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TDMA
(Time-division multiple access)
• When numerous remote sites communicate with
one central hub, this design is similar to packet-
switched networks.
• Because of competition with one another for
access to the central hub, it restrict the
maximum bandwidth to most cases is about 19.2
kbps.
• all VSATs share satellite resource on a time-slot
basis.
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•Usually used in STAR topology as a
transmission technique.
•Offered to domestic needs.
• The VSAT Hub communicates with all
dispersed VSATs (typically a 1.8-meter diameter
parabolic-shaped dish) on an outgoing channel
of up to 512kbps based on the TDM scheme.
•The incoming or return channel from the
dispersed VSATs uses the TDMA channel
technology that enables a large number of the
respective VSATs to share this single return
channel.
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•The incoming routes typically operate at
128kbps, and can go up to a maximum
bandwidth of 256kbps.
•Petrol stations, information providers, retail
chains, financial institutions and any large
corporation that requires transfers of low to
medium rate data applications between its head
office and, local and outstation branch offices.
• Example of Point of Sales (POS transactions),
Credit Card Verification, Reservations Systems
Financial and Banking applications (teller, loans,
ATM) etc.
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SCDC (single-carrier per channel)
• SCPC-based design provides a point-to-point
technology, making VSAT equivalent to
conventional leased lines.
• dedicated bandwidth of up to 2 Mbps
• Usually use in an international VSAT services in
Asia-Pacific.
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•In the Hub-to-Remote configuration, one end of
the VSAT link (normally the customer's HQ) is
connected to the 11-meter VSAT Hub (Earth
Station) via a terrestrial leased line.
•A VSAT antenna at the remote end or the
distant end (normally the branch office) of the
VSAT link is then interconnected to the VSAT
hub via the satellite.
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•VSAT links with a Remote-to-Remote
configuration bypass the VSAT Hub and has a
stand-alone VSAT antenna at both ends of the
link.
• Typical VSAT antenna size ranges from 1.8m
to 2.4m.
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FDMA (Frequency Division Multiple
Access)
• Oldest method for channel allocation
• The satellite channel bandwidth is broken into
frequency bands for different earth stations
• The earth stations must be carefully power-
controlled to prevent the microwave power
spilling into the bands for the other channels.
• Here, all VSATs share the satellite resource on
the frequency domain only.
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PAMA (Pre-Assigned Multiple Access);
• the VSATs are pre-allocated a designated
frequency. Equivalent of the terrestrial (land
based) leased line solutions.
• PAMA solutions use the satellite resources
constantly. Therefore, no call-up delay in the
interactive data applications or high traffic
volumes.
• PAMA connects high data traffic sites within an
organization.
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DAMA (Demand Assigned Multiple
Access);
• The network uses a pool of satellite channels,
which are available for use by any station in that
network.
• On demand, a pair of available channels is
assigned, so that a call can be established.
Once the call is completed, the channels are
returned to the pool for an assignment to
another call.
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•Since the satellite resource is used only in pro-
portion to the active circuits and their holding
times, this is ideally suited for voice traffic and
data traffic in batch mode.
•DAMA offers point-to-point voice, fax, data
requirements and supports video-conferencing.
• Satellite connections are established and
dropped only when traffic demands them.
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CDMA (Code Division Multiple Access)
• Under this, a central network monitoring system
allocates a unique code to each of the VSATs.
• Enabling multiple VSATs to transmit
simultaneously and share a common frequency
band.
• The data signal is combined with a high bit rate
code signal which is independent of the data.
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