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The history of VSAT What is a VSAT? Advantages of VSAT Technology VSAT System Architecture |
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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 analyse 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.
VSATs can typically be divided into two parts- an outdoor unit and an indoor unit. The outdoor unit is generally ground or even wall mounted and the indoor unit which is the size of a desktop computer is normally located near existing computer equipment in your office.
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
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. Back to top 
Advantages of VSATs
If by now you believe that VSATs provide an edge over terrestrial lines only in cases where the land lines are difficult to install, say in the case of remote locations, then consider this. Close to 50 percent of the total VSAT population is installed in the US which also boasts of world's best terrestrial communications.
Networking of business activities, processes and divisions is essential to gain a competitive edge in any industry. 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.
VSAT System Architecture
A VSAT system consists of a satellite transponder, central hub or a master earth station, and remote VSATs. The VSAT terminal has the capability to receive as well as transmit signals via the satellite to other VSATs in the network. Depending on the access technology used the signals are either sent via satellite to a central hub, which is also a monitoring centre, or the signals are sent directly to VSATs with the hub being used for monitoring and control.
The most popular of these is Star topology. Here we have a big, central earthstation known as the hub. Generally the hub antenna is in the range of 6-11metre in diameter. This hub station controls, monitors and communicates with a large number of dispersed VSATs. Since all VSATs communicate with the central hub station only, this network is more suitable for centralized data applications. Large organizations, like banks, with centralized data processing requirements is a case in point.
In a mesh topology a group of VSATs communicate directly with any other VSAT in the network without going through a central hub. A hub station in a mesh network performs only the monitoring and control functions. These networks are more suitable for telephony applications. These have also been adopted to deploy point to point high speed links.
However, in actual practice a number of requirements are catered to by a hybrid network topology. Under hybrid networks a part of the network operates on a star topology while some sites operate on a mesh topology.
However, with a view to maximise the utilisation of orbital slots, Co-located satellites are being deployed. Co-located satelites are separated by 0.1 degree in space or approximately 30 kms. Signal interference from the Co-located satellites is prevented by using orthogonal polarisations. 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 polarisation.
Space segment : Space Segment is available from organisations 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 :
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 and operate in Ext-C band only. Ext-C band is available only on the INSAT series of satellites and is not a standard band available internationally. Link Budgets : Ascertains that the RF equipment would cater to the requirements of the network topology and satellite modems in use. The link Budget estimates the ground station and satellite EIRP required. Equivalent isotropically radiated power (EIRP) is the power transmitted from a transmitting object. Satellite ERP can be defined as the sum of output power from the satellite’s amplifier, satellite antenna gain and losses. Calculations of signal levels through the system (from originating earth station to satellite to receiving earth station) to ensure the quality of service should normally be done prior to the establishment of a satellite link. This calculation of the link budget highlights the various aspects. EIRP required at the transmitting VSAT, Satellite EIRP which will be required for a desired specified gain of this receiving system. Apart from the known losses due to various cables and inter - connecting devices, it is customary to keep sufficient link margin for various extraneous noise which may effect the performance. It is also a safeguard to meet eventualities of signal attenuation due to rain/snow. As mentioned earlier a satellite provides two resources, bandwidth and amplification power. In most VSAT networks the limiting resource in satellite transponder is power rather than bandwidth. With all their advantages, VSATs are taking on an expanding role in a variety of interactive, on-line data, voice and multimedia applications. Whether it is gas station service, rural telephony, environmental monitoring, distance learning / remote training or the Internet, VSATs are truly poised to be the Space Age Technology.
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For comments and suggestions, mail me at vikramr@planetasia.com.
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