VSAT

VSAT

Definition:

A Very Small Aperture Terminal (VSAT), is a two-way satellite ground station with a dish antenna that is smaller than 3 meters (most VSAT antennas range from 75 cm to 1.2 m). VSAT data rates typically range from narrowband up to 4 Mbit/s. VSATs access satellites in geosynchronous orbit to relay data from small remote earth stations (terminals) to other terminals (in mesh configurations) or master earth station "hubs" (in star configurations).

VSATs are most commonly used to transmit narrowband data (point of sale transactions such as credit card, polling or RFID data; or SCADA), or broadband data (for the provision of Satellite Internet access to remote locations, VoIP or video). VSATs are also used for transportable, on-the-move (with phased-array antennas) or mobile maritime (such as Inmarsat or BGAN) communications.

Most VSAT networks are configured in one of these topologies:

·         A star topology, using a central uplink site, such as a network operations center (NOC), to transport data back and forth to each VSAT terminal via satellite,

• A mesh topology, where each VSAT terminal relays data via satellite to another terminal by acting as a hub, minimizing the need for a centralized uplink site,
• A combination of both star and mesh topologies. Some VSAT networks are configured by having several centralized uplink sites (and VSAT terminals stemming from it) connected in a multi-star topology with each star (and each terminal in each star) connected to each other in a mesh topology. Others configured in only a single star topology sometimes will have each terminal connected to each other as well, resulting in each terminal acting as a central hub. These configurations are utilized to minimize the overall cost of the network, and to alleviate the amount of data that has to be relayed through a central uplink site (or sites) of a star or multi-star network.

Star topology services like HughesNet, Spacenet Connexstar/StarBand, WildBlue and others can be used to provide broadband wide area networks, as well as to provide broadband Internet access. Applications of this include intranet networking for front and back office applications, managed store and forward solutions such as digital signage, and interactive distance learning.

Configuration:

VSAT Frequency Spectrum Allocation This table acts as a guide only.
Band	Frequency GHz	Area Foot-print	Delivered Power	Rainfall effect
Band C	3 to 7	Large	Low	Minimum
Band Ku	10 to 18	Medium	Medium	Moderate
Band Ka	18 to 31	Small	High	Severe

Technology:

VSAT was originally intended for sporadic store-and-forward data communications but has evolved into real-time internet services. VSAT uses existing satellite broadcasting technology with higher powered components and antennas manufactured with higher precision than conventional satellite television systems. The satellite antenna at the customer's location includes, in addition to the receiver, a relatively high-powered transmitter that sends a signal back to the originating satellite. A very small portion of a transponder is used for each VSAT return path channel. Each VSAT terminal is assigned a frequency for the return path which it shares with other VSAT terminals using a shared transmission scheme such as time division multiple access.

An innovative feature of VSAT is that the technology has evolved to the point that something that previously could only be done with large, high-powered transmitting satellite dishes can now be done with a much smaller and vastly lower-powered antenna at the customer's premises. In addition, several return-path channels can co-exist on a single satellite transponder, and each of these return-path channels is further subdivided using to serve multiple customers.

In the system used by WildBlue, 31 different spot beams are used to serve the continental United States instead of the one beam used by conventional satellites. Thus, the same Ka-band transponders and frequencies are used for different regions throughout the United States, effectively re-using the same bandwidth in different regions.

The return path is transmitted from the customer's receiver in the L-band to a device called a low-noise block upconverter. There it is converted into the much higher frequency satellite transmission frequency, such as Ku-band and Ka-band, and amplified. Finally the signal is emitted to the dish antenna which focuses the signal into a beam that approximately covers the satellite with its beam. Because the transmission cannot be precise in these smaller dishes there is some effort to use frequencies for the uplink that are not used by adjacent satellites otherwise interference can occur to those other satellites.

Another satellite communications innovation, also used by satellite trucks for video transmission, is that only a small portion of a single satellite transponder is used by each VSAT channel. Previously a single transponder was required for a single customer but now several customers can use one transponder for the return path. This is in addition to time-based subdivision.

Peer to Peer Network

2.    Peer to Peer Network [2012]:
In its simplest form, a peer-to-peer (P2P) network is created when two or more PCs are connected and share resources without going through a separate server computer. A P2P network can be an ad hoc connection—a couple of computers connected via a Universal Serial Bus to transfer files. A P2P network also can be a permanent infrastructure that links a half-dozen computers in a small office over copper wires. Or a P2P network can be a network on a much grander scale in which special protocols and applications set up direct relationships among users over the Internet.
The initial use of P2P networks in business followed the deployment in the early 1980s of free-standing PCs. In contrast to the minimainframes of the day, such as the VS system from Wang Laboratories Inc., which served up word processing and other applications to dumb terminals from a central computer and stored files on a central hard drive, the then-new PCs had self-contained hard drives and built-in CPUs. The smart boxes also had onboard applications, which meant they could be deployed to desktops and be useful without an umbilical cord linking them to a mainframe.
Many workers felt liberated by having dedicated PCs on their desktops. But soon they needed a way to share files and printers. The obvious solution was to save files to a floppy disk and carry the disk to the intended recipient or send it by interoffice mail.
Sneaker Nets
That practice resulted in the term "sneaker net." The most frequent endpoint of a typical sneaker net was the worker who had a printer connected to his machine.
While sneaker nets seemed an odd mix of the newest technology and the oldest form of transportation, the model is really the basis for today's small P2P workgroups.
Whereas earlier centralized computing models and today's client/server systems are generally considered controlled environments in which individuals use their PCs in ways determined by a higher authority, a classic P2P workgroup network is all about openly sharing files and devices.
In general, office and home P2P networks operate over Ethernet (10M bit/sec.) or Fast Ethernet (100M bit/sec.) and employ a hub-and-spoke topology. Category 5 (twisted-pair) copper wire runs among the PCs and an Ethernet hub or switch, enabling users of those networked PCs access to one another's hard drives, printers or perhaps a shared Internet connection.

Both Client and Server
In effect, every connected PC is at once a server and a client. There's no special network operating system residing on a robust machine that supports special server-side applications like directory services (specialized databases that control who has access to what).
In a P2P environment, access rights are governed by setting sharing permissions on individual machines.
For example, if User A's PC is connected to a printer that User B wants to access, User A must set his machine to allow (share) access to the printer. Similarly, if User B wants to have access to a folder or file, or even a complete hard drive, on User A's PC, User A must enable file sharing on his PC. Access to folders and printers on an office P2P network can be further controlled by assigning passwords to those resources.

Navigating a P2P Network
This diagram shows how a P2P network operates. The solid lines indicate physical, hard-wired network cables. The dotted lines indicate that each PC can communicate and share files with every other PC on such a network. A printer attached to one PC can be  used by other PCs on the network—if that printer’s PC allows such use.