TUCN - Faculty of Electronics, Telecommunications and Information Technology

Antennae and Propagation

VSAT s3m2mn
(VERY SMALL APPERTURE TERMINAL)

A brief history of space communication

The idea of radio transmission through space was first conceived in 1911. In 1945 British author-scientist Arthur C Clarke suggested the use of a geosynchronous earth satellite for the purpose. His assumption of a manned space station was later revised by a US engineer, J R Pierce, in April 1955, who was also the first one to analyze unmanned communication satellites. This idea later led to the great success of satellite communications.
The first artificial satellite "SPUTNIK I" was launched by the erstwhile USSR, in 1957. This began a series of space initiatives by USA and USSR.
The first satellite communication experiment was the US government's project SCORE (Signal Communication by Orbiting Relay Equipment), which launched a satellite on December 18, 1958. This satellite circled the earth in an elliptical orbit and retransmitted messages recorded on a magnetic tape. It lasted for about 13 days after which the batteries ran out!
The US Army Signal Corp's Courier IB, launched in October 1960, lasted for about 17 days. It could handle typewriter data and voice and facsimile messages.
It was a balloon, Echo 1, launched in August 1960, which led American Telephone & Telegraph Company (AT&T) to build Telstar. Communication tests carried out by reflecting radio signals from Echo 1's surface were completely successful.
Telstar, launched on July 1962 was the first active satellite with a microwave receiver and transmitter to transmit live television and telephone conversations across the Atlantic. It was turned off in February 1963. Successive initiatives include NASA's Relay 1 satellite was launched in elliptical orbit in December 1962 and Syncom 2, the first synchronous communication satellite was launched in July 1963.
In 1964 a global initiative was undertaken leading to the formation of INTELSAT, which has been one of the major driving forces for the large scale commercial exploitation of satellite technology for communications. Since then there has been no looking back.

What is a VSAT?
The term Very Small Aperture Terminal (VSAT) refers to a small fixed earth station. VSATs provide the vital communication link required to set up a satellite based communication network. VSATs can support any communication requirement be it voice, data, or video conferencing.
The VSAT comprises of two modules - an outdoor unit and an indoor unit. The outdoor unit consists of an Antenna and Radio Frequency Transceiver. (RFT). The antenna size is typically 1.8 metre or 2.4 metre in diameter, although smaller antennas are also in use. The indoor unit functions as a modem and also interfaces with the end user equipment like stand alone PCs, LANs, Telephones or an EPABX.

Outdoor Unit
The antenna system comprises of a reflector, feedhorn and a mount. The size of a VSAT antenna varies from 1.8 metres to 3.8 metres. The feedhorn 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 minimise 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
Upconverters and High Powered Amplifiers (HPA) are also part of the RFT and are used for upconverting and amplifying the signal before transmitting to the feedhorn. The Up/Down converters convert frequencies between intermediate frequency (Usually IF level 70 MHz) and radio frequency. For Extended C band, the downconverter receives the signal at 4.500 to 4.800 GHz and the upconverter converts it to 6.725 to 7.025 GHz. The HPA ratings for VSATs range between 1 to 40 watts
Interlink Facility
The outdoor unit is connected through a low loss coaxial cable to the indoor unit.
Indoor Unit
The IDU consists of modulators which superimpose the user traffic signal on a carrier signal. This is then sent to the RFT for upconversion, amplification and transmission. It also consists 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.
Features:
Ability to target small-dish audiences and meet specialized service requirements
Support for applications including LAN/WAN networking, Internet/intranet, video conferencing, remote site networking
Large in-orbit capacity makes VSAT platforms cost-effective
Star, multi-star, mesh wideband and remote solutions

Network Architectures

Different transmission architectures are used for interactive hubbed VSAT networks:

TDM/TDMA
( def: TDMA = Time Division Multiple Access - a technology for delivering digital wireless service using time-division multiplexing (TDM). TDMA works by dividing a radio frequency into time slots and then allocating slots to multiple calls. In this way, a single frequency can support multiple, simultaneous data channels. )
Demand Assigned SCPC (single channel per carrier)
Point-to-point SCPC links are the satellite equivalent of a terrestrial leased line connection. They are usually set-up on a permanent, 24 hour basis and are thus more costly in satellite capacity and less efficient if not used all the time. However, they do support high bandwidths (typically from 9.6 kbps to 2 Mbps) and can easily be used to carry data, voice and even video traffic.)
CDMA -; contentionless, nr. of simultaneously transmitting users under a threshold.

