaller than VSATs is now available; these
are termed Ultra Small Aperture Terminals (USATs). For most practical
purposes, USATs are just VSATs with smaller antennas. It must always be
remembered, however, that as antenna size decreases, the antenna beam widens
and that a point is rapidly reached when there is no further advantage in
decreasing antenna size because of increased interference with other
systems. The practical current lower limit on antenna size is 55 cm
diameter.
Typical applications for interactive VSAT networks
are:
-
computer communications;
-
reservation systems;
-
database enquiries;
-
billing systems;
-
file transfers;
-
electronic mail;
-
video conferencing;
-
point of sale transactions;
-
credit checks and credit card verification;
-
stock control and management.
The most common VSAT configuration is the TDM/TDMA
star network. These have a high bit rate outbound carrier (TDM) from the hub
to the remote earth stations, and one or more low or medium bit rate Time
Division Multiple Access (TDMA) inbound carriers.
With its star configuration network architecture,
interactive VSAT technology is appropriate for any organisation with
centralised management and data processing.
This configuration has been developed to minimise
overall lifetime costs for the complete network including satellite
transmission costs. The use of a single high performance hub allows the use
of low cost remote VSAT terminals and optimises use of satellite capacity.
Even so, in most VSAT networks, the cost of the VSAT terminals usually far
exceeds the cost of the hub (typically a VSAT terminal is 0.1 to 0.2% of the
price of the hub).
In a typical VSAT network, remote user sites have a
number of personal computers, dumb terminals and printers connected to the
VSAT terminal which connects them to a centralised host computer either at
the organisation's head office or data processing centre. Data sent to the
VSAT terminal from the DTEs is buffered and transmitted to the hub in
packets.
Interactive VSAT
Networks
The principle characteristics of an interactive VSAT
network are:
-
Remote user sites have several low bit rate data terminal equipments (DTEs) operating at 1.2 to 9.6 kb/s. These are connected through the VSAT network to a centralised host processor. The DTEs are connected to the host through an X.25 Packet Assembler/Dissembler (PAD) or through a conventional or statistical multiplexer which concentrates the traffic.
-
The amount of data transferred in each transaction is relatively small, typically between 300 and 105 bits. Interactive VSATs are not usually used for batch file transfer (107 to 1011 bits per transaction) unless the transmission plan is specifically designed to carry large files.
-
Each VSAT terminal only operates with a low duty cycle, i.e. with only a relatively small number of transactions in the peak busy hour compared to the total available capacity.
-
A large number of VSAT terminals (10 to 10000) share the same communications link using random access.
-
Connections between remote VSAT terminals require a double hop through the hub and are rarely used.
VSAT networks are designed to be flexible and to
evolve with user needs. VSAT terminals are controlled by microprocessors and
can generally be reprogrammed remotely using downloaded software from the
hub. If additional interfaces or capacity are required this can usually be
provided by adding or replacing cards in the VSAT terminal.
Three different transmission schemes are used for
interactive hubbed VSAT networks:
·
TDM/TDMA
·
Demand
Assigned SCPC
·
CDMA
Of these TDM/TDMA is by far the dominant technique
with only CDMA being used to a small extent. Demand assigned SCPC has been
virtually abandoned as a transmission scheme for the present.
It is also common for VSAT systems to support one-way
TV transmission from the host to the remote stations.
Two-way, 2 Mb/s transmissions can also be supported by
some VSAT systems.
.
