Very small aperture terminal
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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 Vizada or Eutelsat services) communications.
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[edit] Usage
The first commercial VSATs were C band (6 GHz) receive-only systems by Equatorial Communications using spread spectrum technology. More than 30,000 60 cm antenna systems were sold in the early 1980s. Equatorial later developed a C band (4/6 GHz) 2 way system using 1 m x 0.5 m antennas and sold about 10,000 units in 1984-85.
In 1985, Schlumberger Oilfield Research co-developed the world's first Ku band (12-14 GHz) VSATs with Hughes Aerospace to provide portable network connectivity for oil field drilling and exploration units. Ku Band VSATs make up the vast majority of sites in use today for data or telephony applications.
The largest VSAT network (more than 12,000 sites) was deployed by Spacenet and MCI for the US Postal Service. Other large VSAT network users include Walgreens Pharmacy, Dollar General, Wal-Mart, CVS, Riteaid, Yum! Brands (Taco Bell, Pizza Hut, Long John Silver's and other Quick Service Restaurant chains),Intralot, GTECH and SGI for lottery terminals. VSATs are used by car dealerships affiliated with manufacturers such as Ford and General Motors for transmitting and receiving sales figures and orders, as well as for receiving internal communications, service bulletins, and interactive distance learning courses from manufacturers. The FordStar network, used by Ford and its local dealers, is an example of this.
VSAT technology is also used for two-way satellite Internet providers such as HughesNet, StarBand and WildBlue in the United States; and Bluestream, SatLynx and Technologie Satelitarne in Europe, among others. These services are used across the world as a means of delivering broadband Internet access to locations which cannot get less expensive broadband connections such as ADSL or cable internet access; usually remote or rural locations.
Nearly all VSAT systems are now based on IP, with a very broad spectrum of applications. As of December 2004, the total number of VSATs ordered stood at over 1 million, with nearly 650,000 in service. Annual VSAT service revenues were $3.88 billion (source: www.comsys.co.uk).
[edit] Configurations
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.
[edit] Spectrum allocation and characteristics
| 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 |
[edit] 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.[1]
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.[2] 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.
[edit] Pros and cons of VSAT networks
Advantages
- Availability: VSAT services can be deployed anywhere having a clear view of the Clarke Belt
- Diversity: VSAT provides a wireless link completely independent of the local terrestrial/wireline infrastructure - especially important for backup or disaster recovery services
- Deployability: VSAT services can be deployed in hours or even minutes (with auto-acquisition antennas)
- Homogeneity: VSAT enables customers to get the same speeds and service level agreements at all locations across their entire network regardless of location
- Acceleration: Most modern VSAT systems use onboard acceleration of protocols such as TCP ("spoofing" of acknowledgement packets) and HTTP (pre-fetching of recognized HTTP objects); this delivers high-quality Internet performance regardless of latency (see below)
- Multicast: Most current VSAT systems use a broadcast download scheme (such as DVB-S) which enables them to deliver the same content to tens or thousands of locations simultaneously at no additional cost
- Security: Corporate-grade VSAT networks are private layer-2 networks over the air
Disadvantages
- Latency: Since they relay signals off a satellite in geosynchronous orbit 22,300 miles above the Earth, VSAT links are subject to a minimum latency of approximately 500 milliseconds round-trip. This makes them a poor choice for "chatty" protocols or applications such as online gaming
- Encryption: The acceleration schemes used by most VSAT systems rely upon the ability to see a packet's source/destination and contents; packets encrypted via VPN defeat this acceleration and perform slower than other network traffic
- Environmental concerns: VSATs are subject to signal attenuation due to weather ("Rain Fade"); the effect is typically far less than that experienced by one-way TV systems (such as DirecTV, DISH Network or British Sky Broadcasting) that use smaller dishes, but is still a function of antenna size and transmitter power and frequency band
- Installation: VSAT services require an outdoor antenna installation with a clear view of the sky (southern sky if the location is in the northern hemisphere or northern sky if the location is in the southern hemisphere); this makes installation in skyscraper urban environments or locations where a customer does not have "roof rights" problematic
[edit] Future applications
Advances in technology have dramatically improved the price/performance equation of FSS (Fixed Service Satellite) over the past five years. New VSAT systems are coming online using Ka band technology that promise higher bandwidth rates for lower costs.
FSS satellite systems currently in orbit have a huge capacity with a relatively low price structure. FSS satellite systems provide various applications for subscribers, including: phone conversations; fax; TV broadcast; high speed communication services; Internet access; video conferencing; Satellite News Gathering (SNG); Digital Audio Broadcasting (DAB) and others. These systems are applicable for providing various high-quality services because they create efficient communication systems, both for residential and business users.
[edit] Constituent parts of a VSAT antenna configuration
- Block upconverter (BUC)
- Low-noise block converter (LNB)
- Orthomode transducer (OMT)
[edit] Training
Modern VSAT systems are a prime example of convergence, and hence require skills from both the RF and IP domains. VSAT specific training includes:
[edit] References
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This article does not cite any references or sources. (June 2007) Please help improve this article by adding citations to reliable sources. Unverifiable material may be challenged and removed. |
- ^ http://www.comsys.co.uk/vsatnets.htm VSAT Network FAQ
- ^ http://www.wildblue.com/aboutWildblue/how_it_works_demo.jsp WildBlue spot beaming
