2. OVERVIEW
• A Communication satellite
functions as an overhead
wireless repeater station
• It provides a microwave
communication link between
two geographically remote
sites
3. TRANSPONDERS
• Each Satellite is equipped
with various “transponders”
consisting of a transceiver and
an antenna tuned to the part
of the allocated spectrum
• Most satellites simply
broadcast whatever they
receive and they are called as
“bent pipes”
4. DATA TRANSMISSION
• Use of satellites in packet
data transmission has been
on the rise
• They are typically used in
WAN networks which acts as
the backbone links to
geographically dispersed
LAN’s and MAN’s
5. SATELLITE COMMUNICATION
CHANNELS
• Wide Area coverage of the
earth's surface.
• Long transmission delays.
• Broadcast transmission.
• Large Channel Bandwidth.
• Transmission costs
independent of Distance.
6. MICROWAVE POWER
• The received microwave
power involved in satellite
links is typically very small
(of the order of a few
100picowatts).
• Specially designed earth
stations that keep C/N
(carrier to noise ratio) to a
minimum are used to
transmit/receive satellite
communications.
7. SATELLITE LINKS
• Satellite links can operate in
different frequency bands
and use separate carrier
frequencies for the up-link
and down-link or "spot
beam"
• The area of the earth's
surface covered by a
satellite's transmission
beam is referred to as the
"footprint" of the satellite
transponders.
9. SATELLITE CONSTELLATIONS
• Satellites can be positioned in
orbits with different heights
and shapes
• Depending on the shapes
they are of 3 types:
LEO: Low Earth Orbit
MEO: Medium Earth
Orbit
GEO: Geostationary Earth
Orbit
10. SATELLITE CONSTELLATIONS
• Satellites are also classified in
terms of their payload.
• Satellites that weigh in the
range of 800-1000 kg fall in
the "Small" class
• The heavier class is named as
"Big" satellites. GEO
satellites are typically "Big"
satellites
• LEO satellites can fall in
either class
12. MAC PROTOCOLS FOR SATELLITE
LINKS
• Satellite channels require special considerations at the
DLC (Data Link Control) layer of the OSI model they
are referred to as “Long Fat Pipes”
• Implementing CSMA/CD (Carrier Sense Multiple
Access/Collision Detection) protocol is not possible
• Delay occurs in the channel in which they cause
potential or multiple collisions in the satellite link
13. MAC SCHEMES
• Most satellite MAC schemes usually assign dedicated channels in time and/ or
frequency for each user
• ALOHA
Pure Aloha allows every competing stations to transmit anytime
Its uses S-ALOHA (Slotted ALOHA) to increase its efficiency
• FDMA (Frequency Division Multiple Access)
The available satellite channel bandwidth is broken into
frequency bands for different earth stations
ES controls the microwave power in the band for not spilling
into other channels
14. MAC SCHEMES
• TDMA (Time Division Multiple Access)
Channels are time multiplexed in a sequential fashion.
Time synchronization is required between the Earth
Station
• CDMA (Code Division Multiple Access)
Uses a hybrid of time/frequency multiplexing
Form of spread spectrum modulation
No Time synchronization
15. PRMA
• The Packet Reservation Multiple
Access (PRMA) method combines
TDMA with the techniques of S-
ALOHA
• A TDMA satellite channel consists
of multiple time slots in a framed
structure
• Time Slot carries packets of N
users
• Assignments of time slots are not
fixed
• Data packet carries a VCI (Virtual
Circuit Identifier) field that
indicates its receiving earth station
16. PRMA PROCESS
• TDMA frame consists reserved and empty slots
• When an ES has reserved a slot, the packets are termed as "safe"
packets
• A new ES tries to establish a channel, it sends an "unsafe" data
packet
• ES can transmit "safe" packets only if the packet reaches the
destination without collisions
• The reservation slot is lost if there is no data to be sent during the
reserved time slot
18. APPLICATIONS OF PRMA
• Round trip delay (270ms for
GEO before an ES can
transmit safe packets)
• The application and its
required QoS (Quality of
Service)
• BER rate of the link
19. CDMA
• Code Division Multiple
Access (CDMA) is a type of
spread spectrum
communication
• It was initially used only in
military satellites to overcome
jamming and provide security
to the user
20. CDMA PROCESS
• Binary transmission is given by a spreading code with a zero
one sequence
• The Bit Rate of the code is higher than the chip rate
