VSAT Networks

Second Edition

Gérard MaralEcole Nationale Supérieure des Télécommunications,Site de Toulouse France

Innodata0470866853.jpg

VSAT Networks

VSAT Networks

Second Edition

Gérard MaralEcole Nationale Supérieure des Télécommunications,Site de Toulouse France

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Library of Congress Cataloging-in-Publication Data

Maral, Gérard.VSAT networks / Gérard Maral. – 2nd ed.

p. cm.ISBN 0-470-86684-5 (Cloth : alk. paper)

1. VSATs (Telecommunication) I. Title.TK5104.2.V74 M37 2003384.5′1 – dc22

2003022021

British Library Cataloguing in Publication Data

A catalogue record for this book is available from the British Library

ISBN 0-470-86684-5

Typeset in 11/13pt Palatino by Laserwords Private Limited, Chennai, IndiaPrinted and bound in Great Britain by TJ International, Padstow, CornwallThis book is printed on acid-free paper responsibly manufactured from sustainable forestryin which at least two trees are planted for each one used for paper production.

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Contents

Preface ix

Acronyms and Abbreviations xiii

Notation xvii

1 Introduction 11.1 VSAT network definition 11.2 VSAT network configurations 51.3 User terminal connectivity 91.4 VSAT network applications and types of traffic 11

1.4.1 Civilian VSAT networks 111.4.2 Military VSAT networks 15

1.5 VSAT networks: involved parties 151.6 VSAT network options 17

1.6.1 Star or mesh? 171.6.2 Data/voice/video 211.6.3 Fixed/demand assignment 221.6.4 Frequency bands 241.6.5 Hub options 29

1.7 VSAT network earth stations 301.7.1 VSAT station 301.7.2 Hub station 35

1.8 Economic aspects 391.9 Regulatory aspects 41

1.9.1 Licensing 421.9.2 Access to the space segment 431.9.3 Local regulations 43

1.10 Conclusions 441.10.1 Advantages 441.10.2 Drawbacks 45

2 Use of satellites for VSAT networks 472.1 Introduction 48

vi CONTENTS

2.1.1 The relay function 482.1.2 Transparent and regenerative payload 502.1.3 Coverage 522.1.4 Impact of coverage on satellite relay performance 552.1.5 Frequency reuse 59

2.2 Orbits 602.2.1 Newton’s universal law of attraction 602.2.2 Orbital parameters 61

2.3 The geostationary satellite 652.3.1 Orbit parameters 652.3.2 Launching the satellite 652.3.3 Distance to the satellite 682.3.4 Propagation delay 692.3.5 Conjunction of the sun and the satellite 692.3.6 Orbit perturbations 702.3.7 Apparent satellite movement 722.3.8 Orbit corrections 762.3.9 Doppler effect 77

2.4 Satellites for VSAT services 77

3 Operational aspects 793.1 Installation 79

3.1.1 Hub 793.1.2 VSAT 793.1.3 Antenna pointing 81

3.2 The customer’s concerns 853.2.1 Interfaces to end equipment 863.2.2 Independence from vendor 863.2.3 Set-up time 863.2.4 Access to the service 873.2.5 Flexibility 873.2.6 Failure and disaster recovery 873.2.7 Blocking probability 893.2.8 Response time 903.2.9 Link quality 91

3.2.10 Availability 913.2.11 Maintenance 963.2.12 Hazards 973.2.13 Cost 97

4 Networking aspects 994.1 Network functions 994.2 Some definitions 100

4.2.1 Links and connections 1004.2.2 Bit rate 1014.2.3 Protocols 1034.2.4 Delay 1034.2.5 Throughput 1044.2.6 Channel efficiency 1044.2.7 Channel utilisation 104

4.3 Traffic characterisation 105

CONTENTS vii

4.3.1 Traffic forecasts 1054.3.2 Traffic measurements 1054.3.3 Traffic source modelling 106

4.4 The OSI reference model for data communications 1104.4.1 The physical layer 1124.4.2 The data link layer 1124.4.3 The network layer 1144.4.4 The transport layer 1154.4.5 The upper layers (5 to 7) 116

