BOWLA.T.KCHERUBA DOROTHY.N5th October 2012

SATELLITE DATA NETWORK COMMUNICATIONS

OVERVIEW

• A Communication satellite functions as an overhead wireless repeater station

• It provides a microwave communication link between two geographically remote sites

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”

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

SATELLITE COMMUNICATION CHANNELS

• Wide Area coverage of the earth's surface.

• Long transmission delays.

• Broadcast transmission.

• Large Channel Bandwidth.

• Transmission costs independent of Distance.

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.

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.

EARTH SATELLITE COMMUNICATION

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

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

DIFFERENT SATELLITE CONSTELLATIONS

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

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

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 modulationNo Time synchronization

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

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

SEQUENCE OF STEPS IN A PRMA PROCESS

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

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

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

CDMA BLOCK DIAGRAM

A SIMPLE CDMA PROCESS

APPLICATIONS OF CDMA

• Provides multiple access among users

• Spreading code identifies and authenticates the source transmitter ES

• Provides high security against eavesdropping

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

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

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

COMMUNICATION BETWEEN TWO VSAT TERMINALS

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

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

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

DirecPC SERVICES OPERATIONS

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

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

ATM ON SATELLITE CHANNELS PROCESS

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)

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)

MORDERN SATELLITE NETWORKS WITH LEO,GEO,MEO SATELITE

CONSTELLATION

TELEDESIC

MOBILE SATELLITE SERVICES

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)

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