Short for very small aperture terminal, an earthbound station used in satellite communications of data, voice and video signals. A VSAT consists of two parts, a transceiver that is placed outdoors in direct line of sight to the satellite and a device that is placed indoors to interface the transceiver with the end user's communications device, such as a PC. The transceiver receives or sends a signal to a satellite transponder in the sky. The satellite sends and receives signals from a ground station computer that acts as a hub for the system. Each end user is interconnected with the hub station via the satellite, forming a star topology. The hub controls the entire operation of the network. For one end user to communicate with another, each transmission has to first go to the hub station that then retransmits it via the satellite to the other end user's VSAT. VSAT can handle up to 56 Kbps.

What does VSAT comprise of?

The VSAT comprises of two modules - an outdoor unit and an indoor unit. The outdoor unit consists of an Antenna and Radio Frequency Transceiver. (RFT). The antenna size is typically 1.8 metres or 2.4 metres in diameter, although smaller antennas are also in use. The indoor unit functions as a modem and also interfaces with the end user equipment like stand alone PCs, LANs, Telephones or an EPABX. k3o8og
VSATs can typically be divided into two parts- an outdoor unit and an indoor unit. The outdoor unit is generally ground or even wall mounted and the indoor unit, which is the size of a desktop computer, is normally located near existing computer equipment in your office.
Features:
· Ability to target small-dish audiences and meet specialized service requirements
· Support for applications including LAN/WAN networking, Internet/intranet, video conferencing, remote site networking
· Large in-orbit capacity makes VSAT platforms cost-effective
· Star, multi-star, mesh wideband and remote solutions
Applications:
The great advantage of VSAT is its flexibility. It permits any kind and size of network based around a central hub and remote locations. This makes them particularly pertinent for corporate networks or, for example, communications among educational, government or health-care institutions. Through a VSAT network, a corporation can communicate freely and constantly with branch offices.
· Voice and fax transmissions, Local Area Network interconnection, Data broadcasting , Videoconferencing, In-house trening, Business TV or radio
VSAT Block Diagramm

Interactive Data: Corporate networks, point-of-sale terminal to host, inventory checks, financial data transfer and process control, public networks, database access, inquiry-response, X.25 packet networks.
System control and data distribution: Corporate networks, process control, pipeline monitoring and flow control, resource monitoring and control, electric power grid control.
Broadcast Data: Commercial or corporate networks; transmission at different rates of database information, news, stock market reports, weather reports, etc.
Business Video: Broadcast networks with intra- and inter-company distribution of high quality video, audio and data for news, sales, marketing, training, personnel management.
Advantages of VSAT Technology
· One single network to all sites
· Full availability - all sites on the same network
· Flexible network topology - easy to add, relocate or delete sites
· Transmission costs not distance dependent as with terrestrial networks
· Predictable costs
· One point of contact for all network issues
· More cost-effective than leased or dedicated phone lines to remote locations
· More robust data networks compared to standard telephone lines
· Performance is insensitive to terrain or distance
· Cost-effective emergency back-up for critical data flow
Proactive around-the-clock network support from Network Control Centre, located at Australian Satellite Services. Individual applications can be customised to meet specific requirements.

Why use VSATs?
VSAT networks provide rapid, reliable satellite transmission of data, voice, and video to an unlimited number of geographically dispersed sites or from these sites to headquarters. With a satellite network, there are no routers and no switches - nothing between the user and the source of the information, except the sky. There are no physical limitations in terms of geography or distance to make deployment difficult or too expensive. And since VSAT satellite communication systems often provides a complete end-to-end infrastructure. Furthermore, VSATs have a reliability rate and a network availability that is significantly higher than terrestrial systems. Particularly effective for broadband applications, the high throughput VSATs offer - of 40 Mbps downstream and 76.8 Kbps upstream - means that rich-content material can be delivered live and on-line, or downloaded for later viewing.
VSAT networks offer value-added satellite-based services capable of supporting the Internet, Data, LAN, Voice/Fax communications, Video Conferencing, Distance Learning, and can provide powerful, dependable private and public network communications solutions.

Is there an advantage of VSAT over cable?
When it comes down to cost, making general comparisons between VSAT services and their terrestrial equivalents is almost impossible. Charges for terrestrial services are nearly always distance-dependent, while VSAT connections cost the same whether sites are 1 or 1,000 miles apart. And with most VSAT services, the cost per connection comes down considerably when a customer addes users.

Installation:

Geography:

Bandwidth:

VSAT networks provide an efficient, cost-effective method for reliable distribution of data through a multiple site organization, regardless of location.
VSAT Data Capacity
The indoor data processing unit (DPU) or satellite modem will typically have one or more user etherent ports for connection either direct to PCs or to a local area network router.The number of ports is typically 1 or 4.
If your modem has just one etherent port you can extend it to 4, 8, 16 etc as required using local area network hub devices or switches.
Download bit rate
In the outlink (download) direction from the hub to the VSATs continuous carriers are used, typically more than 256k bit/s and up to 60 Mbit/s. The type of carrier is almost identical with a digital TV carrier called DVB-S. Each VSAT is restricted to extract from the data stream only those packets of data intended for it. One outlink may be shared by 5 to 32,000 VSAT sites, according to the traffic load.
In some cases you can use a DVB-S type receiver to receive only and still extract just what is intended for you. This is the basis for DVB-S/SCPC systems where the VSAT uses a separate transmit modem to transmits a low speed continuous carrier back to he hub. The DVB-S arrangement also applies to so called one-way satellite internet systems that use a DVB-S plug-in card in your PC. In this case the customer sends request packets to the hub using a terrestrial phone line/modem. The benefit is high speed downloads.
Upload bit rate
The data transmission rate on the return link, in the direction from the VSAT to the hub, is typically from a few 100 bit/s to 512 kbit/s. Each VSAT typically transmits in short occasional bursts, interleaved in time. The number of VSAT sites sharing a return link and the number of return links is adjustable to match actual traffic patterns, hopefully without unacceptable congestion. Sharing ratios vary from 5:1 to 60:1 The higher sharing ratios correspond to least cost services, suitable for the occasional undemanding user. Shared systems are vulnerable to greedy users who may quickly overload the network.
Where reliable return link bit rates are required such as for business, cyber cafe or VoIP applications a continuous return link carrier is used (SCPC = Single channel per carrier). It is also possible to set up a dedicated repeating time slot in a TDMA system which gives the same equivalent result - a continuous guaranteed bit rate for the customer. This has the advantage that any congestion that does occur is fully under the customer's control.
VoIP requires approx 20 kbit/s each way for the duration of the call and most satisfactory results are obtained with dedicated capacity.
Other arrangements
There are many further possibilities. Point to point VSAT networks (2 terminals only) are possible with say 2 Mbit/s each way, carrying a mixture of data and voice traffic for example. Mesh networks are possible, e.g. several hotels with common ownership might share a common pool of satellite capacity and have the options to communicate direct to each other when desired.