4.5 Application to VSAT networks 1164.5.1 Physical and protocol configurations of a VSAT network 1164.5.2 Protocol conversion (emulation) 1164.5.3 Reasons for protocol conversion 118

4.6 Multiple access 1274.6.1 Basic multiple access protocols 1294.6.2 Meshed networks 1314.6.3 Star-shaped networks 1344.6.4 Fixed assignment versus demand assignment 1414.6.5 Random time division multiple access 1494.6.6 Delay analysis 1554.6.7 Conclusion 161

4.7 Network design 1634.7.1 Principles 1634.7.2 Guidelines for preliminary dimensioning 1644.7.3 Example 168

4.8 Conclusion 169

5 Radio frequency link analysis 1715.1 Principles 172

5.1.1 Thermal noise 1735.1.2 Interference noise 1745.1.3 Intermodulation noise 1745.1.4 Carrier power to noise power spectral density ratio 1765.1.5 Total noise 176

5.2 Uplink analysis 1795.2.1 Power flux density at satellite distance 1805.2.2 Effective isotropic radiated power of the earth station 1815.2.3 Uplink path loss 1885.2.4 Figure of merit of satellite receiving equipment 194

5.3 Downlink analysis 1955.3.1 Effective isotropic radiated power of the satellite 1975.3.2 Power Flux density at earth surface 1975.3.3 Downlink path loss 1985.3.4 Figure of merit of earth station receiving equipment 198

5.4 Intermodulation analysis 2055.5 Interference analysis 207

5.5.1 Expressions for carrier-to-interference ratio 2075.5.2 Types of interference 2085.5.3 Self-interference 2095.5.4 External interference 2195.5.5 Conclusion 225

viii CONTENTS

5.6 Overall link performance 2265.7 Bit error rate determination 2295.8 Power versus bandwidth exchange 2315.9 Example 231

Appendices 239Appendix 1: Traffic source models 239Appendix 2: Automatic repeat request (ARQ) protocols 242Appendix 3: Interface protocols 245Appendix 4: Antenna parameters 250Appendix 5: Emitted and received power 254Appendix 6: Carrier amplification 257Appendix 7: VSAT products 260

References 265

Index 267

Preface

Satellites for communication services have evolved quite signifi-cantly in size and power since the launch of the first commercialsatellites in 1965. This has permitted a consequent reduction inthe size of earth stations, and hence their cost, with a consequentincrease in number. Small stations, with antennas in the order of1.2–1.8 rn, have become very popular under the acronym VSAT,which stands for ’Very Small Aperture Terminals’. Such stationscan easily be installed at the customer’s premises and, consideringthe inherent capability of a satellite to collect and broadcast signalsover large areas, are being widely used to support a large range ofservices. Examples are broadcast and distribution services for data,image, audio and video, collection and monitoring for data, imageand video, two-way interactive services for computer transactions,data base inquiry, internet access and voice communications.

The trend towards deregulation, which started in the UnitedStates, and progressed in other regions of the world, has triggeredthe success of VSAT networks for corporate applications. Thisillustrates that technology is not the only key to success. Indeed,VSAT networks have been installed and operated only in thoseregions of the world where demand existed for the kind of servicesthat VSAT technology could support in a cost effective way, andalso where the regulatory framework was supportive.

This book on VSAT networks aims at introducing the reader tothe important issues of services, economics and regulatory aspects.It is also intended to give detailed technical insight on networkingand radiofrequency link aspects, therefore addressing the specificfeatures of VSAT networks at the three lower layers of the OSIReference Layer Model for data communications.

x PREFACE

From my experience in teaching, I felt I should proceed from thegeneral to the particular. Therefore, Chapter 1 can be consideredas an introduction to the subject, with rather descriptive contentson VSAT network configurations, services, operational and reg-ulatory aspects. The more intrigued reader can then explore thesubsequent chapters.