Comparison of Interactive VSAT Network
Characteristics
|
|||||
Supplier
|
Hardware
|
Type
|
Inbound Data Rate (kb/s)
|
Outbound Data Rate (kb/s)
|
Modulation
|
Gilat/Spacenet
|
Skystar Advantage
|
TDM/TDMA
|
9.6, 19.2, 38.4, 56, 64, 76.8, 128
|
64, 128, 256, 512, 1024, 2048
|
DPSK or MSK
|
Hughes
|
ISBN/PES
|
TDM/TDMA
|
64, 128, 256
|
128, 512
|
BPSK
|
Indra Espacio
|
Arcanet
|
CDMA
|
|
|
|
NEC
|
Nextar V
|
TDM/TDMA
|
64, 128, 256
|
64, 128, 256, 512, 768, 1536, 2048
|
BPSK/QPSK
|
STM
|
X.Star
|
TDM/TDMA
|
96, 192, 384
|
64, 128, 256, 512, 1024, 1544
|
BPSK
|
TSAT
|
TSAT 2000
|
TDM/TDMA
|
0.3, 0.6, 1.2, 2.4, 4.8
|
0.3, 0.6, 1.2, 2.4, 4.8
|
4FSK, 2-4PSK
|
TSAT
|
TSAT 2100
|
TDM/TDMA
|
2.4 - 9.6, 14.4, 16.8
|
2.4 - 9.6, 14.4, 16.8
|
QPSK
|
ViaSat
|
Sky Relay
|
TDM/TDMA
|
|
|
|
To make VSAT networks more affordable it
is possible to share the hub between several users, thereby
spreading the cost. In this case the hub is usually owned by a
service provider who retains overall control of the network and
who manages the hub itself.
Each user, however, is allocated his own
time slots or carriers and can so operate his own private
network using the shared hub facility without any loss of
privacy. The operation and management of these sub networks is
performed by the users themselves completely independently of
the service supplier.
VSAT Shared Hub Network Configuration
In this configuration, each user has his
own "mini-hub" which is much smaller and simpler, and hence
cheaper, than a conventional hub. An approximate price for a
mini-hub is 250 k Euro. The antenna diameter is typically only
2.4 m. Each user organization has complete control over his own
communications. Overall management of the complete network is
provided by the service supplier who has a "super hub" which
provides network supervision and diagnostic support.
VSAT
Mini Hub Network Configuration
Current interactive VSAT networks
generally have distributed, rather than a centralised,
network management. Multiple points of control and
intelligent operator interfaces are common features. The
network manager not only has the ability to perform
diagnostics on the network, but can also reconfigure the
network from his own console. Where multiple consoles are
available, the network can be configured, monitored and
operated either locally or remotely. In addition, many VSAT
network management systems have interfaces available for
working with other vendor's network management systems such
as IBM's Netview and DEC's EMA.
Many VSAT systems can be configured to
support virtual subnetworks within a VSAT network. These can
be set up to give closed groups of users their own private
networks.
This facility allows groups of users
to have complete control over their own subnetwork and to be
able to manage it independently of the main network.
Virtual subnetworks are exploited by
many VSAT service vendors in "shared hub" networks. Within a
single organisation, however, virtual subnetworks can be
used, for example, for each division in the organisation, so
that communications costs can be accurately charged.
All the established interactive hubbed
VSAT systems use TDM/TDMA access as the primary access
technique (TDM on the outbounds and TDMA on the inbounds).
Network Configuration
TDM/TDMA
VSAT Network
Signal Types and Characteristics
The outbound data stream from the hub
is transmitted at a relatively high data rate (typically 56
to 1024 kb/s) using TDM. The bit stream consists of a
synchronisation word followed by a series of messages in
time slots directed towards individual VSAT terminals.
Broadcast messages to all remote VSAT terminals are also
generally permitted.
Outbounds are transmitted continuously
(i.e. duty cycle 100%) as a TDM stream. The number of
outbounds per network is determined by the traffic
statistics, packet length as well as the outbound data rate.
The outbounds for a network are
generally grouped together at either the top or the bottom
of the leased bandwidth.
The inbound carrier is often accessed
using ALOHA or Slotted ALOHA. If a higher capacity is
required, a separate channel can be dedicated to ALOHA or
Slotted ALOHA access requests and a demand assigned TDMA
access scheme established.
Inbound slotted ALOHA carriers
information rates are usually between 2.4 and 16 kb/s.
