Very small aperture terminal
From Wikipedia, the free encyclopedia
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. Data rates typically range from 56 Kbit/s 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 (utilising phased array antennas) or mobile maritime communications.
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[edit] Usage
The first commercial VSATs were C band 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 Hughes Network Systems, Spacenet and MCI for the US Postal Service. Other large VSAT network users in the USA 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, GI Terminal http://gitech.in 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 Hughes Network Systems, StarBand and WildBlue in the United States; and ASTRA2Connect across Europe. 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[update], 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] Technology
[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 36,000 km (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: telephony, fax, television, high speed data communication services, Internet access, 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 configuration
- Antenna
- Block upconverter (BUC)
- Low-noise block converter (LNB)
- Orthomode transducer (OMT)
- Interfacility Link Cable (IFL)
- Indoor unit (IDU)
All the outdoor parts on the dish are collectively called the ODU (Outdoor Unit), i.e. OMT to split signal between BUC and LNB. The IDU is effectively a Modem, usually with ethernet port and 2 x F-connectors for the coax to BUC(Transmit) and from LNB (Receive). The Astra2Connect has an all-in-one OMT/BUC/LNA that looks like a QUAD LNB in shape and size which mounts on a regular TV sat mount. As a consequence it is only 500 mW compared with the normal 2W, thus is poorer in rain.
[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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[edit] External links
- [1]
- Information about Vsat
- Satellite Internet and VSAT Information Centrum
- VSAT FAQ
- The Role of Satellite Comms in Remote Area Mining Operations
- ASTRA2Connect website
- Gilat
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