What does VSAT stand for?
VSAT stands for Very Small Aperture Terminal.

What is a VSAT?
A VSAT is a small-sized telecommunications earth station that transmits and receives via satellite. The terminal size is 1.2 to 2.4 meters in diameter. VSAT stands for Very Small Aperture Terminal - its a catchy acronym and as such its been adopted by all and sundry for every type of satellite product from small components of a system to complete systems. Because the term really hinges around the small size of the antenna it has been used to describe both one-way and interactive systems. Specifically, we in the industry, isolate television broadcast receivers because counting these as well would simply distort the numbers in the marketplace, but data, audio and, to some extent, voice systems are included. Generally, these systems operate in the Ku-band and C-band frequencies. As a rule of thumb C-band (which suffers less from rain attenuation, but requires larger antennas) is used in Asia, Africa and Latin America whilst Ku-band (which can use smaller antennas, but suffers from rain fade in a monsoon-like downpour) is used in Europe and North America. Typically, interactive Ku-band antenna sizes range from 75 centimetres to 1.8 metres and C-band from 1.8 metres to 2.4 metres. One way systems can use antennas as small as 45 centimetres.
 

For what are VSATs used?
VSAT systems generally connect a large number of geographically dispersed sites to a central location. VSAT networks may transmit voice, data, fax, or video conferencing. The VSAT market has been going since the early 1980s and the launch of the first one-way VSAT system by Equatorial of California. Towards 1985 the first interactive star systems began to be seen and it wasn't until 1989 that the first mesh telephony products were really sold.You name it really, car dealerships, gas stations, banks, insurance companies, drug stores, general stores, supermarkets, healthcare companies, manufacturers, couriers, hotel chains, car rental businesses, food manufacturers, heavy industries, mines, electrical utilities, oil and gas pipelines, energy production and exploration, timber companies, plantations, various government departments and agencies ....... any others you can think of, just add to the list.

What are the components in a VSAT site?
A typical VSAT site consists of a parabolic-shaped antenna mounted on the roof of a building, connected by a cable to a chassis inside the building. Operators install these antennas at customer sites and buy transmission capacity on satellites.

What does the VSAT unit contain?
A typical VSAT unit contains a modem for translating satellite transmissions back into data (and vice versa) and terrestrial interfaces for connecting customer equipment.

What is a satellite transponder?
A satellite transponder is a combination receiver, frequency converter, and transmitter package. It is physically part of a communications satellite. Communications satellites typically have 12 to 24 onboard transponders.

What are the typical VSAT network configurations?
VSAT networks can be arranged in point to point, star, mesh, star/mesh, and broadcast configurations. The preferred arrangement depends on the kind of information flow the network will service.VSAT networks come in various shapes and sizes ranging from star data system users with one site connected to an operator's shared hub to many thousands based on a dedicated facility located at their own site. Mesh systems have traditionally been somewhat smaller in size than star systems - 5 to 30 sites used to be a good rule of thumb - but the average size of orders has risen as prices have come down and some rural telephony networks now comprise as many as several hundred or even thousands of sites.

What is a point to point VSAT network?
A point to point network allows two-way communications between two VSAT sites.

What is a star VSAT network?
A star network allows any number of VSAT sites to have two-way communication with a central hub.

What is a mesh VSAT network?
A mesh network allows two-way communications between any VSAT sites in a network. A central hub is not necessary. Each site communicates to another site with a single satellite hop.

What are the different VSAT transmission methods?
There are three basic VSAT transmission types: TDMA, time-division multiple access; DAMA, demand-assigned multiple access; and SPCP/MCPC, single/multiple channels per carrier. Interactive VSAT systems come in two main network topologies - star and mesh. The former tends to be based either on a shared access scheme (TDM/TDMA), which is designed to support transactional processing applications, or on a dedicated link (the satellite equivalent to a leased line). The latter usually uses links which are set-up and torn-down on request to establish a direct link between two sites on a demand assigned basis. These mesh systems were initially designed to support corporate and public network telephony links, but are being increasingly used to serve high data rate services, such as file downloads, at rates of 64 kbps or greater.

What are the characteristics of a TDMA transmissions type?
TDMA is a form of multiple access in which a single carrier is shared by many users. When signals from earth stations reach the satellite, they are processed in time segments without overlapping. TDMA is typically used in a packet switched environment when small or moderate amounts of data are to be transferred.

What are the characteristics of a DAMA transmission type?
The DAMA protocol is used to share bandwidth in a time division mode. Typically DAMA transmission is used in a packet-switched environment when large amounts of data are to be transferred. Is a highly efficient means of instantaneously assigning telephony channels in a transponder according to immediate traffic demands. DAMA is also applicable in a circuit-switched environment and is usually characterized by allowing each user a variable slot of time on a demand (or request) basis.

What are the characteristics of a SCPC/MCPC transmission type?
SCPC/MCPC systems use a dedicated satellite link between a few distinct locations. These links can support either a single telephone line or several telephone or data lines. Such links generally are permanently assigned with no carrier switching or rerouting over the satellite.

What is SCPC-DAMA transmission?
SCPC/DAMA systems provide a control network on top of an SCPC network. When a particular station wishes to make a telephone call, the control network is used to forward that request to a central processor that sets up a dedicated SCPC link between the two sites. When the call is finished, the link is taken down and the satellite resources can be used for a different call.

What is TDM-TDMA transmission?
TDM-TDMA networks are designed for interactive data applications. TDM-TDMA systems feature a large expensive hub that provides basic data communications to very inexpensive remote sites. The architecture supports many remote stations using a small amount of satellite bandwidth. Data rates supported at the remote sites are typically between 1.2 kbps and 9.6 kbps; however, this type of traffic has a very low average data rate. Each station may transmit bursts of 9.6 kbps data, but they generally average less than 100 kbps. Typical applications are transactional in nature. Examples include credit card verifications, point of sale systems, SCADA systems, and inventory control.

What is TDMA-DAMA transmission?
TDMA-DAMA networks have the same demand assignment capability as SCPC-DAMA networks, but also have division multiplexing to reduce the need for multiple modems at each site. TDMA-DAMA networks allow many telephone calls to be placed simultaneously to different destinations through a single station.

What is FDMA transmission?
FDMA (Frequency Division Multiple Access) uses a single modem for all communication from a site and transmits for very short interval and at higher rates. FDMA uses multiple carriers within the same transponder within which each uplink has been assigned frequency slot and bandwidth. It is usually used in conjunction with frequency modulation.

What is the advantage of a TDMA-DAMA system?
TDMA-DAMA systems support many telephone lines with very little incremental cost. E1 or T1 interfaces can be provided for direct digital connections to PBXs or telephony switches. TDMA-DAMA systems are also flexible in supporting applications such as data, video conferencing, broadcast, and the like. In addition, TDMA networks are hubless, which eliminates the high cost of a hub and a single point of failure within the network.

What applications do TDMA-DAMA networks best support?
TDMA-DAMA networks support applications with mesh connectivity and applications that require multiple services that are integrated into a single network such as telephony, low to high-speed data imaging, fax, and interactive video conferencing.