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Seminar on VSat

Hello friends,
This article presenting you the vsat technology.It is an interesting seminar topic for computer science students.



Low cost business terminal with small antennas (generally less than 2 meters in diameter) are often termed Very Small Aperture Terminals (VSATs). VSATs are software driven earth station used for reliable transmission of data video or voice via satellite.

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, 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.


Vsat n/w provides rapid, reliable satellite transmission of data, voice & video to an unlimited no. of geographical dispersed sites or from these sites to main stations, No matter how remote or dispersed your headquarters. VSAT can provide remote diagnostics, remote monitoring, and data streaming services from remote or hazardous sites.


In a typical satellite Internet deployment, the remote site(s) communicate with the satellite hub and through the hub to other sites on the Internet, sometimes including a VPN between the satellite Internet hub and a corporate data center.
Sometimes two satellite remote sites will need to connect directly to each other. Subscribing each site to the VSAT Systems LLC broadband satellite service will allow direct communication while allowing each site to access the Internet in the usual manner.

This configuration will, however, produce double the amount of satellite latency when communicating directly between the sites due to the second satellite “hop.” See the diagram below.


1. Satellite
2. Parabolic shaped antenna
3. An outdoor unit (ODU)
4. An Indoor Unit (IDU)

Satellite: The ideal orbit for a communications satellite is geostationary, or motionless relative to the ground. Satellites used for communications are almost exclusively to the ground. Satellites used for communications are almost exclusively in the geostationary orbit, located at 36000Km above the equator. In line with ITU stipulations, for avoiding interference, all satellites are placed 2 degree apart. This places a maximum limit of 180 satellites operating in a geostationary orbit.

However, with a view to maximize the utilization of orbital slots, Co-located satellites are being deployed. Co-located satellites are separated by 0.1 degree in space or approximately 30 Kms. Signal interference from the Co-located satellites is prevented by using orthogonal polarizations. Hence a ground station equipment can receive signals from two Co-located satellites without any reorientation of the antenna. The signals can be differentiated based on their polarizations.

Space segment: Space Segment is available from organizations which have procured satellites, arranged launches and conducted preliminary tests in-orbit and who then operate these satellites on commercial basis.

Transponders: Contained in the satellite body are a number of transponders or repeaters. These transponders perform the following functions :

• Signal Reception – It receives the signal up linked by a VSAT and/or hub.
• Frequency Translation – The frequency, known as the received signal is translated to a different frequency translation ensures that there is no positive feedback and also avoid interference related issues.
• Amplification – The transponder also amplifies the downlink signal.

The number of transponders determines the capacity of a satellite. The INSAT series of transponders in various frequency bands. Each transponder typically has a bandwidth of 40 MHz. The various frequency bands are as below –

Frequency Band Uplink (GHz)
Earth Station to satellite Downlink (GHz)
Satellite to Earth Station
C Band 5.925 to 6.425 3.700 to 4.200
Extended C Band 6.725 to 7.025 4.500 to 4.800
Ku Band 14.000 to 14.500 10.950 to 11.700

Internationally Ku-Band is a popular frequency band in use. The Ku-Band by virtue of its higher frequency can support traffic with smaller antenna sizes in comparison to C/Ext-C Band. It is, however, susceptible to rain outages making it unsuitable for use in South East Asian regions. Indian service providers are presently allowed to hire space segment only on the INSAT series of satellites and is not a standard band available internationally.

Parabolic shaped antenna: Typically, interactive Ku-band antenna sizes range from 75 centimeters to 1.8 meters and C-band from 1.8 meters to 2.4 meters. One way systems can use antennas as small as 45 centimeters.

Outdoor Unit (ODU):

The outdoor unit consists of an Antenna and Radio Frequency Transceiver. (RFT). The antenna size is typically 1.8 meter or 2.4 meter in diameter, although smaller antennas are also in use. The antenna system comprises of a reflector, feed horn and a mount. The size of a VSAT antenna varies from 1.8 meters to 3.8 meters. The horn is mounted on the antenna frame at its focal point by support arms. The FEED HORN directs the transmitted power towards the antenna dish or collects the received power from it. It consists of an array of microwave passive components. Antenna size is used to describe the ability of the antenna to amplify the signal strength.

The RFT is mounted on the antenna frame and is interconnected to the feed horn. Also termed as outdoor electronics, RFT, in turn, consists of different subsystems.

These include low noise Amplifiers (LNA) and down converters for amplification and down conversion of the received signal respectively. LNAs are designed to minimize the noise added to the signal during this first stage of the converter as the noise performance of this stage determines the overall noise performance of the converter unit. The noise temperature is the parameter used to describe the performance of a LNA.

