VSAT – Very Small Aperture Terminal – a micro-Earth station that
uses satellite communications technology for reliable satellite communications.
Earth To Satellite Communication
Indoor Unit (IDU)
Satellite Modem: The router transmits modulated carrier either SCPC or TDMA to the radio frequency transceiver using a coaxial cable and with the Intermediate Frequency (IF) band (typically, L-band).
Outdoor Unit (ODU)
A typical ODU consists of the following:
- Connecting RF cable
- RFT – Radiofrequency Transceiver – includes U/C up-converter, Solid State Power Amplifier (SSPA), Down Converter
- LNA – Low noise amplifier – It could be a separate unit esp. in DAMA system.
The transceiver is responsible for converts IF frequency band to the required C-, X-, Ku- or Ka-higher frequency band and transmits the converted band and amplified signal to the satellite transponder.
A satellite dish antenna does the further amplification of the signal and forms the focused beam directed to the satellite.
Geostationary satellites are used generally for building a VSAT network.
VSAT Antenna do hot have to track the Geo Satellites as they are stationary due to placement of it in Geostationary orbit.
Satellite to Earth Communication
The satellite amplifies the received signals and transmits it applying different frequencies. Communication satellites re-transmit signals received from the satellite dishes back to the Earth.
An unlimited number of VSAT antennas whining a coverage zone of the satellite can receive the signal transmitted by the satellite.
Three layers of Protocol in the VSAT network:
- Satellite Access Protocol
- Satellite Network Access Protocol
- User Data Protocol
Satellite Access Protocol
VSAT shares the Satellite bandwidth and there three ways to do it:
1. Frequency Division Multiple Access FDMA
2. Time Division Multiple Access TDMA
3. Code Division Multiple Access CDMA
FDMA allow sharing of satellite capacity by using a different frequency assignment for each carrier.
In this way, each VSAT terminals shares the available capacity by carriers transmission at a distinct frequency. The carriers can have distinct power or bandwidth, but the total sum must be within designated capacity.
TDMA allows accessing the allocated capacity in a time-shared mode.
It is a multiplexing technique in which sub-channels within a single carrier are used to streams of multiple bit transmission simultaneously. There are timeslots available for each sub-channels to give separation between Data Transmission. It allows several VSAT stations to share the same frequency channel by dividing the signal into different time slots.
CDMA, a pseudo-random sequence encodes the initial signal by overlaying the signal over a larger bandwidth.
The receiver compares the composite input with the original encoding sequence stored in its memory, in order to restore the original signal.
Satellite Network Access Protocols:
Network Controller at Hub assigned the capacity requested by the VSATs.
There are three ways:
- at random,
on-demand assignment protocol:
The VSAT sends the request to hub for a capacity which is pre-assigned, which may be either carriers or a time slot.
random assignment protocol:
In this type of assignment, once the traffic is available at one of its data port VSAT transmits.
permanent assignment protocol:
The VSAT can access permanently to a small portion of the satellite capacity.
Time Division Multiplex Time Division Multiple Access
- Very efficient
- Use mostly in interactive data applications
- Data in packetized form
- Each packet contains an address that represents a data terminal within the VSAT network domain.
- Hub to VSAT link is a single carrier. It is the result of multiplexing all the packets from different customers and directing them to the various VSATs in the network.
- Each VSAT listens to the total traffic carried by the outbound carrier.
- However, each VSAT will only decode those packets having control information or addresses to one of the interfaces.
There may be one or many incoming carriers.
The incoming carriers carry traffic from the VSAT terminal to the hub.
It will transmit to the hub if a VSAT needs to communicate with a peer.
Demand Assignment Multiple Access (DAMA)
It is an access protocol that allows each channel to use one carrier pair in a Single Channel Per Carrier – SCPC mode to establish a link.
They are used for voice circuits.