DAMA

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.

TDM/TDMA Interactive VSAT Networks
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).
A typical TDMA network employs a large satellite hub system that manages all network terminal access and routing. Data is transmitted to and from the hub in short bursts on satellite channels that are shared with 30 to 40 other terminals (depending upon network loading parameters). The hub communicates with these VSAT terminals over a higher-speed "outbound" TDM satellite carrier. The terminals transmit back to the hub on their assigned "inbound" carriers using TDM protocols.
This is how a star data, TDM/TDMA VSAT network works using a hub station, usually six metres or more in size and small VSAT antennas (between 75 centimetres and 2.4 metres). All the channels are shared and the remote terminals are online, offering fast response times. Consequently, TDM/TDMA systems are comparable with terrestrial X.25 or frame relay connections:
Initial systems were designed to offer fast response times for predicable, bursty traffic patterns - typified by credit verification transactions and lottery systems. As the internet and broadband generally began to drive demand, the manufacturers introduced completely new IP-centric platforms designed to serve broadband applications. In essence, current systems are now also able to trade a short initial delay to allow the hub to allocate dedicated capacity within the inbound (return) channel to a VSAT. This capacity is dynamically sized by the system based on the traffic demand seen by the VSAT. In addition, all systems also incorporate frequency hopping, allowing the network to be load-balanced by moving VSATs between inbound carriers to ensure that capacity is used efficiently and congestion does not occur.
Broadband VSAT systems are sometimes criticised for poor performance with the blame often being laid at the door of the product. However, almost every VSAT platform of which we are aware, is very capable of meeting the demands of the most demanding user - the culprit for poor performance is usually a result of the amount of bandwidth a subscriber is allocated as part of the service. High rates of over-subscription are mostly a consequence of low prices - satellite bandwidth is both a finite and expensive resource unfortunately.
On a brighter note, the industry has been working hard towards introducing greater efficiencies at all levels - the satellite (Ka-band and on-board processing), the inbound/return links (turbo coding and modulation), the outbound/forward channel (DVB-S2) and with more advanced techniques to manage IP and web traffic (acceleration, compression and caching).

Network Configuration

TDMA is a Contentionless ( the users transmit in an orderly scheduled manner), fixed assignment ( each user has a predetermined and fixed allocation of resources) fashion protocol.

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 -; contention ( possibility of collision exists), random access ( there is no distributed scheduling of transmission) . 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.
Demand assigned SCPC (point -;to -;multipoint)
SCPC is used for economical distribution of broadcast data, digital audio and video materials, as well as for full-duplex or two-way data, audio or video communications.
In an SCPC system, user data is transmitted to the satellite continuously on a single satellite carrier. The satellite signal is received at a single location, in the case of a point-to-point system, or at many locations in a broadcast application, providing hubless connectivity among multiple sites.
Mesh networks which use capacity on a demand assigned multiple access (DAMA) basis take a different approach. The master control station merely acts as a controller and facilitator rather than a hub through which traffic passes as in a star network. However, these connections take a little time to set-up and thus, mesh/DAMA systems are often equated to a terrestrial dial-up connection.
There are also mesh systems which use a TDMA access scheme where all of the terminals in a network receive and transmit to the same channel, selecting different time slots because each terminal is aware of what the others have reserved. In the past this type of system has been costly and therefore, reserved for large scale trunking applications, but, more recently, costs have come down considerably and now they can be cost competitive with SCPC/DAMA systems for thin route applications as well.

Point -;to -; point SCPC
Point-to-point SCPC (single channel per carrier) links are the satellite equivalent of a terrestrial leased line connection. They are usually set-up on a permanent, 24 hour basis and are thus more costly in satellite capacity and less efficient if not used all the time. However, they do support high bandwidths (typically from 9.6 kbps to 2 Mbps) and can easily be used to carry data, voice and even video traffic.


All other systems are usually a variation on one of the themes described above, either in a star, mesh or hybrid (star and mesh) configuration. Most of the TDM/TDMA manufacturers also offer a mesh product which can be deployed in a hybrid-ised configuration, sharing common components such as antennas and RF units, at a remote site.

