Sistemul de comunicatii VSAT (Very Small Aperture Terminal) se bazeaza pe tehnologia wireless satellite. Este format dintr-o statie terestra satelit „mica” si o antena tipica cu diametrul de 1.8 m.

O retea VSAT are trei componente:

1. Statie terestra principala (master earth station) -; centrul de control al retelei. Aici are loc configurarea, monitorizarea si intretinerea retelei. Statia are o antena de 6 m, sistem de alimentare de rezerva, sistem de aer conditionat si este supravegheata 24x7 de un operator uman.

2. Statie terestra secundara (remote earth station) -; echipamentul instalat la locatia clientului. l8o22ok
Componente:
* unitate de exterior ODU (outdoor unit) -; antena standard 1.8 m (offset feed antenna), un amplificator SSPA (solid state), un amplificator cu zgomot mic (LNA low noise amplifier) si un horn (feedhorn).
* unitatea de interior IDU (indoor unit) -; echipamentul electronic de comunicatie, inclusiv interfata cu echipamentul utilizatorului (pc, telefon).
* interfacility link IFL -; cabluri coaxiale ce fac legatura dintre IDU si ODU.

3. Satelitul ! -; Toate semnalele trimise intre statiile terestre VSAT sunt transmise prin satelit. Sistemul VSAT foloseste un satelit geostationar ce orbiteaza la 36,000 km deasupra Pamantului.
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Taking the USA as an example, approximately half of all installed VSATs are only used for one way data links.
ETSI ( "European Telecommunications Standards Institute") takes a different definition for a VSAT as 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.
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.
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 centralised management and data processing. The use of a single high performance hub allows the use of low cost remote VSAT terminals and optimises use of satellite capacity. Even so, in most VSAT networks, the cost of the VSAT terminals usually far exceeds the cost of the hub (typically a VSAT terminal is 0.1 to 0.2% of the price of the hub). In a typical VSAT network, remote user sites have a number of personal computers, dumb terminals and printers connected to the VSAT terminal which connects them to a centralised host computer either at the organisation's head office or data processing centre. Data sent to the VSAT terminal from the DTEs is buffered and transmitted to the hub in packets.

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 centralised 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. Demand assigned SCPC has been virtually abandoned as a transmission scheme for the present.
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.

VSAT Shared Hub Network Configuration

VSAT Mini-Hub Network Configuration

TDM/TDMA Interactive VSAT Networks
All the established interactive hubbed VSAT systems use TDM/TDMA access as the primary access technique (TDM on the outbounds and TDMA on the inbounds).
Network Configuration

Signal Types and Characteristics
The outbound data stream from the hub is transmitted at a relatively high data rate (typically 56 to 1024 kb/s) using TDM. The bit stream consists of a synchronisation word followed by a series of messages in time slots directed towards individual VSAT terminals. Broadcast messages to all remote VSAT terminals are also generally permitted.
Outbounds are transmitted continuously (i.e. duty cycle 100%) as a TDM stream. The number of outbounds per network is determined by the traffic statistics, packet length as well as the outbound data rate.
The outbounds for a network are generally grouped together at either the top or the bottom of the leased bandwidth.
The inbound carrier is often accessed using ALOHA or Slotted ALOHA. If a higher capacity is required, a separate channel can be dedicated to ALOHA or Slotted ALOHA access requests and a demand assigned TDMA access scheme established.
Inbound slotted ALOHA carriers information rates are usually between 2.4 and 16 kb/s. Inbound TDMA or SCPC carriers used for file transfer usually have information data rates between 56 kb/s and 256 kb/s. All carriers are BPSK or QPSK modulated and have rate 1/2 or 2/3 Forward Error Correction (FEC). This ensures that bit error rates are low (typically 10-6 or 10-7 which is comparable to ISDN).
Remote terminals transmit in TDMA bursts in either a pre-assigned inbound channel slot or in any inbound channel slot depending on the manufacturer.
Several different inbound TDMA access systems are used depending on traffic characteristics and the manufacturer.
In a shared hub network, individual customers are often, but not always, allocated one or more dedicated outbounds and several inbounds.
If the traffic mix is a combination of short interactive messages and long file transfers it is often worthwhile to use a technique called Adaptive ALOHA/TDMA. VSATs which have large blocks of data to transmit request dedicated TDMA time slots and use TDMA. The other VSAT terminals in the network use slotted ALOHA and avoid the assigned time slots. Alternatively, dedicated SCPC carriers can be temporarily assigned for file transfer.

