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URL:
http://www.gvf.org/
A satellite circuit has five elements - two
terrestrial links, an uplink, a downlink, and a satellite repeater. The
satellite itself consists of six subsystems: physical structure, transponder,
attitude control apparatus, power supply, telemetry equipment, and
station-keeping apparatus. Earth stations vary from simple, inexpensive
receive-only stations to elaborate two-way communications stations.
An earth station includes microwave relay equipment,
terminating multiplex equipment, and a satellite communications controller. The
relay equipment is different from terrestrial microwave in that the transmitter
has a much higher power output and very large antennas (up to 30m in diameter
for GEO earth stations) are used. The satellite communications controller
apportions the satellite’s bandwidth, processes signals for satellite
transmission, and interconnects the earth station’s radio equipment to
terrestrial circuits.
Satellites employ several techniques to increase the
traffic carrying capacity and to provide access, namely: FDMA (frequency
division multiple access), TDMA (time division multiple access), and DAMA
(demand assigned multiple access). The ¼ second time delay between two earth
stations is noticeable in voice communication circuits. Data communication
circuits that use a block transmission protocol will drop to an unacceptably
low throughput via a satellite because a station can transmit a new block only
after the receiver acknowledges the preceding block.
Several satellite-based services are available. The
one most often used by utilities is called Very Small Aperture Terminal (VSAT)
that uses a very small transmitting antenna (0.6 to 3.8 meter), and is
star-connected with a hub at the center of the network and with dedicated lines
running to the host computer. The hub has a large antenna aimed at the
satellite. The hub is very expensive and is usually owned by the VSAT vendor.
TDMA and spread spectrum technologies are the most common ways of allocating
access to the hub by the VSATS. VSAT provides bandwidth as high as T1/E1 or as
low as what the customer needs for video, voice and data. CONSAT and GTE have
formed a partnership that allows air passengers to make air telephone calls via
satellite.
Generally, these systems operate in the Ku-band and
C-band frequencies. Ku-band based networks are used primarily in Europe and
North America and utilize the smaller sizes of VSAT antennas. C-band, used
extensively in Asia, Africa and Latin America, require
larger antenna.
VSAT networks come in various shapes and sizes ranging from point-to-point,
point-to-multipoint, and on demand for thousands of sites based on a dedicated
facility located at their own site. Mesh systems have traditionally been
somewhat smaller in size than star systems - 5 to 30 sites used to be a good
rule of thumb - but since prices have come down, some networks now comprise as
many as several hundred or even thousands of sites.
Keywords:
Wireless, satellite, broadband, very small aperture terminal
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