|What is a VSAT?|
In our satellite basics discussion, we’ve now described what communication satellites are, but how does a satellite network work? A primary method for providing Wide Area Networks (WANs) to governments and businesses via satellites is something called a VSAT network.
WHAT IS A VSAT?
A Very Small Aperture Terminal (VSAT) is a device, known as an earth station, that is used to receive satellite transmissions.
The "very small" component of the VSAT acronym refers to the size of the VSAT dish antenna, typically about 0.55-1.2 m (2 to 4 feet ) in diameter, that is mounted on a roof or wall, or is placed on the ground. That size is appropriate for Ku band communications which, as mentioned in “What is Satellite Communications”, is most used for current systems. A slightly larger antenna would be needed for C-band communications, 1.8 m (4 ft).
The antenna, along with the attached low-noise block converter or LNB (which amplifies the received satellite signals) and the transmitter (which sends signals) make up the VSAT outdoor unit (ODU), one of the two components of a VSAT.
The second component of the VSAT is the indoor unit (IDU). The indoor unit is a small desktop appliance that converts between satellite analog communications and appropriate protocols for local devices such as telephones, computer networks, PCs, TVs, kiosks, etc. On top of basic conversion routines, IDUs can also contain value-added functionality such as security, network acceleration and other features. The indoor unit is connected to the outdoor unit with a pair of cables.
The key advantage of a VSAT earth station, versus a typical terrestrial network connection, is that VSATs are not limited by the reach of buried cable. A VSAT earth station can be placed anywhere — as long as it has an unobstructed view of the satellite. VSATs are capable of sending and receiving all sorts of video, data and audio content at the same high speed regardless of their distance from terrestrial switching offices and infrastructure.
HOW DOES A VSAT NETWORK WORK?
A VSAT network has three components:
Content typically originates at the hub. It is also where equipment and software used to control the satellite network are located. The hub is usually connected to the main communications network, whether a big city PSTN, a company’s central computing network or the internet backbone.
The most obvious component of the hub is a very large, 4,5-11 m (15-36 ft ), antenna. The indoor components include multiple devices that control two-way communications through the antenna, conversions between satellite and terrestrial protocols, and other technical matters. A Network Management System (NMS) server controls the operation of all the devices, as well as allocates communications priorities to applications depending on customer defined rules for quality of service.
The VSATs, as already described, are the devices used in remote locations to provide communications to the central site through the network hub.
In the simplest configuration, outbound information (from the hub to the VSATs) is sent up to the communications satellite's transponder, which receives it, amplifies it and beams it back to earth for reception by the remote VSATs. The remote VSATs send information inbound (from the VSATs to the hub) via the same satellite transponder to the hub station.
This arrangement, where all network communication passes through the network's hub processor, is called a "star" configuration, with the hub station at the center of the star. One major advantage of this configuration is that there is virtually no limit on the number of remote VSATs that can be connected to the hub.
As just described, the star topology is the simplest way to configure a satellite network. However, it has one issue that affects performance. Remember that a satellite in geostationary orbit is 35 400 km above the earth. That means transmission time matters. Because of the distance, sending a bit from one location through a satellite to another location (a single “hop”) averages around a quarter of a second. If communications is going from one VSAT to another, the star topology requires two hops, with a half second delay.
That time delay doesn’t typically matter when sending data between two computers, to update a database for instance. In addition, the star topology allows VSATs to use smaller antennas and lower power transmitters, since they’re communicating only with the large hub antenna.
However, star topology delay can become noticeable for voice communications. Therefore, the star topology is best when communications is primarily between a central system and remote locations in a single hop, or when VSAT to VSAT communications doesn’t require immediate response.
Mesh topologies provide the ability for VSATs to communicate directly to other VSATs, minimizing delay for distributed communications. That means, for example, a telephone conversation between people on VSAT connected telephones only has a single hop, a delay not noticeable to most people. Mesh IP supports single hop performance for computer applications such as client/server software which require frequent two-way connectivity between computers at remote locations. The trade-off is the requirement for a larger antenna and a higher powered transmitter at the VSAT, both of which increase costs.
Multi-Star topologies provide a mix of star and mesh solutions, where the hub sends information to the VSATs, but VSATs are also capable of direct, mesh connections. That provides the capability, for instance, of a VoIP phone on one VSAT to communicate directly with a PSTN network accessible via a second VSAT. In another example, a corporate server for an international company can send database updates from the hub to national headquarters behind one VSAT, which can then forward the information on to regional offices.
Because of the trade-offs in price and performance, cost-benefit analysis must always be performed in order to understand the appropriate topology necessary for each site in order to design the appropriate network topology for your needs.