Chapter 2 deals with those aspects of satellite orbit and technologywhich influence the operation and performance of VSAT networks.

Chapter 3 details the operational aspects which are importantto the customer. Installation problems are presented, and a listof potential concerns to the customer is explored. Hopefully, thischapter will not be perceived as discouraging, but on the contraryas a friendly guide for avoiding misfortunes, and getting the bestfrom a VSAT network.

The next two chapters are for technique oriented readers. Actually,I thought this would be a piece of cake for my students, and areference text for network design engineers.

Chapter 4 deals with networking. It introduces traffic character-isation, and discusses network and link layers protocols of the OSIReference Layer Model, as used in VSAT networks. It also presentssimple analysis tools for the dimensioning of VSAT networks fromtraffic demand and user specifications in terms of blocking proba-bility and response time.

Chapter 5 covers the physical layer, providing the basic radio fre-quency link analysis, and presenting the parameters that conditionlink quality and availability. An important aspect discussed here isinterference, as a result of the small size of the VSAT antenna, andits related large beamwidth.

Appendices are provided for the benefit of those readers who maylack some background and have no time or opportunity to refer toother sources.

The second edition of this book takes into account my experi-ence while using the first edition as a support for my lectures. Itincorporates some theoretical developments that were missing inthe first edition, which constitute useful tools for the dimensioningand the performance evaluation of VSAT networks. In particular,Chapter 4 provides a more detailed treatment on how to evaluateblocking probability and expands on the information transfer delayanalysis of the first edition. This second edition also underplays theregulatory aspects, as during the seven year interval between thissecond edition and the first, many administrations have simplifiedand harmonised their regulatory framework. I felt this topic was notperhaps as important as it used to be.

PREFACE xi

I would like to take this opportunity to thank all the students I havetaught, at the Ecole Nationale Supérieure des Télécommunications,the University of Surrey, CEI-Europe and other places, who, byraising questions, asking for details and bringing in their comments,have helped me to organise the material presented here.

Gérard Maral, Professor.

Acronyms andAbbreviations

ABCS Advanced Business Communications via SatelliteACI Adjacent Channel InterferenceACK ACKnowledgementAMP AmplifierARQ Automatic repeat ReQuestARQ-GB(N) Automatic repeat ReQuest-Go Back NARQ-SR Automatic repeat ReQuest-Selective RepeatARQ-SW Automatic repeat ReQuest-Stop and WaitASYNC ASYNChronous data transferBEP Bit Error ProbabilityBER Bit Error RateBITE Built-In Test EquipmentBPF Band Pass FilterBPSK Binary Phase Shift KeyingBSC Binary Synchronous Communications (bisync)BSS Broadcasting Satellite ServiceCCI Co-Channel InterferenceCCIR Comité Consultatif International des

Radiocommunications (International RadioConsultative Committee)

CCITT Comité Consultatif International du Télégrapheet du Téléphone (The International Telegraphand Telephone Consultative Committee)

CCU Cluster Control UnitCDMA Code Division Multiple Access

xiv ACRONYMS AND ABBREVIATIONS

CFDMA Combined Free/Demand Assignment MultipleAccess

CFRA Combined Fixed/Reservation AssignmentCOST European COoperation in the field of Scientific

and Technical researchDA Demand AssignmentDAMA Demand Assignment Multiple AccessdB deciBelD/C Down-ConverterDCE Data Circuit Terminating EquipmentDEMOD DEMODulatorDTE Data Terminal EquipmentDVB-S Digital Video Broadcasting by SatelliteEIA Electronic Industries AssociationEIRP Effective Isotropic Radiated PowerEIRPES Effective Isotropic Radiated Power of earth

station (ES)EIRPSL Effective Isotropic Radiated Power of satellite

(SL)ES Earth StationETR ETSI Technical ReportETS European Telecommunications Standard, created

within ETSIETSI European Telecommunications Standards

InstituteEUTELSAT European Telecommunications Satellite

OrganisationFA Fixed AssignmentFCC Federal Communications Commission, in the

USAFDM Frequency Division MultiplexFDMA Frequency Division Multiple AccessFEC Forward Error CorrectionFET Field Effect TransistorFIFO First In First OutFODA FIFO Ordered Demand AssignmentFSK Frequency Shift KeyingFSS Fixed Satellite ServiceGBN Go Back NGVF Global VSAT ForumHDLC High level Data Link ControlHEMT High Electron Mobility TransistorHPA High Power AmplifierIAT InterArrival Time