Inbound TDMA or SCPC carriers used for file transfer usually
have information data rates between 56 kb/s and 256 kb/s.
All carriers are BPSK or QPSK modulated and have rate 1/2 or
2/3 Forward Error Correction (FEC). This ensures that bit
error rates are low (typically 10-6 or 10-7
which is comparable to ISDN).
Remote terminals transmit in TDMA
bursts in either a pre-assigned inbound channel slot or in
any inbound channel slot depending on the manufacturer.
Several different inbound TDMA access
systems are used depending on traffic characteristics and
the manufacturer.
In a shared hub network, individual
customers are often, but not always, allocated one or more
dedicated outbounds and several inbounds.
If the traffic mix is a combination of
short interactive messages and long file transfers it is
often worthwhile to use a technique called Adaptive ALOHA/TDMA.
VSATs which have large blocks of data to transmit request
dedicated TDMA time slots and use TDMA. The other VSAT
terminals in the network use slotted ALOHA and avoid the
assigned time slots. Alternatively, dedicated SCPC carriers
can be temporarily assigned for file transfer.
Typical
Interactive Hubbed VSAT Network Spectrum
Typical Interactive Hubbed VSAT Frame and
Packet Format
Each TDM outbound carries a continuously
transmitted bitstream which is divided into frames.
The start of a frame is denoted by a
framing packet contain a unique word (UW) and a control word (CNTRL)
which, together, provide framing, timing and control
information.
The rest of the frame is filled by
(generally) fixed length data packets which each contain:
-
F preamble
-
HDR header - giving IDU address and control information
-
FCS frame check sequence
-
F postamble
Outbound data packets typically contain
between 50 and 250 bytes in transactional networks.
Each TDMA inbound contains frames which
are synchronised to the outbound frames. Each inbound frame is
divided into slots. Individual IDUs transmit in these slots in a
manner depending on the access modes available to the particular
system and how the network has been set up.
Each inbound packet consists of:
-
F preamble
-
HDR header - giving IDU address and control information
-
FCS frame check sequence
-
F postamble
Inbound data packets typically contain
between 50 and 250 bytes in transactional networks.
The main inbound transmission modes used
are:
Aloha,
in which an IDU can transmit data packets at any time in a
particular inbound frequency slot. Transmissions in any
particular frequency slot are intermittent with a peak traffic
duty cycle of 10 to 15%.
Slotted Aloha,
in which an IDU can transmit data packets in any slot (or any of
a predetermined number of slots) in a particular inbound
frequency slot. Transmissions in any particular frequency slot
are intermittent with a peak traffic duty cycle of 25 to 30%.
Fixed
Assignment,
in which specific time slots in an inbound frequency slot are
permanently, or for the duration of a particular transmission,
assigned to a particular IDU. This is often used for batch
transmission and for telephony. Transmissions in any particular
frequency slot are intermittent but can have a peak traffic duty
cycle of 100% if that particular inbound is carrying telephony
traffic or several batch file transfers from different IDUs.
Dynamic
Assignment,
in which time slots in an inbound frequency slot are dynamically
assigned to a particular IDU in line with ongoing traffic
demands. Transmissions in any particular frequency slot are
intermittent with a peak traffic duty cycle of from 25 to 30% to
approaching 100%, depending on the traffic mix.
Most interactive hubbed VSATs now have
protocol stacks which map, at least notionally, onto the OSI
stack.
Network layer spoofing is provided by many
VSATs to minimise the impact of the data layer protocol and,
particularly, the satellite transmission delay, on the
throughput of the satellite link.
When the network is established, or when
additional remote terminals are added to the network, remote
terminal addresses and characteristics (i.e. card fits and port
addresses) are entered into a network database which is used as
a routing table by the operational system. This database
establishes permanent virtual circuits between ports at the user
interface of the hub and the ports at the user interfaces of the
remote terminals. In those products which permit the dedication
of the assignment of capacity on request, or dynamic variable
assignment, the database also establishes permanent virtual
circuits between the IDU controllers at the remote terminals and
the NCC.
