Very Small Aperture Terminal Computer Science Essay

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In today's era, satellites have become a vital part of global communication. They bring a lot of services that benefit us. They let us to have radio and cable television. They allow us to use mobile phones for communicating. Besides that, today, we are using Global Positioning System (GPS) is just because of satellites. It lets us to know exactly where we are and we can find the destination we want to go. Our government also uses them to spy on other countries in order to protect us. Weather can be predicted through satellites too, this is very useful for fishermen and farmers, whose their lives are dependent on weather. Without satellites, fishermen may risk their lives in the middle of sea if they do not know the weather of that day. Satellite helps in exploring the space and even our earth. This service helps for space research. Other than that, there are still many services that are using satellites to work. These all get possible due to the satellite rotating around the earth in different orbits. All of the services above are valuable to us.

Satellites have brought a great change in our life. That is why we are interested in this topic. Next, we are going to talk about one of the satellite technologies, which is VSAT (Very Small Aperture Terminal).

What is VSAT? VSAT is an abbreviation for a Very Small Aperture Terminal. A VSAT is a two-way satellite ground station. A class of very small aperture (typically 0.5m to 2.4m), intelligent satellite earth station suitable for easy on-premise installation that is usually operating in conjunction with a larger central hub. The class can be said as a VSAT too. A VSAT terminal transmits and receives data via satellites. VSATs that are called as TSAT. It is because they handle the T1 data rate which is up to 1.544 Mbits per second.

An automated teller machine (ATM) is an electronic machine that eases financial transactions. ATMs are often called as automated banking machine, cash machine, bank machine, money machine, hole-in-the-wall, and others. The ATM does not need a human clerk or bank employee to operate it, bank customers or users can access their accounts by using a debit card with a magnetic stripe or a plastic smartcard with a chip that contains a unique card number and entering a PIN (Personal Identification Number) code.

The ATM can be used to verify your bank account balance, to deposit cash or cheques in your bank account, to transfer money between accounts, to withdraw cash or to pay bills as well as purchasing mobile phone prepaid credit from the account. ATMs are located in outside of bank branches, or in popular locations like shopping malls, petrol station, airports and others. ATM machines are very convenient nowadays because they can be accessed in anytime and anywhere.

In the rest of this report, we are going to talk about the history of VSAT and ATM, VSAT system components, VSAT network architecture and how VSAT and ATM work together.

VSAT System Components

There are various components of a typical VSAT network. There are seven components which are antenna, high-power amplifiers, solid state power amplifier, low-noise converters, modem and codecs, up/down converter and network interface unit.

Firstly, the main component is antenna. The size and cost of the antenna are the main differences between the hub and the VSAT. Normally, the typical C- or Ku- band hub terminal has an antenna with diameter between 5.6m and 11m and it costs between $300,000 and $5,000,000. VSATs can be characterized by much smaller antennas, typically less than 2m in diameter, and cost less than $12,000.

Next main component is high-power amplifier. One of the high-power amplifier is Traveling Wave Tube Amplifier (TWTA). It can release output levels of several hundred watts. It is also capable of being tuned across an individual satellite uplink band. Therefore, it is generally used in hub earth stations. Klystrone are used for single frequency uplinks, e.g., TV.

Solid state power amplifiers are used for providing output powers of 50W to 100W in C-band and 20W in Ku-band and they are taking advantage of recent improvements in semiconductor technology. For final-stage amplification in VSAT terminals, these power levels are good to use.

Besides that, low-noise converters are used to amplify and down convert the received signal while minimizing the added noise. It is very vital to make that no spurious signals are generated and that phase noise in the oscillators is kept to acceptable low levels for data transmission.

Up/Down Converters are a part of main components too. The function of the up converters is to translate the signal from an intermediate frequency which is typically 70 MHz to a microwave signal [6GHz (C-band), 14 GHz (Ku-band)]. The function of down converters is used for the reverse operation that is translating the received microwave signal [4 GHz (C-band), 12 GHz (Ku-band)] to a lower intermediate frequency.

Furthermore, modems and codecs are important in VSAT network too. Encoders and decoders are built into a single codec unit (modem). Normally, a modulator converts a digital signal into a Phase Shift Keying (PSK) signal and the demodulator reverses the process.

Last but not least, Network Interface Unit (NIU) sometimes is called as network interface device. Usually, it is used to implement user protocol interface and access to the satellite.

VSAT Network Architecture

VSAT uses radio frequency links via satellite to connect. Those links are radio frequency links with an is-called 'uplink' and a so-called 'downlink' Uplink means the link from the station to the satellite. Downlink means the link from the satellite to the station. Sometimes we also call the overall links as hop which consists of an uplink and a downlink. Basically, a radio frequency link is a modulated carrier transporting information. First, the satellite receives the uplinked carriers from earth stations. Then it amplifies those carriers and changes their frequency to a lower band with the purpose of preventing output or input interference to the station.

Nowadays, VSAT networks use geostationary satellites and carriers can be relayed by the satellite from VSAT to any other VSATs in the network. Regarding meshed VSAT networks, there are some limitations. First, the 200dB carrier power attenuation on the uplink and the downlink as a result of the distance to and from a geostationary satellite, limited satellite radio frequency power which is a few tens of watts and the small size of the VSAT, which limits receiving sensitivity and transmitted power. Therefore, direct links from VSAT to VSAT are not allowable. The only way to solve this problem is to install the network into a station larger than a VSAT, called hub. The hub station is able to receive all carriers transmitted by the VSATs and to transport the information to all VSATs.

There are two types of VSAT network architecture which are star shaped network and meshed network. Star shaped network is the most commonly used architecture. It uses central hub stations and VSATs. This architecture is usually worked for both point-to-multi-point and multi-point-to-point communication services. However, there is a delay of 540ms. This is because the complete communication has to go through the central hub stations and VSATs.

Next page, there are some pictures of the VSATs network which are star shaped network and meshed network.

There are two types of star shaped VSAT networks. The first one is one-way network (Figure 1.6). The hub transmits carriers to VSATs which are used to receive only. This is used to supports the broadcasting services from the central site where the hub is located. Next is a two-way network (Figure1.7), where VSATs can transmit and receive the data. This kind of networks supports interactive traffic and so on.

The two-way connectivity between VSATs can be achieved in two ways:

Point-to-point between two VSATs via satellite (Figure 1.5)

or by double hob links via satellite in a star shaped VSAT network (Figure 1.7)

In conclusion, star shaped network are forced by power requirements resulting from the reduced size and hence the low cost of the VSAT earth station in the conjunction with power limitation of the satellite. Meshed networks have the advantage compared to star shaped network which can reduce propagation delay. Single hop delay is 0.25s instead of 0.5s for double hop.

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