ACRONYMS AND ABBREVIATIONS xv

IBO Input Back-OffIDU InDoor UnitIF Intermediate FrequencyIM InterModulationIMUX Input MultiplexerIP Internet ProtocolIPE Initial Pointing ErrorISDN Integrated Services Digital NetworkISO International Organisation for StandardisationITU International Telecommunication UnionLAN Local Area NetworkLAP Link Access ProtocolLNA Low Noise AmplifierLO Local OscillatorMAC Medium Access ControlMCPC Multiple Channels Per CarrierMIFR Master International Frequency RegisterMOD MODulatorMTBF Mean Time Between FailuresMUX MUltipleXerMX MiXerNACK Negative ACKnowledgementNMS Network Management SystemOBO Output Back-OffODU OutDoor UnitOMUX Output MUltipleXerOSI Open System InterconnectionPABX Private Automatic Branch eXchangePAD Packet Assembler/DisassemblerPBX Private (automatic) Branch eXchangePC Personal ComputerPDF Probability Density FunctionPDU Protocol Data UnitPOL POLarisationPSD Power Spectral DensityPSK Phase Shift KeyingQPSK Quaternary Phase Shift KeyingRCVO ReCeiVe-OnlyRec RecommendationRep ReportRF Radio FrequencyRX ReceiverS-ALOHA Slotted ALOHA protocolSCADA Supervisory Control and Data Acquisition

xvi ACRONYMS AND ABBREVIATIONS

SCPC Single Channel Per CarrierSDLC Synchronous Data Link ControlSKW Satellite-Keeping WindowSL SateLliteSNA Systems Network Architecture (IBM)SNG Satellite News GatheringSR Selective RepeatSSPA Solid State Power AmplifierSW Stop and WaitTCP Transmission Control ProtocolTDM Time Division MultiplexTDMA Time Division Multiple AccessTTC Telemetry, Tracking and CommandTV TeleVisionTWT Travelling Wave TubeTX TransmitterVSAT Very Small Aperture TerminalXPD Cross Polarisation DiscriminationXPI Cross Polarisation Isolation

Notation

A attenuation (larger than onein absolute value, thereforepositive value in dB), alsolength of acknowledgementframe (bits)

ARAIN attenuation due to rainAz azimuth angle (degree)a semi-major axis (m)

B bandwidth (Hz)Bi interfering carrier

bandwidth (Hz)Binb inbound carrier bandwidth

(Hz)BN receiver equivalent noise

bandwidth (Hz)Boutb outbound carrier

bandwidth (Hz)BXpond transponder bandwidth

(Hz)BU burstiness

c speed of light:c = 3 × 108m/s

C carrier power (W)CD carrier power at earth

station receiver input (W)CU carrier power at satellite

transponder input (W)Cx received carrier power on

X-polarisation (W)Cy received carrier power on

Y-polarisation (W)C/N carrier to noise power ratio(C/N)D downlink carrier to noise

power ratio

(C/N)Dsat same as above, at saturation(C/N)IM carrier to intermodulation

noise power ratio (Hz)(C/N)U uplink carrier power to

noise power ratio(C/N)Usat same as above, at saturation(C/N)T overall link (from station to

station) carrier to totalnoise power ratio

C/Ni carrier to interferencepower ratio

(C/Ni)D downlink carrier tointerference power ratio

(C/Ni)U uplink carrier tointerference power ratio

(C/Ni)T overall link (from station tostation) carrier tointerference power ratio

C/N0 carrier power to noisepower spectral densityratio (Hz)

