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A Tutorial on VSATs

The history of VSAT
What is a VSAT?
Advantages of VSAT Technology
VSAT System Architecture


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.
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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.

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. The typical limit of an IFL cable is about 300 feet.

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.
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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.

Last Mile Problem
Let us begin with the situation 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.

Reach
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.

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

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

Network Management
Network monitoring and control of the entire VSAT network is much simpler than a network of leased lines, involving multiple carriers at multiple locations. A much smaller number of elements needs to be monitored incase of a VSAT network and also the number of vendors and carriers involved in between any two user terminals in a VSAT network is typically one. This results in a single point of contact for resolving all your VSAT networking issues. A VSAT NMS easily integrates end-to-end monitoring and configuration control for all network subsystems.

Maintenance
A single point contact for operation, maintenance, rapid fault isolation and trouble shooting makes things very simple for a client, using VSAT services. VSATs also enjoy a low mean time to repair (MTTR) of a few hours, which extends upto a few days in the case of leased lines. Essentially, lesser elements imply lower MTTR.

Flexibility
VSAT networks offer enormous expansion capabilities. This feature factors in changes in the business environment and traffic loads that can be easily accommodated on a technology migration path. Additional VSATs can be rapidly installed to support the network expansion to any site, no matter however remote.

Cost
A comparison of costs between a VSAT network and a leased line network reveals that a VSAT network offers significant savings over a two to three years timeframe. This does not take into account the cost of downtime, inclusion of which would result in the VSAT network being much more cost - effective. Pay-by-mile concept in case of leased line sends the costs spiraling upwards. More so if the locations to be linked are dispersed all over the country. Compare this to VSATs where the distance has nothing to do with the cost.Additionally, in case of VSATs, the service charges depend on the bandwidth which is allocated to your network in line with your requirements. Whereas with a leased line you get a dedicated circuit in multiples of 64Kbps whether you need that amount of bandwidth or not.
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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.

Topologies
The network of VSATs at different locations adopts different topologies depending on the end applications traffic flow requirements. These topologies could be Star or Mesh.

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.

Access Technologies
The primary objective and advantage of these networks is to maximise the use of common satellite and other resources amongst all VSAT sites. The method by which these networks optimise the use of satellite capacity, and spectrum utilisation in a flexible and cost effective manner are referred to as satellite access schemes. Each of the above topologies is associated with an appropriate satellite access scheme. The most commonly used satellite access schemes are:

  • Time Division Multiple Access(TDMA)
  • Frequency Division Multiple Access(FDMA)
  • Code Division Multiple Access(CDMA)
  • Demand Assigned Multiple Access(DAMA)
  • Pre-Assigned Multiple Access(PAMA)
  • Frequency-Time Division Multiple Access(FTDMA)
These technologies are explained in another article VSAT access technologies.

Space Segment Support
The ideal orbit for a communications satellite is geostationary , or motionless relative to the ground. Satellites used for communications are almost exclusively in the geostationary orbit, located at 36000 km 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 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 :

  • Signal Reception - it receives the signal uplinked by a VSAT and/or hub
  • Frequency Translation - the frequency of the received signal is translated to a different frequency, known as the downlink frequency. The 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 satellites have typically 12 / 18 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 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.