This arrangement allows the normal
transactional traffic carried by the network to be switched
without an individual call set up procedure.
A packet sent by a particular IDU carries
addressing information identifying both the source and
destination. This allows the hub switch to route the packet to
the correct user interface port without additional signalling
traffic.
This same procedure is used for intra
network signalling to set up assignments for the temporary or
permanent assignment of channels to a particular IDU port/hub
port pair (for example, telephony or batch data transfers). Call
set up information is sent as a transactional data packet as
described above, except that the destination address at the hub
is the NCC.
The hub station is usually a relatively
large, high performance earth station with an antenna diameter
of anything between 6 and 9m. The hub consists of a control
centre which manages the network as well as microwave equipment,
including an outdoor antenna, for the transmission and reception
of signals. A substantial amount of interfacing equipment
necessary to support the wide range of terrestrial interfaces
required at the hub completes the installation. This equipment
is usually mounted in several racks.
VSAT Hub Station Block Diagram
Hub stations are expensive and typically
cost upwards of 1 MEuro. Hub stations can be shared between
several networks, resulting in a sharing of costs. Two principal
options for network implementation can be adopted. Firstly, some
very large users will wish to purchase their own dedicated VSAT
networks including a hub. Other users will choose to buy or
lease the user terminals and to lease access to a hub which will
be owned by the system operator.
The hub station consists of several main
subsystems, except for the antenna these are usually fully
redundant with automatic switchover in the event of failure:
-
A switch (generally a packet switch) which controls routing between host ports and the modulator and demodulator ports, as well as adding and reading header address information which controls routing to and from individual IDUs.
-
One or more modulators which modulate the outbound carriers with the TDM stream generated by the switch (each outbound carrier has a dedicated modulator).
-
A bank of demodulators which receive the inbound carriers and extract the data packets and feed them to the switch.
-
An RFT (radio frequency terminal), which contains:
-
The transmit subsystem containing upconverters which change the 70 or 140 MHz IF to the required transmit frequency before feeding it to the High Power Amplifier (HPA). If the hub only uses a single carrier for data it is possible to use a solid state power amplifier (SSPA), otherwise a more powerful Travelling Wave Tube Amplifier (TWTA) must generally be used. Uplink power control is often provided so that the power transmitted by the hub can be increased to compensate for high link attenuation due to precipitation in bad weather.
-
The receive subsystem consisting of a Low Noise Amplifier (LNA) with a noise temperature usually between 150 and 175° K (Ku band) and a downconverter to change the received frequency to the IF frequency (70 or 140 MHz).
-
The antenna subsystem consisting of a large antenna (6 to 9 m in diameter) on a mount with a tracking system which allows the antenna to follow the satellite as it moves very slightly in the sky. A feed horn is fitted at the focus of the dish to collect the received signals from the antenna and to feed the transmit signals to it.
-
-
An NCC (network control centre) which controls and monitors the operation of the hub and the IDUs in the network.
-
The primary power subsystem which guarantees the quality and continuity of the power supply for the hub. It typically contains power switching, an uninterruptable power supply with a large battery bank and a diesel generator.
The hub is usually very expensive, costing
typically between 0.5 million Euro to 2 million Euro, depending
on the configuration and manufacturer. This cost excluded the
price of the RFT, antenna and civil works.
A few small, simple VSAT systems intended
for very low data rate applications such as SCADA (for example
the TSAT) have low cost hubs, costing of the order of 25,000 to
50,000 Euro.
In contrast to the hub station, the remote
terminals are much simpler. To minimise total system costs, VSAT
networks are designed to have a single expensive hub and a large
number of much smaller remote terminals.
VSAT Remote Terminal Block Diagram
Remote terminals consist of:
-
A dish antenna, generally 0.55 to 2.4 m in diameter (though larger dishes are sometimes required), which can be wall, roof or ground mounted.