DAMA
Demand Assigned Multiple Access (DAMA)
A DAMA system is typically a single hop satellite transmission network which allows direct connection between any two nodes in the network among many users sharing a limited "pool" of satellite transponder space.
DAMA supports full mesh, point-to-point or point-to-multipoint communications -- any user can connect directly to any other user anywhere within the network. The result is economical and flexible bandwidth sharing with any mix of voice, FAX, video and data traffic. DAMA optimizes the use of satellite capacity by automatically allocating satellite resources to each active node upon demand. Users typically pay for satellite capacity used, plus a network maintenance fee.
Network Control System....
In a DAMA system, the network allocates communications bandwidth to each call from a pool of frequency channels on a demand-assigned basis. When a caller at a remote terminal requests service (e.g. picks up the telephone to make a call) the request is made to a Network Control System (NCS) over the shared DAMA common signaling channel. The NCS functions as a "switchboard in the sky". The NCS determines if the call is valid and then establishes the channel (including bandwidth) between the originating site and the called number. Circuits remain active only as long as needed, then are broken to free bandwidth for other users. When the call is completed, the NCS is informed by the remote terminals and the freed bandwidth is returned to the frequency pool. By using a DAMA system a single transponder can support several thousand subscribers.
Any remote unit can be configured to perform as the Network Control System with the addition of some hardware and Network Management System (NMS) software. The hardware, together with the NMS, serves as the single focal point for system level control within the satellite communications network. The NCS can be located anywhere within the satellite footprint.
What are the benefits of using a DAMA system?
DAMA systems quickly and transparently assign communication links or circuits to users on a call-by-call basis. After use (hang up), channels are immediately returned to the central pool, for reuse by others. By using DAMA, many subscribers can be served using only a fraction of the satellite resources required by dedicated, point-to-point Single-Channel-Per-Carrier (SCPC) networks.
The business case for DAMA is a strong one. DAMA saves satellite users money, optimizes use of scarce satellite resources, and can increase service provider revenues. Service providers using DAMA are able to pack more services into the same satellite bandwidth, with a resulting increase in their profit-making opportunities. For military applications, many users compete for very limited communications resources. DAMA’s inherent network management capabilities provide positive control of those resources.

VSAT Network Design and Simulation

In this section we present a few articles and applications regarding studies over the VSAT network and also some software tools we found.

1. A Simulation Study of a Broadband Multimedia VSAT network

- is a simulation approach for assessment of quality of service and bandwidth utilisation for a meshed broadband VSAT system over a geostationary satellite.
- A multi-frequency timedivision multiple access is used (MF_TDMA) with an integrated call admission control (CAC) and bandwidth on demand (BOD) algorithm for uplink from the VSATs.

- Traffic source models of variation multimedia services are implemented along with the CAC and BOD scheme using OPNET. Using this simulation platform, we measure the quality of service for different applications and uplink bandwidth utilization.
- After extensive simulation experiment for various traffic mixes, the results refer to uplink throughput, SIT queing delay and end-to-end queing delai.
- Based on the results several issues can be effectively addressed. For example, the system designers can determine how to mix different applications to obtain maximum capacity utilization network revenue, guarantee the QoS for all applications with limited network resources, and implement buffers of sufficient size to reduce loss.

2. Optimal resource Utilisation in VSAT Networks

- Provides optimal resource utilization for various traffic streams in VSAT networks.
- Presents various types of traffic such as bursty, demand based traffic and multimedia traffic and worked on the problem of optimal utilization of the satellite bandwidth under various traffic conditions. A GUI oriented discrete-event simulator in Visual C++, is decvelopped and implemented many MAC layer protocols like ALOHA, R-ALOHA, FDMA, TDMA, frequency hopping and their combinations for simulation and study of various scenarios.
- An ALOHA protocol simulator written in Visual C++. All the parameters are adjustable and it shows the packets animation in real-time.

- A Visual MAC Simulator was that automatically does bandwidth utilization analysis and outputs a report file. Sources can be configured and any number can be added. It supports many different kinds of sources and also many different MAC protocols like Aloha, FDMA, TDMA, FTDMA etc. It activates the sources and records actual bandwidth utilization. Purpose was to find efficient ways of bandwidth utilization in VSAT networks.

