Two developments in the twentieth century changed the way people lived: the automobile and telecommunications. Prior to the widespread availability of personal automobiles, individuals had to travel on foot, by bicycle, or on horseback. Trains provided faster travel between cities, but the lives of most people were centered on their hometowns and immediate surroundings. A journey of 100 miles was a major expedition for most people, and the easy mobility that we all take for granted in the twenty-first century was unknown. Before the telegraph and telephone came into widespread use, all communication was face-to-face, or in writing. If you wanted to talk to someone, you had to travel to meet with that person, and travel was slow and arduous. If you wanted to send information, it had to be written down and the papers hand carried to their destination.
Telecommunication systems have now made it possible to communicate with virtually anyone at any time. Early telegraph and telephone systems used copper wire to carry signals over the earth’s surface and across oceans, and high frequency (RF) radio, also commonly called short wave radio, made possible intercontinental telephone links. Artificial earth satellites have been used in communications systems for more than 35 years and have become an essential part of the world’s telecommunications infrastructure. Satellites allow people to talk by telephone and exchange electronic mail from anywhere in the world and to receive hundreds of TV channels in their homes.
The geostationary orbit is preferred for all high capacity communication satellite systems because a satellite in GEO appears to be stationary over a fixed point on the ground. It can establish links to one-third of the earth’s surface using fixed antennas at the earth stations. This is particularly valuable for broadcasting, as a single satellite can serve an entire continent. Direct broadcast satellite television (DBS-TV) and the distribution of video signals for cable television networks are the largest single revenue source for geostationary satellites, accounting for $17 B in revenues in 1998. By year 2001, nearly 200 GEO communication satellites were in orbit, serving every part of the globe. Although television accounts for much of the traffic carried by these satellites, international and regional telephony, data transmission, and Internet access are also important. In the populated parts of the world, the geostationary orbit is filled with satellites every 2° or 3° operating in almost every available frequency band.
(Satellite Communications “Second Edition” – Timothy Pratt, Charles Bostian and Jeremy Allnutt – WILEY)
Today, there are several companies in the world which related to satellite communication. They could lease the satellite’s space segment to their customers. There are several ways to use the satellite services. NetSkies is one of the companies in the world that can provide a good and reliable satellite service to its customers. With NetSkies services you will be able to connect to the Internet with real bandwidth. You can connect to your other sites across the space either. You also can talk and watch through the satellite. There are different types of satellite connectivities in the world that we explain them as follow:
VSAT Network Architectures
l VSATs 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. Most network architectures apply both to one-way and to two-way VSATs.
l A point-to-point link is the simplest form of VSAT connection, consisting, in its most basic form, of a modem and an outdoor unit (ODU) at each end of the link. Point-to-point links are the satellite equivalent of a digital leased terrestrial line. The point-to-multipoint architecture provides a powerful platform for the implementation of video broadcasting services. With regard to cost, satellites are unrivaled when it comes to broadcast TV signals to language regions or continents.
l A star VSAT network consists of a large high performance hub earth station and a large number of smaller, low cost remote VSAT terminals. Meshed, or hub-less, VSAT networks offer a means of establishing a switched point-to-point connection from remote to remote.
Point-to-Multipoint Communication
l The main characteristic of communication systems using satellite technology, compared with traditional wide-area networks, is the ability of broadcasting and multicasting. The point-to-multipoint architecture provides a powerful platform for the implementation of video broadcasting services without the need of user interaction. With regard to cost, satellites are unrivaled when it comes to broadcast TV signals to language regions or continents.
Star Network
l A star VSAT network consists of a large high performance hub earth station and a large number of smaller, low cost remote VSAT terminals. This configuration has been developed to minimize overall lifetime costs for the complete network including satellite capacity.
l In its most basic form, several point-to-point links can form a star network when they focus on a central hub site. At the hub, each link has its own modem. It is also possible to superimpose Demand Assigned (DAMA) control over a star network. The DAMA controller at each site switches the carrier on and off as required by the hub and if necessary may change the uplink frequency. However, today, TDM/TDMA is by far the dominant technique for star networks. A large number of VSAT terminals share the same communications link using random access.
l The hub station is usually a relatively large, high performance earth station with an antenna diameter between 6 and 9 m. The hub also consists of a control center which manages the network (NOC). In contrast to the hub station, remote terminals are much simpler. They feature an outdoor unit with dish antenna, generally 0.75 to 2.4 m in diameter, a BUC and LNB. HPA output powers are usually in the range 1 – 8 W (Ku band). An indoor unit provides the modulation, demodulation, multiplexing, de-multiplexing and synchronization with the rest of the network and supports the user interfaces.
l A star network architecture is appropriate for organizations with centralized management and data processing allowing remote user sites to be connected to a centralized host computer. Typical applications for starred VSAT networks include:
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Reservation Systems
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Database Inquiries
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Electronic Mail
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Video Conferencing
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Point of sale transactions
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Credit Card verification
TDM/TDMA Transmission
l Today, the most common star network transmission scheme for shared bandwidth applications is based on a Time Division Multiplex (TDM) high data rate outbound carrier (from the hub to the remotes; forward channel) and a low or medium data rate Time Division Multiple Access (TDMA) inbound carrier (from the remotes to the hub; return channel).
l A variant on TDMA is multi-frequency TDMA (MF-TDMA), in which several parallel TDMA carriers are used for inbound traffic. This has the advantage that network capacity can be high but each terminal only requires a relatively low data rate transmit capability, hence minimizing cost of remote stations.
l Each inbound and outbound circuit is typically assigned a Committed Information Rate (CIR) and a maximum burstable rate, depending on the application.
Meshed Network
l Meshed or hub-less VSAT networks offer a means of establishing a switched point-to-point data and telephony network. Links are set up directly between remote terminals, often on a call by call basis.
l Although meshed VSAT networks do not have a hub, they generally have a network control station which controls the allocation of satellite resources across the network. The network control center is only involved in the signaling for call set up, and in monitoring the operation of the network.
l Typical applications for meshed VSAT networks include:
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Connections between PABXs
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Voice-over-IP (VoIP) traffic between remote sites
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LAN-LAN interconnections
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Video-Conferencing
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Back-up for terrestrial circuits