-
The antennas are usually offset-fed parabolic dishes, although larger dishes tend to be centre-fed. Recently, to gain higher performance (in particular sidelobe performance) dual reflector, Gregorian designs have started to become common. Several different materials are used for the dishes with spun aluminium, steel, fibreglass and reinforced plastic being the most popular.
-
An outdoor unit, which contains the microwave electronics for the terminal. This is usually the size of a shoe box, but it may be much smaller. If the ODU is large it is normally supported on the antenna mount behind the dish. Smaller ODUs can be attached directly to the rear of the feed assembly in front of the dish.
-
The outdoor unit is usually all solid state with GaAs FETs used in the Low Noise Receiver and the High Power Amplifier. LNA noise temperatures are typically in the range 190 - 225° K (Ku band) and HPA output powers are usually in the range 0.1 - 6 W (Ku band).
-
An indoor unit, which provides the modulation, demodulation, multiplexing, demultiplexing and synchronisation with the rest of the network and supports the user interfaces. This box is usually about the size of a domestic video recorder.
Remote terminals usually support a wide
range of common electrical interfaces such as RS-232, RS-422,
V.35, as well as voice and TV. Several common protocols are also
generally supported including SDLC, 3270 bisync, X.25, asynch
and Ethernet. Asynchronous data rates are typically available up
to 9.6 kb/s. Synchronous data rates between 1.2 and 32 or 64
kb/s are also generally available.
Remote terminals have now become very
reliable, with MTBFs of typically 25000 hours. Link availability
is also usually designed to be high, with an end to end
availability of better than 99.7% being quite common.
The price of a remote terminal, like that
of a hub station, can vary a great deal, but typical prices are
in the range 3 to 8 kEuro (for a complete installation
consisting of antenna, mount, ODU and IDU).
Network Configuration
This is similar to TDM/TDMA networks.
Signal Types and Characteristics
This technique is used in networks which,
unlike most interactive VSAT networks, are required to transfer
large files.
When the VSAT terminal wants to transmit
it requests an SCPC inbound channel over an ALOHA or Slotted
ALOHA access request channel. The hub assigns a specific SCPC
channel to the VSAT terminal which then has full use of that
channel until it stops transmission. The SCPC channel is then
allocated to the next terminal requesting a channel.
The outbound channel can also use DA/SCPC
or TDMA depending on the traffic statistics.
Hub Station
This is similar to the hub in TDM/TDMA
networks.
Remote Terminals
These are similar to the remote terminals
in TDM/TDMA networks.
Network Configuration
This is similar to TDM/TDMA networks.
Signal Types and Characteristics
Each VSAT in the network is assigned a
unique pseudorandom number (PN) which is used to code and decode
its transmissions. Several VSATs can transmit simultaneously on
the same frequency and be separated on reception by the hub.
The outbound transmission from the hub is
also usually coded in a similar way, except only a single PN
code is used allowing reception by all the VSATs in the network.
CDMA is an inefficient method of using
satellite capacity, however it has great resistance to external
interference and generates substantially lower levels of
interference than other methods. CDMA is therefore used
primarily where external interference restricts the use of other
solutions.
Hub Station
This is similar to the hub in TDM/TDMA
networks.
Remote Terminals
These are similar to the remote terminals
in TDM/TDMA networks.
What is VoIP?
VoIP stands for Voice over Internet Protocol. Voice over
IP is a form of communication much different than circuit
switching because VoIP sends information through IP packets over
the internet. Years ago it was found that sending a signal to a
remote destination could also be done digitally which brought
about the evolution of VoIP. A typical VoIP call uses an ADC or
analog to digital converter, then transmits the data over the
internet in packets and at the end of transmission formats the
data again with a DAC or digital to analog converter. Basically
VoIP digitalizes voice in data packets, sends them and
reconverts them in voice at the call destination.
