What is a VSAT?
A very small aperture terminal (VSAT) is a device (known as an earth station) that is used to receive satellite transmissions. The "very small" component of the VSAT acronym refers to the size of the VSAT dish antenna — typically 3 to 6 feet in diameter — which is mounted on a roof on a wall, or placed on the ground. This antenna, along with the attached low-noise blocker or LNB (which receives satellite signals) and the transmitter (which sends signals) make up the VSAT outdoor unit — one of the two components of a VSAT earth station.
The second component of VSAT earth station is the indoor unit. The indoor unit is a small desktop box or PC that contains receiver and transmitter boards and an interface to communicate with the user’s existing in-house equipment — LANs, servers, PCs, TVs, kiosks, etc. The indoor unit is connected to the outdoor unit with a pair of cables. These systems operate in the Ku and C band of the frequency spectrum.
BAND UP-LINK
(GHz) DOWN-LINK(GHz) ISSUES
C
4 (3.7-4.2)
6 (5.925-6.425) Interference with
ground links
Ku
11 (11.7-12.2) 14 (14.0-14.5)
Attenuation due to
Ka
20 (17.7-21.7) 30 (27.5-30.5)
High Equipment cost
Table 1: Frequency spectrum allocation for some common bands used in Satellite networks
A transponder is a combination receiving and transmitting antenna on a communications satellite. A frequency converter is also including in the transmit/receive package which converts the uplinked signal frequency to a transmission or downlink frequency. A typical satellite has about 32 transponders and each transponder is capable of handling approximately 100 million bits per second.
The advantage of a VSAT earth station, versus a typical terrestrial network connection, is that VSATs are not limited by the reach of buried cable. A VSAT earth station can be placed anywhere — as long as it has an unobstructed view of the satellite. VSATs are capable of sending and receiving all sorts of video, data, fax and audio content at the same high speed regardless of their distance from terrestrial switching offices and infrastructure. You can do point-to-multipoint communications at reasonable cost without dialling.
A VSAT network has three components: a central hub (or master earth station), the satellite itself, and a virtually unlimited number of VSAT earth stations in various locations across the country.
Content
originates at the hub, which features a very large (15 to 36-foot) antenna.
The hub controls the network through a network management system (NMS)
server, which allows a network operator to monitor and control all components
of the network. The NMS operator has the ability to view, modify, and download
individual configuration information to the individual VSATs.
Outbound information (from the hub to the VSATs) is sent up to the communications satellite's transponder, which receives it, amplifies it, and beams it back to earth for reception by the remote VSATs. The VSATs at the remote locations send information inbound (from the VSATs to the hub) via the same satellite transponder to the hub station.
Satellite’s true advantage over standard terrestrial networks is derived from its inherent capabilities as a broadcast medium.
Some of the users of such systems include large retailers for card authorization point of sale applications, lottery, petroleum marketers, healthcare (to broadcast updates from FDA and pharmaceutical cos, advertising sponsored TV health programming in waiting rooms), education industries, auto makers, car dealers, banks, insurance companies, drug stores, supermarkets, govt., and financial services firms. These days have IP-based VSAT products which allow the delivery of broadband video and data directly to the LAN or PC. Organizations are using them for software downloads, file transfers, transmission of press agency news items with pictures, and broadcasting paging messages for terrestrial transmission to the pagers themselves, WAN/LAN interconnections.
Operation of VSAT Networks
VSAT networks could be arranged in a star based topology, where each remote user is supported by a VSAT or "mesh" topology where VSAT's can communicate directly without going through the central hub. In star topology, the Earth hub station acts as the central node and employs a large size dish antenna with a high quality transceiver. The satellite provides a broadcast medium acting as a common connection point for all the remote VSAT earth stations.
In star VSAT networks, for two VSATs to communicate, two satellite hops are required, since all connections must pass through the hub ES (earth station) node. Figure below shows how two VSAT terminals can communicate in a star VSAT network.
Figure Communication
between two VSAT terminals
The most common access schemes
used on VSAT's are S-ALOHA (slotted alhoha), TDMA, and DAMA. DAMA is a
method of allocating variable time slots
to users on demand. Idle
channels are kept in a pool; when capacity is requested
an idle channel is allocated
the requested bandwidth and assigned to the user.. The most commonly used
network protocol on VSAT links is X.25. A user can select how much bandwidth
should be leased for each side. In other words, bandwidth on uplink could
be different than bandwidth on the downlink. One would select this asymmetric
configuration if the traffic patterns between VSATs and the central hub
are not the same in the two directions.
Recent advances in satellite
technology have enhanced the functionality of the satellites to provide
full point-point mesh connectivity between VSAT ES with larger bandwidth
in both directions.