• Each code is unique
• The resulting signal is obtained by the product of the input
data stream and the spreading code
• The incoming bit code is correlated with the receivers
spreading code ,the data is retrieved otherwise it appears as
noise
23. APPLICATIONS OF CDMA
• Provides multiple access
among users
• Spreading code identifies and
authenticates the source
transmitter ES
• Provides high security against
eavesdropping
24. VSAT NETWORKS
• Very Small Aperture Terminals
(VSAT) provides a solution to the
complication, high cost and need
of Earth Station
• According to the VSAT networks,
either the transmitter or the
receiver antenna on a satellite link
must be larger
• Lower performance microwave
transceiver and lower gain dish
antenna (smaller size) is used
25. OPERATION OF VSAT NETWORKS
• VSAT Networks are star based topology
• The Earth hub station acts as the central node
• The satellite provides a broadcast medium acting as a common
connection point for all the remote VSAT earth stations
• VSAT networks are ideal for centralized networks with a central
host and a number of geographically dispersed terminals
26. OPERATION OF VSAT NETWORKS
• The weaker signal from the ES is amplified by the satellite and
received by the hub ES
• Lower gain at the uplink is compensated at the downlink by the
high performance Hub ES
• When two VSAT s need to communicate it requires two hopes
since the connection must pass through the hub ES
• The most common MAC schemes used on VSAT's are S-
ALOHA and TDMA
• The most commonly used network protocol on VSAT links is
X.25
28. DirecPC SERVICES
• These services comes with an ISA
computer card, a RF dish antenna (2 ft
dia) equipped with an LNA, and
supporting software
• Services needs an IBM compatible with
486 or higher processor and Windows
OS
• A cable runs from the dish antenna and
connects to the ISA card inside the PC
• The receiver processes of demodulation,
decoding and de-multiplexing are carried
out inside the card
29. KINDS OF SERVICES
DIGITAL PACKAGE
DELIVERY
• This service allows the end
user to download files at
speeds 100 times faster than
the public telephone network
• Multiple DirecPC end points
downloads large files using
the standard analog modem
over telephone lines
30. KINDS OF SERVICES
TURBO INTERNET
• A connection is setup with the local ISP using
the analog telephone line modem.
• All mouse and keyboard actions in the web
browser are communicated to the web server
on the other end using this link.
• Instead of directing the data to the requesting
node, data is directed to the DirecPC Network
Operations Centre (NOC).
• The data is transferred from the NOC to the
end user via a satellite link
32. ATM ON SATELLITE CHANNELS
PROCESS
• The performance of ATM
(Asynchronous Transfer Mode) on
Satellite channels is worthy
• ATM is capable of providing the
requested QoS (Quality of Service)
required by multimedia services
• The basic transfer unit is 53 byte
unit
• 5 bytes for header and 48 for
payload
33. ATM ON SATELLITE CHANNELS
PROCESS
• HEC (Header Error Control) byte, detects
error and corrects all single bit errors in the
header
• ATM performance factors is the Cell Loss
Ratio (CLR)
• COMSAT [ALE] and is known as the ALE
(ATM Link Enhancer) was developed to
overcome the burst noise on satellite
channel
• ALE performs selective interleaving of the
cells
• Helps in isolating the ATM cells from burst
errors
35. MODERN SATELLITE NETWORKS
• Future satellites will no longer act
as "bent pipes“
• Inter-Satellite Links are
incorporated on board switching,
data buffering and signal
processing
• Moreover communication
satellites are expected to provide
global PCS (Personal
Communication Services)
36. MODERN SATELLITE NETWORKS
• Design of modern satellite
networks are to be used
• Satellite networks in global
trend of user instead of
network oriented services.
• The telecommunication
industry is making large
investments in Mobile
Satellite Services (MSS)
39. CONCULSIONS
• The role of satellites is changing from the
traditional telephony and TV broadcast
services to user oriented data services
• Small and large LEO constellations are
expected to become a candidate in the
cellular market at the same time GEO will
not diminish
• MAC protocols strongest candidates are
TDMA and CDMA
• The evolution of satellite technology along
with the fixed and mobile terrestrial
communications is expected to merge into
Universal Personal Telecommunications
(UPT)