Typical Interactive Hubbed VSAT Network Spectrum

Typical Interactive Hubbed VSAT Frame and Packet Format
Each TDM outbound carries a continuously transmitted bitstream which is divided into frames.
The start of a frame is denoted by a framing packet contain a unique word (UW) and a control word (CNTRL) which, together, provide framing, timing and control information.
The rest of the frame is filled by (generally) fixed length data packets which each contain:
F preamble
HDR header - giving IDU address and control information
FCS frame check sequence
F postamble
Outbound data packets typically contain between 50 and 250 bytes in transactional networks.
Each TDMA inbound contains frames which are synchronised to the outbound frames. Each inbound frame is divided into slots. Individual IDUs transmit in these slots in a manner depending on the access modes available to the particular system and how the network has been set up.
Each inbound packet consists of:
F preamble
HDR header - giving IDU address and control information
FCS frame check sequence
F postamble
Inbound data packets typically contain between 50 and 250 bytes in transactional networks.
The main inbound transmission modes used are:
Aloha, in which an IDU can transmit data packets at any time in a particular inbound frequency slot. Transmissions in any particular frequency slot are intermittent with a peak traffic duty cycle of 10 to 15%.
Slotted Aloha, in which an IDU can transmit data packets in any slot (or any of a predetermined number of slots) in a particular inbound frequency slot. Transmissions in any particular frequency slot are intermittent with a peak traffic duty cycle of 25 to 30%.
Fixed Assignment, in which specific time slots in an inbound frequency slot are permanently, or for the duration of a particular transmission, assigned to a particular IDU. This is often used for batch transmission and for telephony. Transmissions in any particular frequency slot are intermittent but can have a peak traffic duty cycle of 100% if that particular inbound is carrying telephony traffic or several batch file transfers from different IDUs.
Dynamic Assignment, in which time slots in an inbound frequency slot are dynamically assigned to a particular IDU in line with ongoing traffic demands. Transmissions in any particular frequency slot are intermittent with a peak traffic duty cycle of from 25 to 30% to approaching 100%, depending on the traffic mix.
Most interactive hubbed VSATs now have protocol stacks which map, at least notionally, onto the OSI stack.
Network layer spoofing is provided by many VSATs to minimise the impact of the data layer protocol and, particularly, the satellite transmission delay, on the throughput of the satellite link.
TDM/TDMA Connection Set Up
When the network is established, or when additional remote terminals are added to the network, remote terminal addresses and characteristics (i.e. card fits and port addresses) are entered into a network database which is used as a routing table by the operational system. This database establishes permanent virtual circuits between ports at the user interface of the hub and the ports at the user interfaces of the remote terminals. In those products which permit the dedication of the assignment of capacity on request, or dynamic variable assignment, the database also establishes permanent virtual circuits between the IDU controllers at the remote terminals and the NCC.
This arrangement allows the normal transactional traffic carried by the network to be switched without an individual call set up procedure.
A packet sent by a particular IDU carries addressing information identifying both the source and destination. This allows the hub switch to route the packet to the correct user interface port without additional signalling traffic.
This same procedure is used for intra network signalling to set up assignments for the temporary or permanent assignment of channels to a particular IDU port/hub port pair (for example, telephony or batch data transfers). Call set up information is sent as a transactional data packet as described above, except that the destination address at the hub is the NCC.