The data network involved might be the Internet itself, or a
corporate intranet, or managed networks used by local or long
distance carriers and ISPs. Who runs the network isn’t
important-- what is is the fact that you're taking voice (i.e.,
analog information) and encoding it digitally, converting it
into packets, and then using a data network to move those
packets along the most efficient path to their destination,
where they get reassembled and transmitted in the format they
started in: voice. This way of packet switching is more
efficient than the previous way of circuit switching because the
information is sent in groups and there is no dead air time. If
no one is speaking during a VoIP call then no information is
being sent, however with a circuit switching call if no one is
speaking you are still being charged for the dead air time on
the line.
Why VoIP?
VoIP could be applied to almost any voice communications requirement, ranging from a simple inter-office intercom to complex multi-point teleconferencing/shared screen environments.
Widespread deployment of a new technology seldom occurs without a clear and sustainable justification, and this is also the case with VoIP. Demonstrable benefits to end-users are also needed if VoIP products (and services) are to be a long-term success. Generally, the benefits of technology can be divided into the following four categories:
VoIP could be applied to almost any voice communications requirement, ranging from a simple inter-office intercom to complex multi-point teleconferencing/shared screen environments.
Widespread deployment of a new technology seldom occurs without a clear and sustainable justification, and this is also the case with VoIP. Demonstrable benefits to end-users are also needed if VoIP products (and services) are to be a long-term success. Generally, the benefits of technology can be divided into the following four categories:
• Cost Reduction. Reducing long distance telephone costs
is a good reason for implementing VoIP. Today flat rate long
distance pricing is available with the Internet and can result
in considerable savings for both voice and facsimile (at least
currently). The sharing of equipment and operations costs across
both data and voice users can also improve network efficiency
since excess bandwidth on one network can be used by the other,
thereby creating economies of scale for voice (especially given
the rapid growth in data traffic).
• Simplification. An integrated infrastructure that supports all forms of communication allows more standardization and reduces the total equipment complement. This combined infrastructure can support dynamic bandwidth optimization and a fault tolerant design. The differences between the traffic patterns of voice and data offer further opportunities for significant efficiency improvements.
• Consolidation. Since people are the most significant cost elements in a network, any opportunity to combine operations, to eliminate points of failure, and to consolidate accounting systems would be beneficial. In the enterprise, SNMP-based management can be provided for both voice and data services using VoIP. Universal use of the IP protocols for all applications holds out the promise of both reduced complexity and more flexibility. Related facilities such as directory services and security services may be more easily shared.
• Other Advanced Applications. Even though basic telephony and facsimile are the initial applications for VoIP, the longer term benefits are expected to be derived from multimedia and multi-service applications. For example, Internet commerce solutions can combine WWW access to information with a voice call button that allows immediate access to a call center agent from the PC. Needless to say, voice is an integral part of conferencing systems that may also include shared screens, white boarding, etc. Combining voice and data features into new applications will provide the greatest returns over the longer term. Videoconferencing also can be greatly enhanced.
• Simplification. An integrated infrastructure that supports all forms of communication allows more standardization and reduces the total equipment complement. This combined infrastructure can support dynamic bandwidth optimization and a fault tolerant design. The differences between the traffic patterns of voice and data offer further opportunities for significant efficiency improvements.
• Consolidation. Since people are the most significant cost elements in a network, any opportunity to combine operations, to eliminate points of failure, and to consolidate accounting systems would be beneficial. In the enterprise, SNMP-based management can be provided for both voice and data services using VoIP. Universal use of the IP protocols for all applications holds out the promise of both reduced complexity and more flexibility. Related facilities such as directory services and security services may be more easily shared.
• Other Advanced Applications. Even though basic telephony and facsimile are the initial applications for VoIP, the longer term benefits are expected to be derived from multimedia and multi-service applications. For example, Internet commerce solutions can combine WWW access to information with a voice call button that allows immediate access to a call center agent from the PC. Needless to say, voice is an integral part of conferencing systems that may also include shared screens, white boarding, etc. Combining voice and data features into new applications will provide the greatest returns over the longer term. Videoconferencing also can be greatly enhanced.