Low cost business terminals with small antennas (generally less than 2 meters in diameter) are often termed Very Small Aperture Terminals (VSATs). These are usually perceived as being two way data terminals, though strictly speaking many of the systems used for data broadcast are really one-way VSATs. 

One definition for a VSAT is; a one or two-way terminal used in a star, mesh or point to point network. Antenna size is restricted to being less than or equal to 3.8 m at Ku band and 7.8 m at C band. A network is considered a VSAT network if it consists of a large high performance hub earth station (with an antenna of up to 9 m in diameter) and a large number of smaller, lower performance terminals. Being completely general, these small terminals can be receive only, transmit only or transmit/receive. 

As terminal technology advances, the size of the antenna required to achieve a particular link quality (bit error rate) decreases. A class of terminals smaller than VSATs is now available; these are termed Ultra Small Aperture Terminals (USATs). For most practical purposes, USATs are just VSATs with smaller antennas. It must always be remembered, however, that as antenna size decreases, the antenna beam widens and that a point is rapidly reached when there is no further advantage in decreasing antenna size because of increased interference with other systems. The practical current lower limit on antenna size is 55 cm diameter.

Typical applications for interactive VSAT networks are:

• computer communications;
• reservation systems;
• database enquiries;
• billing systems;
• file transfers;
• electronic mail;
• video conferencing;
• point of sale transactions;
• credit checks and credit card verification;
• stock control and management.

Overview of Technology

The most common VSAT configuration is the TDM/TDMA star network. These have a high bit rate outbound carrier (TDM) from the hub to the remote earth stations, and one or more low or medium bit rate Time Division Multiple Access (TDMA) inbound carriers.

With its star configuration network architecture, interactive VSAT technology is appropriate for any organisation with centralized management and data processing.

The principle characteristics of an interactive VSAT network are:

• Remote user sites have several low bit rate data terminal equipments (DTEs) operating at 1.2 to 9.6 kb/s. These are connected through the VSAT network to a centralized host processor. The DTEs are connected to the host through an X.25 Packet Assembler/Dissembler (PAD) or through a conventional or statistical multiplexer which concentrates the traffic.
• The amount of data transferred in each transaction is relatively small, typically between 300 and 10⁵ bits. Interactive VSATs are not usually used for batch file transfer (10⁷ to 10¹¹ bits per transaction) unless the transmission plan is specifically designed to carry large files.
• Each VSAT terminal only operates with a low duty cycle, i.e. with only a relatively small number of transactions in the peak busy hour compared to the total available capacity.
• A large number of VSAT terminals (10 to 10000) share the same communications link using random access.
• Connections between remote VSAT terminals require a double hop through the hub and are rarely used.

VSAT networks are designed to be flexible and to evolve with user needs. VSAT terminals are controlled by microprocessors and can generally be reprogrammed remotely using downloaded software from the hub. If additional interfaces or capacity are required this can usually be provided by adding or replacing cards in the VSAT terminal.

Three different transmission schemes are used for interactive hubbed VSAT networks:

• TDM/TDMA
• Demand Assigned SCPC
• CDMA

Of these TDM/TDMA is by far the dominant technique with only CDMA being used to a small extent, SCPC is seldom used.

It is also common for VSAT systems to support one-way TV transmission from the host to the remote stations. Two-way, 2 Mb/s transmissions can also be supported by some VSAT systems.

Shared Hub Networks

To make VSAT networks more affordable it is possible to share the hub between several users, thereby spreading the cost. In this case the hub is usually owned by a service provider who retains overall control of the network and who manages the hub itself.

Each user, however, is allocated his own time slots or carriers and can so operate his own private network using the shared hub facility without any loss of privacy. The operation and management of these subnetworks is performed by the users themselves completely independently of the service supplier.

Mini-Hub Networks

In this configuration, each user has his own "mini-hub" which is much smaller and simpler, and hence cheaper, than a conventional hub. An approximate price for a mini-hub is 250 kEuro. The antenna diameter is typically only 2.4 m. Each user organization has complete control over his own communications. Overall management of the complete network is provided by the service supplier who has a "super hub" which provides network supervision and diagnostic support.

Network Management

Current interactive VSAT networks generally have distributed, rather than a centralized, network management. Multiple points of control and intelligent operator interfaces are common features. The network manager not only has the ability to perform diagnostics on the network, but can also reconfigure the network from his own console. Where multiple consoles are available, the network can be configured, monitored and operated either locally or remotely. In addition, many VSAT network management systems have interfaces available for working with other vendor's network management systems such as IBM's Netview and DEC's EMA.

TDM/TDMA Interactive VSAT Networks

Established interactive hubbed VSAT systems use TDM/TDMA access as the primary access technique (TDM on the outbounds and TDMA on the inbounds).