(C/N0)D downlink carrier power tonoise power spectraldensity ratio (Hz)

(C/N0)Dsat same as above, atsaturation (Hz)

(C/N0)IM carrier power tointermodulation noisepower spectral densityratio (Hz)

(C/N0)U uplink carrier power tonoise power spectraldensity ratio (Hz)

(C/N0)Usat same as above, atsaturation (Hz)

xviii NOTATION

(C/N0)T overall link (from station tostation) carrier power tototal noise power spectraldensity ratio (W/Hz)

C/N0i carrier power tointerference noise powerspectral density ratio (Hz)

(C/N0i)D downlink carrier power tointerference noise powerspectral density ratio (Hz)

(C/N0i)U uplink carrier power tointerference noise powerspectral density ratio (Hz)

(C/N0i)T overall link (from station tostation) carrier power tototal interference noisepower spectral densityratio (W/Hz)

D antenna diameter (m), alsonumber of data bits perframe to be conveyed fromsource to destination

dBx value in dB relative to x

E elevation angle (degree),also energy per bit (J)

Eb energy per informationbit (J)

Ec energy per channel bit (J)e eccentricityEIRP equivalent isotropic

radiated power oftransmittingequipment (W)

EIRPES EIRP of earth station (W)EIRPESmax maximum value of

EIRPES (W)EIRPESsat value of EIRPES, at

transponder saturation (W)EIRPESi EIRP of interfering earth

station (W)EIRPESi,max maximum value of earth

station EIRP allocated tointerfering carrier (W)

EIRPESw EIRP of wanted earthstation (W)

EIRPSL EIRP of satellitetransponder (W)

EIRPSLsat EIRP of satellitetransponder atsaturation (W)

EIRPSL1sat EIRP of satellitetransponder in beam 1 atsaturation (W)

EIRPSL2sat EIRP of satellitetransponder in beam 2 atsaturation (W)

EIRPSLi,max maximum value ofinterfering satellite EIRPallocated to interferingcarrier (W)

EIRPSLw,max maximum value of wantedsatellite EIRP for wantedcarrier (W)

EIRPSLww wanted satellite EIRP forwanted carrier in directionof wanted station (W)

EIRPSLiw interfering satellite EIRPfor interfering carrier indirection of wantedstation (W)

EIRPSL1ww EIRP of satellitetransponder in beam 1 forwanted carrier in directionof wanted station (W)

EIRPSL2iw EIRP of satellitetransponder in beam 2 forinterfering carrier indirection of wantedstation (W)

EIRPSL1wsat EIRP of satellitetransponder in beam 1 indirection of wanted stationat saturation (W)

EIRPSL2wsat EIRP of satellitetransponder in beam 2 indirection of wanted stationat saturation (W)

f frequency (Hz): f = c/λfD downlink frequency (Hz)fIM frequency of an

intermodulation product(Hz)

fLO local oscillator frequency(Hz)

fU uplink frequency (Hz)

G power gain (larger than onein absolute value, thereforepositive value in dB), alsonormalised offered traffic,also gravitational constant:G = 6.672 × 10−11 m3/kg s2

NOTATION xix

Gcod coding gain (dB)GD power gain from

transponder output to earthstation receiver input

GIF intermediate frequencyamplifier power gain

GLNA low noise amplifier powergain

Gmax maximum gainGMX mixer power gainGR antenna receive gain in

direction of transmittingequipment

GRmax antenna receive gain atboresight

GRX receiving equipmentcomposite receive gain:GRX = GRmax/LRLpolLFRX

GRX max maximum value of GRXGRXi receiving equipment

composite receive gain forinterfering carrier

GRXw receiving equipmentcomposite receive gain forwanted carrier

GT antenna transmit gain indirection of receivingequipment

GTmax antenna transmit gain atboresight

GTi,max antenna transmit gain atboresight for interferingcarrier

GT1w satellite beam 1 transmitantenna gain in direction ofwanted station

GT2w satellite beam 2 transmitantenna gain in direction ofwanted station

GTE power gain from satellitetransponder input to earthstation receiver input

GXpond transponder power gainG1 gain of an ideal antenna

with area equal to1 m2 : G1 = 4π/λ2

G/T figure of merit of receivingequipment (K−1)