- There are also available some graphs generated by the simulator with some of the MAC protocols we tried.

3. Digisim 3.0 Satellite Communications Software

- DIGISIM can help planning for new satellite digital transmission services, preserving link quality of existing services and supporting spectrum coordination processes.
- DIGISIM can serve satellite operators, teleports, VSAT-operators etc., in areas as diverse as link analysis, transmission planning, system engineering, systems architecture, spectrum coordination, interference control, and more.
- DIGISIM is a time-domain simulation tool, simultaneously generating up to 3 modulated carriers, passed through individual satellite transponders or sharing one transponder. Each carrier can be independently configured for offset frequency, symbol rate and modulation type. Supported modulation formats are QPSK, Offset-QPSK, BPSK, 8-PSK and 16-QAM. Roll-off factor of pulse-shaping filters can be independently set for transmitter and receiver. Automatic receiver clock synchronisation can be overridden by manual control. Additional carriers can be added to create a desired interference or inter-modulation scenario. Each transponder can be individually configured with user-supplied filter and (TWT-)nonlinearity response files. The built-in filter set-up algorithm performs GD-to-phase conversion and re-interpolation necessary for filter computation. Transponder input back-off and relative output levels are individually adjustable. Multipath interference from adjacent uplinks is included by default, can be disabled. Dynamic uplink pre-distortion can be individually enabled to compensate for Imux-, TWT- and/or Omux-responses of each active transponder. Carrier phase noise can be enabled, combining cascaded contributor spectra (up-converter, on-board LO, LNB) read from file. Phase noise spectrum can also be manually entered as discrete points on a logarithmic scale, and stored to file if desired. Result of each simulation run is presented as a Symbol-Error-Rate(SER)-curve plotted against Eb/N0, Es/N0, C/N in Nyquist bandwidth or user bit rate. Multiple SER-curves can be overlaid, edited and annotated for comparison between runs. Waveform plots (constellation plot, output spectrum, eye-diagram, I/Q waveform plot), as well as possibly generated phase noise spectrum plots can be called from menu functions. The computed output spectrum can be exported to file. The user interface is fully GUI-based, facilitating parameter entry and file loading.
- It is not a freeware software.

BENEFITS OF VSAT
VSAT technology offers many advantages:
Ease of Implementation
After the order is placed, putting up a VSAT network can be done in a matter of days depending on the number of sites. The antenna dish can be installed virtually anywhere - on rooftops, in parking lots, or on the sides of the buildings. The size of the antenna dish ranges from 1.8m to 2.4m in diameter, depending on the type of application.
Likewise, moving the VSAT unit to a new location can be done very quickly. Once the antenna site has been established, the field technicians will be able to erect the antenna in hours and usually complete the installation of the Indoor Unit in very short time.
Availability
The wireless nature of VSAT system allows its installation at virtually any location within the footprint of the satellite. With this advantage, Customers may setup offices in a promising location without being constrained by the availability of terrestrial lines. At the same time, Customers may relocate offices to a lower cost area and still maintain the communication link with the use of our VSAT links.
Reliability
Satellite communications is extremely reliable. VSAT system has a Bit Error Rate (BER) of approaching 1 x 10-9. This BER is similar to that available on many of today's advanced digital terrestrial circuits. Historical network uptime is approaching 99.98%, exceeding typical terrestrial cable lines. VSAT Master Earth Station has built-in redundancy that ensures continuous operations in case of failure.
Regardless of where the sites are located, each VSAT remote unit receives the same level of performance and signal quality.
Network Management
VSAT equipment can be remotely managed avoiding any risks that might occure.
Recovery
One of the great advantages of wireless VSAT technology is its direct connectivity between end-points. Every VSAT link is established directly through the satellite, bypassing intermediate terrestrial lines. Hence, faulty problems with the network can be identified and fixed quickly
Security

