VSAT stands for "Very Small Aperture Terminal;" it refers to receive/transmit terminals, installed at remote sites and connecting to a central hub via satellite using small diameter antenna dishes (0.75 to 3.8 meter).

VSAT networks offer value-added satellite-based services capable of supporting the Internet, data, LAN, and voice/fax communications, and can provide powerful, dependable private and public network communications solutions

VSATs are used for a wide variety of telecommunications applications, including Internet/Intranet access, corporate networks, SCADA/Line Monitoring, Environmental Monitoring, Seismic Monitoring,

Remote Video Monitoring, rural telecoms, distance learning, telemedicine, disaster recovery, ship-board communications, transportable "fly-away" systems, and much more. It’s becoming increasingly popular, because they are a single, flexible communications platform that can be installed quickly and cost effectively to provide telecoms solutions for consumers, governments and corporations.

 VSAT Features

• Ability to target small-dish audiences from space and meet specialized service requirements
• High powered, fully steerable Ku-band spot beams
• Applications include POS, banking, SCADA, LAN/WAN networking, Internet/intranet, video conferencing, remote site networking
• End users with a 90 cm to 120 cm dish can download Internet data 20 times faster than PSTN
• Large in-orbit capacity makes VSAT platforms cost-effective
• Star, multi-star, mesh wideband and remote solutions
• Solutions using all the most widely used VSAT equipment

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 customized to meet specific requirements

VSAT COMPONENTS

ODU (Outdoor Unit): Consists of the dish/reflector antenna and Feed horn which contains either an LNB for receive only systems or a LNB for bi-directional systems, which amplify the signal received by the unit. Other electronic components consist of a Transmit Reject Filter, Up/Down Converter (Transceiver), High Frequency Power Amplifier, and Transmit Type N Transition. These components are either packaged separately or packaged together in a single unit, depending on the manufacturer.

IDU (Indoor Unit): Consists of the electronic interface that allows connection to the network. It provides the interface
 

Frequency Bands

• Generally VSAT operates on either Ku-band, or C-band.
• Ku-band based networks are used primarily in North America, Europe and the Middle East.
• C-band, used extensively in Asia, Africa and Latin America.
• Ku-band generally requires a smaller antenna diameter than C-band which generally requires a larger diameter antenna
   

Types of Operations

Bi-Directional Operation - The dish both spends (uplinks) and receives (downlinks) information.
Receive-Only Operation - The dish receives (downlinks) information only.
 

VSAT Networks

There are basically five types of networks employed in VSAT operations.

-Multipoint Network:
The Multipoint Network Structure is used in data oriented networks that require voice. The network structure provides for two-way data, voice and multi-media operations. The network configuration is a star type network that connects one or more main sites to various remote sites. It employs a central hub station and a communication satellite. Each main site uplinks and downlinks from the central hub through terrestrial (land) links. Each remote site can only uplink to the hub. If a remote site has data to send to any other location, it must first pass the information to the hub, and then it can be routed to its final destination.

This type of network is very flexible, supporting multiple interfaces available for LAN, Voice and Data connections and can support numerous transport protocols. The network employs TDMS (Time Division Multiple Access) as the means to send data to each remote site. This provides a secure means of transporting data, as each data packet contains the specific address of the station that it is addressed to and only that station can receive the data and pass it on to its network.

-Full-Meshed Network:
For Voice oriented networks a Full-Meshed Designed Network is utilized. In a Full-Meshed there is no central hub and each site can communicate directly with all network nodes. It employs single satellite hops between each network nodes, which enables superior voice quality, and efficient fast-response data connections. Since the network does not employ a central hub, each station requires increased processing and transmission power. Each remote must uplink and downlink with other remote sites with exactly the same size antenna over the same radio frequency.

In this network one site on the network must provide a Network Control System. It is responsible for setting up calls between the sites and monitors the traffic and bandwidth use. From this site all operating statistics are generated and billing information is produced.  The Network Control System established the allocation of bandwidth as each connection is established through bandwidth-on-demand, that is as one node initiates a call to another node, the Network Control System demands a certain amount of bandwidth from the bandwidth pool and then assigns it to the two nodes for the duration of the communication . Once a call is terminated, the bandwidth is then free, and is back available from the bandwidth pool. This type of network is capable of providing interconnection of dissimilar communication devices. It provides the capability of providing connections between must all currently available communication devices. The system can provide a connection for an individual handset, PBX (Public Branch Exchange) or can provide a gateway system.

-Hybrid Voice and Data Network:
This type of network is a hybrid between the Multipoint and Full-Meshed network. Like the Full-Meshed Network it does not contain a central hub, though it provides all remote sites with the capability to communicate direct with all other sites . It provides the ability to offer voice, fax, and videoconferencing opportunities between the sites. Data and video can be broadcast is bi-directional between the central hub and the remote site. This system takes separate voice and data interfaces at the remote site and then processes this and combines them onto a single cable connecting to the antenna. The remote HES VSAT acts as a switch routing all connections to their appropriate destination. The system is capable of a wide-range of advance telephony and PBX voice services and provides an efficient LAN/WAN network service.
 

-Single Channel Per Carrier (SCPC) - Point-to-Point:
SCPC circuits are point-to-point circuits that provide two-way communication between VSAT terminals located at two sites. It is a very flexible system and has the capability to handle multiple data types over multiple protocols. The system can handle voice, fax, and video application. the data rates range from 9.6 kbps up to 8.4 Mbs. It is ideally suited for bringing Internet to remote ISP sites. Internet access is provided through one satellite dish connected to the Internet through a point-to-point line and transmission is accomplished between it and another terminal at the remote ISP through the use of a satellite. This system is capable of both asymmetric and well as symmetric connections.

-Broadcast Networks:
Broadcast Networks provide for the transmission of data, video and audio files to any number of users. The system "broadcasts" from the central site to the end user remote sites. This one-way system provides an uplink from the central site and each remote site downlinks (receives) only. It provides high speed channel that is capable of up to 24Mbs up linked, providing this information to numerous units.
 

VSAT Network illustration

This is how a star data, TDM/TDMA VSAT network works using a hub station, usually six meters or more in size and small VSAT antennas (between 0.75 to 3.8 meters). All the channels are shared and the remote terminals are online, offering fast response times. Consequently, TDM/TDMA systems are comparable with terrestrial X.25 or frame relay connections.
 

However, mesh networks which use capacity on a demand assigned multiple access (DAMA) basis take a different approach. The master control station merely acts as a controller and facilitator rather than a hub through which traffic passes as in a star network. However, these connections take a little time to set-up and thus, mesh/DAMA systems are often equated to a terrestrial dial-up connection.

There are also mesh systems which use a TDMA access scheme where all of the terminals in a network receive and transmit to the same channel, selecting different time slots because each terminal is aware of what the others have reserved. In the past this type of system has been costly and therefore, reserved for large scale trunking applications, but, more recently, costs have come down considerably and now they can be cost competitive with SCPC/DAMA systems for thin route applications as well.

Point-to-point SCPC (single channel per carrier) links are the satellite equivalent of a terrestrial leased line connection. They are usually set-up on a permanent, 24 hour basis and are thus more costly in satellite capacity and less efficient if not used all the time. However, they do support high bandwidths (typically from 9.6 kbps to 2 Mbps) and can easily be used to carry data, voice and even video traffic.
 

All other systems are usually a variation on one of the themes described above, either in a star, mesh or hybrid (star and mesh) configuration. Most of the TDM/TDMA manufacturers also offer a mesh product which can be deployed in a hybrid-ised configuration, sharing common components such as antennas and RF units, at a remote site.