VSAT A TUTORIAL
VSAT stands for “Very Small Aperture Terminal” and refers to receive/transmit
terminals installed at dispersed sites connecting to a central hub via satellite
using small diameter antenna dishes (0.6 to 3.8 meter).
Generally, these systems operate in the Ku-band and C-band frequencies.
Frequency Bands
- Generally VSAT operates on either Ku-band, or C-band.
- Ku-band based networks, are used primarily in Europe and North America.
- C-band, used extensively in Asia, Africa and Latin America.
- Ku-band generally requires a smaller antenna diameter
than C-band which generally requires a larger diameter antenna
Types of Operations
Bi-Directional Operation - The dish both spends (uplinks)
and receives (downlinks) information.
Receive-Only Operation - The dish receives (downlinks)
information only.
VSAT Networks
There are basically five types of networks employed in
VSAT operations.
Multipoint Network
The Multipoint Network Structure is used in data oriented
networks that require voice. The network structure provides for two-way data,
voice and multi-media operations. The network configuration is a star type
network that connects one or more main sites to various remote sites. It employs
a central hub station and a communication satellite. Each main site uplinks and
downlinks from the central hub through terrestrial (land) links. Each remote
site can only uplink to the hub. If a remote site has data to send to any other
location, it must first pass the information to the hub, and then it can be
routed to its final destination.
This type of network is very flexible, supporting multiple
interfaces available for LAN, Voice and Data connections and can support
numerous transport protocols. The network employs TDMS (Time Division Multiple
Access) as the means to send data to each remote site. This provides a secure
means of transporting data, as each data packet contains the specific address of
the station that it is addressed to and only that station can receive the data
and pass it on to its network.
Full-Meshed Network
For Voice oriented networks a Full-Meshed Designed Network
is utilized. In a Full-Meshed there is no central hub and each site can
communicate directly with all network nodes. It employs single satellite hops
between each network nodes, which enables superior voice quality, and efficient
fast-response data connections. Since the network does not employ a central hub,
each station requires increased processing and transmission power. Each remote
must uplink and downlink with other remote sites with exactly the same size
antenna over the same radio frequency.
In this network one site on the network must provide a Network Control System.
It is responsible for setting up calls between the sites and monitors the
traffic and bandwidth use. From this site all operating statistics are generated
and billing information is produced. The Network Control System
established the allocation of bandwidth as each connection is established
through bandwidth-on-demand, that is as one node initiates a call to another
node, the Network Control System demands a certain amount of bandwidth from the
bandwidth pool and then assigns it to the two nodes for the duration of the
communication . Once a call is terminated, the bandwidth is then free, and is
back available from the bandwidth pool. This type of network is capable of
providing interconnection of dissimilar communication devices. It provides the
capability of providing connections between must all currently available
communication devices. The system can provide a connection for an individual
handset, PBX (Public Branch Exchange) or can provide a gateway system.
Hybrid Voice and Data Network
This type of network is a hybrid between the Multipoint
and Full-Meshed network. Like the Full-Meshed Network it does not contain a
central hub, though it provides all remote sites with the capability to
communicate direct with all other sites . It provides the ability to offer
voice, fax, and videoconferencing opportunities between the sites. Data and
video can be broadcast is bi-directional between the central hub and the remote
site. This system takes separate voice and data interfaces at the remote site
and then processes this and combines them onto a single cable connecting to the
antenna. The remote HES VSAT acts as a switch routing all connections to their
appropriate destination. The system is capable of a wide-range of advance
telephony and PBX voice services and provides an efficient LAN/WAN network
service.
Single Channel Per Carrier (SCPC) - Point-to-Point
SCPC circuits are point-to-point circuits that provide
two-way communication between VSAT terminals located at two sites. It is a very
flexible system and has the capability to handle multiple data types over
multiple protocols. The system can handle voice, fax, and video application. the
data rates range from 9.6 kbps up to 8.4 Mbs. It is ideally suited for bringing
Internet to remote ISP sites. Internet access is provided through one satellite
dish connected to the Internet through a point-to-point line and transmission is
accomplished between it and another terminal at the remote ISP through the use
of a satellite. This system is capable of both asymmetric and well as symmetric
connections.
Broadcast Networks
Broadcast Networks provide for the transmission of data,
video and audio files to any number of users. The system "broadcasts" from the
central site to the end user remote sites. This one-way system provides an
uplink from the central site and each remote site downlinks (receives) only. It
provides high speed channel that is capable of up to 24Mbs up linked, providing
this information to numerous units.
VSAT COMPONENTS
ODU (Outdoor Unit): consists of the dish/reflector
antenna and Feed horn which contains either an LNB for receive only systems or a
LNA for bi-directional systems, which amplify the signal received by the unit.
Other electronic components consists of a Transmit Reject Filter, Up/Down
Converter (Transceiver), High Frequency Power Amplifier, and Transmit Type N
Transition. These components are either packaged separately or packaged together
in a single unit, depending on the manufacturer.
IDU (Indoor Unit): consists of the electronic
interface that allows connection to the network. It provides the interface.
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