Satellite networks offer excellent security against unauthorized access. All transmissions in any VSAT system are scrambled in digital format. Gaining access to a VSAT system is virtually impossible without authorization. Every remote VSAT earth station is controlled and monitored.
Banks and financial institutions around the world predominantly use VSAT system to carry critical and sensitive financial transactions. The Canadian and U.S. Armed Forces also use VSAT for their communication networks, further testifying the security of the network.
Scalability
One of the strengths of our VSAT network is the ability to scale according to the Customer's requirements with low incremental cost. This is especially important as business grows over time. A new site can be added to the network by installing a remote VSAT unit. A VSAT network can support thousands of VSAT remotes.
Customers may start with data service. Adding more bandwidth over time can be done by a simple reconfiguration at the Network Operations Center. In addition, Customers are able to add voice capabilities or video applications at a later stage. These add-on services can be done on the existing VSAT remote unit by installing additional modules to the Indoor Unit.
Flexibility
VSAT streamlines Customers' communications into one powerful and dynamic network. Changes in the communication requirements can be adapted quickly and cost-effectively by reconfiguring the network with ease.
VSAT also gives Customers complete freedom in designing the data network whether it is based on Centralized or Distributed architecture. The VSAT network can be tailored to optimize response times to satisfy the application requirements. Unlike terrestrial networks, there are no geographic constraints in configuring VSAT multi-point links, thus minimizing network traffic peaks and transmission capacity.
Lower Cost
VSAT services bring about cost savings to the Customers. The advantages of the VSAT technology already give tangible and intangible benefits to our Customers. As Customers add more services, they will find that the incremental cost is very low compared to terrestrial networks.
The service costs are distance insensitive. The monthly service fees are fixed and priced according to the network capacity, not the distance between the head-office and branch locations. As a result, Customers pay only for the amount of data throughput used.

TYPICAL VSAT APPLICATIONS
The great advantage of VSAT is its flexibility. It permits any kind and size of network based around a central hub and remote locations. This makes them particularly pertinent for corporate networks or, for example, communications among educational, government or health-care institutions. Through a VSAT network, a corporation can communicate freely and constantly with branch offices.
Most companies develop their private wide area networks to interconnect the network of the remote offices with the main computer center. While data communications is the primary use of VSAT, there are companies that require internal telephone network for inter-office communications.
Corporate communications can be divided into the following categories:
Interactive applications
The interactive applications can be based on centralized or distributed concept. In a centralized system, all terminals in the offices operate "on-line" and communicate intermittently with the host or servers at the data center. These include client-server applications and Web-based intranet applications. In a distributed system, each remote office has terminals linked to its local host or servers. The servers then communicate with each other in a Wide Area Network (WAN). Besides real-time data applications, the VSAT network supports two-way voice capability for telephone or facsimile. Remote offices can make phone calls to each other or to the central headquarters, bypassing the local public phone network.
File Transfer
These applications send a large amount of data in one transaction. These include the use of the TCP/IP File Transfer Protocol (FTP) to transfer files and the printing of large reports.
File Broadcast
A recent file transfer application requires support of file broadcast or IP multicast. These applications send the data to multiple sites in one transaction. Typical applications include audio and video broadcast, distance learning, data distribution, and software updates. A VSAT network is inherently broadcast in nature. Thus a VSAT network naturally and efficiently support these new broadcast applications. Using a VSAT network, a large file can be distributed to hundreds of sites simultaneously. The VSAT network supports IP multicast, which improves broadcast performance even more.
A few applications:
Receive Only..... o Stock market & other news broadcasting o Training or continuing education from a distance o Distribute financial trends & analyses o Introduce new products at geographically dispersed locations o Update market related data, news, and catalog prices o Distribute video or TV programs o Distribute music in stores & public areas o Relay advertising to electronic signs in retail stores

Transmit/Receive..... o Interactive computer transactions o Internet o Video Teleconferencing o Database inquiries o Bank transactions, ATM o Reservation systems o Distributed remote process control and telemetry o Voice communications o Emergency services o Electronic fund transfer at Point-of-Sale o E-mail o Medical data transfer o Sales monitoring & stock control
Why use VSATs?
VSAT networks provide rapid, reliable satellite transmission of data, voice, and video to an unlimited number of geographically dispersed sites or from these sites to headquarters. With a satellite network, there are no routers and no switches - nothing between the user and the source of the information, except the sky. There are no physical limitations in terms of geography or distance to make deployment difficult or too expensive. And since VSAT satellite communication systems often provides a complete end-to-end infrastructure. Furthermore, VSATs have a reliability rate and a network availability that is significantly higher than terrestrial systems. Particularly effective for broadband applications, the high throughput VSATs offer - of 40 Mbps downstream and 76.8 Kbps upstream - means that rich-content material can be delivered live and on-line, or downloaded for later viewing.
VSAT networks offer value-added satellite-based services capable of supporting the Internet, Data, LAN, Voice/Fax communications, Video Conferencing, Distance Learning, and can provide powerful, dependable private and public network communications solutions.

