Satellite and VSAT Technology FAQ Frequently Asked Questions A

Frequently Asked Questions


1. What does VSAT stand for?
VSAT stands for Very Small Aperture Terminal

2. What is a VSAT?
VSATs are small earth stations (normally 1.4 - 2.4 meters) that are utilized for reliable transmission of data, voice and fax via satellite. VSAT Terminal Equipment consists of two units - an indoor unit and an outdoor unit.
The outdoor unit is placed outdoors for a line-of-sight to the satellite and the indoor unit interfaces with the user's communications device (e.g. data terminal equipment). The outdoor unit consists of a small antenna, mount and electronics for signal reception and transmission. The indoor unit consists of a satellite modem and demodulators and a network access device to interface with data LANs and PBXs.

3. For what are VSAT is used?
VSAT systems generally connect a large number of geographically dispersed sites to a central location. VSAT networks may transmit voice, data, fax, or video conferencing.

4. What are the components in a VSAT site?
A typical VSAT site consists of a parabolic-shaped antenna mounted on the roof of a building, connected by a cable to a chassis inside the building. Operators install these antennas at customer sites and buy transmission capacity on satellites.

5. What does the VSAT unit contain?
A typical VSAT unit contains a modem for translating satellite transmissions back into data (and vice versa) and terrestrial interfaces for connecting customer equipment.

6. What is a satellite transponder?
A satellite transponder is a combination receiver, frequency converter, and transmitter package. It is physically part of a communications satellite. Communications satellites typically have 12 to 24 onboard transponders.

7. What are the typical VSAT network configurations?
VSAT networks can be arranged in point to point, star, mesh, star/mesh, and broadcast configurations. The preferred arrangement depends on the kind of information flow the network will service.

8. What is a point to point VSAT network?
A point to point network allows two-way communications between two VSAT sites.

9. What is a star VSAT network?
A star network allows any number of VSAT sites to have two-way communication with a central hub.

10.What is a mesh VSAT network?
A mesh network allows two-way communications between any VSAT sites in a network. A central hub is not necessary. Each site communicates to another site with a single satellite hop.

11.What are the different VSAT transmission methods?
There are three basic VSAT transmission types: TDMA, time-division multiple access; DAMA, demand-assigned multiple access; and SPCP/MCPC, single/multiple channels per carrier.

12.What are the characteristics of a TDMA transmissions type?
TDMA is a form of multiple access in which a single carrier is shared by many users. When signals from earth stations reach the satellite, they are processed in time segments without overlapping. TDMA is typically used in a packet switched environment when small or moderate amounts of data are to be transferred.

13.What are the characteristics of a DAMA transmission type?
The DAMA protocol is used to share bandwidth in a time division mode. Typically DAMA transmission is used in a packet-switched environment when large amounts of data are to be transferred. Is a highly efficient means of instantaneously assigning telephony channels in a transponder according to immediate traffic demands. DAMA is also applicable in a circuit-switched environment and is usually characterized by allowing each user a variable slot of time on a demand (or request) basis.

14.What are the characteristics of a SCPC/MCPC transmission type?
SCPC/MCPC systems use a dedicated satellite link between a few distinct locations. These links can support either a single telephone line or several telephone or data lines. Such links generally are permanently assigned with no carrier switching or rerouting over the satellite.

15.What is SCPC-DAMA transmission?
SCPC/DAMA systems provide a control network on top of an SCPC network. When a particular station wishes to make a telephone call, the control network is used to forward that request to a central processor that sets up a dedicated SCPC link between the two sites. When the call is finished, the link is taken down and the satellite resources can be used for a different call.

16. What is TDM-TDMA transmission?
TDM-TDMA networks are designed for interactive data applications. TDM-TDMA systems feature a large expensive hub that provides basic data communications to very inexpensive remote sites. The architecture supports many remote stations using a small amount of satellite bandwidth. Data rates supported at the remote sites are typically between 1.2 kbps and 9.6 kbps; however, this type of traffic has a very low average data rate. Each station may transmit bursts of 9.6 kbps data, but they generally average less than 100 kbps. Typical applications are transactional in nature. Examples include credit card verifications, point of sale systems, SCADA systems, and inventory control.

17. What is TDMA-DAMA transmission?
TDMA-DAMA networks have the same demand assignment capability as SCPC-DAMA networks, but also have division multiplexing to reduce the need for multiple modems at each site. TDMA-DAMA networks allow many telephone calls to be placed simultaneously to different destinations through a single station.

18. What is FDMA transmission?
FDMA (Frequency Division Multiple Access) uses a single modem for all communication from a site and transmits for very short interval and at higher rates. FDMA uses multiple carriers within the same transponder within which each uplink has been assigned frequency slot and bandwidth. It is usually used in conjunction with frequency modulation.

19. What is the advantage of a TDMA-DAMA system?
TDMA-DAMA systems support many telephone lines with very little incremental cost. E1 or T1 interfaces can be provided for direct digital connections to PBXs or telephony switches. TDMA-DAMA systems are also flexible in supporting applications such as data, video conferencing, broadcast, and the like. In addition, TDMA networks are hubless, which eliminates the high cost of a hub and a single point of failure within the network.

20. What applications do TDMA-DAMA networks best support?
TDMA-DAMA networks support applications with mesh connectivity and applications that require multiple services that are integrated into a single network such as telephony, low to high-speed data imaging, fax, and interactive video conferencing.

21. What are the advantages of VSAT?

1. A VSAT Network can be provided through a lease arrangement with fixed transmission costs regardless of distance. This provides for substantial communication cost savings. The user retains complete control of the way information is communicated within the network.

2. This generates flexibility, particularly where new VSAT sites need to be added, or existing sites need to be moved or removed from the network.

3. High availability and excellent transmission quality is provided by the VSAT Network.

4. VSAT Networks also guarantee the highest performance levels (99.5%) among all communication alternatives.

5. VSAT Networks provide fast data transmission for POS applications to improve customer service.

6. VSATs can be installed anywhere the business has a site irrespective of the terrestrial communications infrastructure.

7. VSAT Networks offer - a single vendor for equipment service, installation & maintenance of the entire network.

8. VSAT Networks offer superior flexibility and performance. Adding a site is quick and easy, and higher network availability levels are easy to obtain.

9. VSAT as a broadcast medium supports business objectives to improve customer service and quality. Reliable data broadcast, audio broadcast for in-site music and video broadcast for sales training are all available on a single platform.

10. Prices of VSAT Networks compete favorably against terrestrial alternatives including dial-up. The costs are predictable and stable.

11. VSAT Networks can be secured with VPNs (Virtual Private Networks) using industrial strength encryption.