UP converters and High Powered Amplifiers (HPA) are also part of the RFT and are used for up converting and amplifying the signal before transmitting to the feed horn. The Up/Down converters convert frequencies between intermediate frequency (Usually IF level 70 MHz) and radio frequency. For Extended C band, the down converter receives the signal at 4.500 to 4.800 GHz and the up converter converts it to 6.725 to 7.025 Ghz. The HPA ratings for VSATs range between 1 to watts.

Indoor Unit (IDU):

The indoor unit functions as a modem and also interfaces with the end user equipment like stand alone PCs, LANs, Telephones or an EPABX. The IDU consists of modulators which superimpose the user traffic signal on a carrier signal. This is then sent to the RFT for up conversation, amplification and transmission. It also consist of demodulators which receive the signal from the RFT in the IF range and demodulates the same to segregate the user traffic signal from the carrier. The IDU also determines the access schemes under which the VSAT would operate. The IDU also interfaces with various end user equipment, ranging from stand alone computers, LAN’s, routers, multiplexes, telephone instruments, EPABX as per the requirement. It performs the necessary protocol conversion on the input data from the customer end equipment prior to modulation and transmission to the RFT. An IDU is specified by the access technique, protocols handled and number of interface ports supported.

The VSAT is configurable via software downloads without site visits. The VSAT software, which includes the TCP acceleration and the routing functionalities, (such as Static routing, RIP and IRDP) is embedded into the IDU. A satellite modem is different than a telephone modem, and is used to convert the data, video, or voice generated by the customer application for transmission over satellite. The modem takes the signals from your computer, phone or other device and changes them so they can be sent to the ODU which transmits them out to the satellite and eventually to other ground stations.

The IDU is responsible for the transfer of the data and video images between your network and the ODU. IDUs may even use a hard drive to provide content storage, caching of data and video. One side of the IDU connects to the coaxial cable from the ODU and the other attaches to network equipment. IDUs come with various port configurations to meet a variety of network applications requirements. For instance, IDUs come with Ethernet, USB, and/or audio and video ports for audio and video


Configurations Used

• Point to point configuration
• Broadcast configuration
• STAR configuration
• MESH configuration
• Hybrid configuration (Star & Mesh both)

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 synchronization 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.

Out bounds are transmitted continuously (i.e. duty cycle 100%) as a TDM stream. The number of out bounds per network is determined by the traffic statistics, packet length as well as the outbound data rate.

The out bounds 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 aaa56 Kb/s and 256 Kb/s. All carriers are BPSK or QPSK modulated and have rate ½ or 2/3 Forward Error Correction (FEC). This ensures that bit error rates are low (typically 10 or 10 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 out bounds 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.
Each TDM outbound carries a continuously transmitted bit stream 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 synchronized to the outbound frames. Each inbound frame is divided into slots. Individual IDUs transmit in these slits 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 duey 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 nix.

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 minimize the impact of the data layer protocol and, particularly, the satellite transmission delay, on the throughput of the satellite link.

TDM/TDMA Connection Set Up

When the network is established, or when additional remote terminals are added to the network, remote 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 signaling traffic.

This same procedure is used for intra network signaling 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.

Hub Station
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.

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 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
• Abank 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 up converters 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 Traveling 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 down converter 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 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 uninterruptible power supply with a large battery band and a diesel generator

Remote Terminals

In contrast to the hub station, the remote terminals are much simpler. To minimize total system costs, VSAT networks are designed to have a single expensive hub and a large number of much smaller remote terminals.

• 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 ishes, although larger dishes tend to be centre-fed. Recently, to gain higher performance (in particular side lobe performance) dual reflector, Gregorian designs have started to become common. Several different materials are used for the dishes with spun aluminum, steel, fiberglass 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 290 -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 synchronization 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 bisyne, 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.

Advantages of VSATs

VSATs are an ideal option for networking because they enable Enterprise Wide Networking with high reliability and a wide reach which extends even to remote sites.

Last Mile Problem

Let us begin with the situstion where you have reliable high-speed links between city exchanges for meeting your communication requirements. But before you begin to feel comfortable, connections from the nearest exchange to your company’s office often fail. Consequently, stretching what is technically called the last mile problem into much longer distances. VSATs located at your premises guarantee seamless communication even across the last mile.


You must be well aware of the limitations faced by terrestrial lines in reaching remote and other difficult locations. VSATs, on the other hand, offer you unrestricted and unlimited reach.


Uptime of upto 99.5 % is achievable on a VSAT network. This is significantly higher than the typical leased line uptime of approximately 80 to 85%.


VSAT deployment takes no more than 4-6 weeks as compared to 4 to 6 months for leased lines.


Management Maintenance


Disadvantages of VSATs








VSAT N/W provides rapid, reliable satellite transmission of data, voice & video to an unlimited no. of geographical dispersed sites or from these sites to main station, No matter how remote or dispersed your operations are, VSATs provide a link to your headquarters. VSAT N/Ws are to every type of architecture like point to point configuration, star, mesh etc.

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