Is there an advantage of VSAT over cable?
When it comes down to cost, making general comparisons between VSAT services and their terrestrial equivalents is almost impossible. Charges for terrestrial services are nearly always distance-dependent, while VSAT connections cost the same whether sites are 1 or 1,000 miles apart. And with most VSAT services, the cost per connection comes down considerably when a customer addes users.
Installation:

Geography:

Bandwidth:

VSAT networks provide an efficient, cost-effective method for reliable distribution of data through a multiple site organization, regardless of location.
VSAT Data Capacity
The indoor data processing unit (DPU) or satellite modem will typically have one or more user etherent ports for connection either direct to PCs or to a local area network router.The number of ports is typically 1 or 4.
If one modem has just one etherent port you can extend it to 4, 8, 16 etc as required using local area network hub devices or switches.
Download bit rate
In the outlink (download) direction from the hub to the VSATs continuous carriers are used, typically more than 256k bit/s and up to 60 Mbit/s. The type of carrier is almost identical with a digital TV carrier called DVB-S. Each VSAT is restricted to extract from the data stream only those packets of data intended for it. One outlink may be shared by 5 to 32,000 VSAT sites, according to the traffic load.
In some cases you can use a DVB-S type receiver to receive only and still extract just what is intended for you. This is the basis for DVB-S/SCPC systems where the VSAT uses a separate transmit modem to transmits a low speed continuous carrier back to he hub. The DVB-S arrangement also applies to so called one-way satellite internet systems that use a DVB-S plug-in card in your PC. In this case the customer sends request packets to the hub using a terrestrial phone line/modem. The benefit is high speed downloads.

Upload bit rate
The data transmission rate on the return link, in the direction from the VSAT to the hub, is typically from a few 100 bit/s to 512 kbit/s. Each VSAT typically transmits in short occasional bursts, interleaved in time. The number of VSAT sites sharing a return link and the number of return links is adjustable to match actual traffic patterns, hopefully without unacceptable congestion. Sharing ratios vary from 5:1 to 60:1 The higher sharing ratios correspond to least cost services, suitable for the occasional undemanding user. Shared systems are vulnerable to greedy users who may quickly overload the network.
Where reliable return link bit rates are required such as for business, cyber cafe or VoIP applications a continuous return link carrier is used (SCPC = Single channel per carrier). It is also possible to set up a dedicated repeating time slot in a TDMA system which gives the same equivalent result - a continuous guaranteed bit rate for the customer. This has the advantage that any congestion that does occur is fully under the customer's control.
VoIP requires approx 20 kbit/s each way for the duration of the call and most satisfactory results are obtained with dedicated capacity.
Other arrangements
There are many further possibilities. Point to point VSAT networks (2 terminals only) are possible with say 2 Mbit/s each way, carrying a mixture of data and voice traffic for example. Mesh networks are possible, e.g. several hotels with common ownership might share a common pool of satellite capacity and have the options to communicate direct to each other when desired.
VSATs have been in use for more than 10 years and, with more than 500,000 systems operating in more than 120 countries, VSATs are a mature and proven technology.

References :

· “Simulation Study for a Broadband Multimedia VSAT Network” -; Yi Qian, Rose Hu and Hosame Abu-Amara
· “Optimal Resource Utilisation in VSAT Networks” -; AshishGupta https://www.cs.northwestern.edu/Iagupta/_projects/visual_mac_simulator/
· https://www.ses-astra.com/corpSite/site_en/SupportServices/TechCom/index.php
· https://www.angelfire.com/electronic/vikram/tech/vsattut.html#1
· https://www.comsys.co.uk/vsat_rep.htm
· https://www.telesat.ca/telecom/anikomVSAT.htm
· https://www.qpcomm.com/vsat_info.html
· https://www.satcoms.org.uk/vsat/vsat_tutorial.asp
· https://www.telekom.com.my/business/corporate_government/data_services/vsat_faq.htm#01