(G/T)ES figure of merit of earthstation receivingequipment (K−1)

(G/T)ESmax maximum value of (G/T)ES(G/T)SL figure of merit of satellite

receiving equipment (K−1)

H total number of bits in theframe header (and trailer ifany)

i orbit inclinationIx received cross polar

interference onX-polarisation (W)

IBO input back-offIBOinb input back-off for inbound

carrierIBOoutb input back-off for

outbound carrierIBO1 input back-off per carrier

with multicarrier operationmode

IBOt total input back-off withmulticarrier operationmode

Jx cross polar interference onX-polarisation generatedby receive antenna (W)

k Boltzmann constant:k = 1.38 × 10−23J/K;k(dBJ/K) = 10 log k =−228.6 dBJ/K

l Earth station latitude withrespect to the satellitelatitude (degrees)

L loss (larger than one inabsolute value, thereforepositive value in dB), alsoearth station relativelongitude with respect to ageostationary satellite(degrees), also length of aframe (bits), also length of amessage (bits)

La Earth station relativelongitude with respect tothe adjacent satellite(degrees)

Lw Earth station relativelongitude with respect tothe wanted satellite(degrees)

LD downlink path lossLFRX feeder loss from antenna to

receiver inputLFTX feeder loss from transmitter

output to antenna

xx NOTATION

Lpol antenna gain loss as a resultof antenna polarisationmismatch

LR off-axis receive gain lossLR max maximum value of LRLU uplink path lossLUi Uplink path loss for

interfering carrierLUw Uplink path loss for

wanted carrier

Me mass of the Earth:Me = 5.974 × 1024kg

N noise power (W)Ni interference power (W)NIM intermodulation noise

power (W)N0D downlink thermal noise

power spectral density(W/Hz)

N0U uplink thermal noise powerspectral density (W/Hz)

N0iD downlink interferencepower spectral density(W/Hz)

N0IM intermodulation noisepower spectral density(W/Hz)

N0iU uplink interference powerspectral density (W/Hz)

N0T total noise power spectraldensity at the earth stationreceiver input (W/Hz)

OBO output back-offOBO1 output back-off per carrier

with multicarrier operationmode

OBOi output back-off forinterfering carrier

OBOt total output back-off withmulticarrier operationmode

OBOw output back-off for wantedcarrier

P power (W)Pf probability for a frame to

be in errorPR received power at antenna

output (W)PT power fed to transmitting

antenna (W)PTX transmitter output power

(W)

PTX max transmitter output powerat saturation (W)

Px transmitted carrier poweron X-polarisation (W)

Py transmitted carrier poweron Y-polarisation (W)

PSD power spectral density(W/Hz)

PSDi interfering carrier powerspectral density (W/Hz)

PSDw wanted carrier powerspectral density (W/Hz)

Qx cross polar interference onX-polarisation generated bytransmit antenna (W)

r distance from centre ofearth to satellite

R range, also bit rateRa slant range from earth

station to adjacent satelliteRb information bit rate (b/s)Rbinb information bit rate on

inbound carrier (b/s)Rboutb information bit rate on

outbound carrier (b/s)Rc transmission bit rate (b/s)Rcinb transmission bit rate on

inbound carrier (b/s)Rcoutb transmission bit rate on

outbound carrier (b/s)Re earth radius: Re = 6378 kmR0 geostationary satellite

altitude: R0 = 35786 kmRw slant range from earth

station to wanted satelliteS normalised throughputSKW satellite station keeping

window halfwidth(degrees)

T interval of time (s), alsoperiod of orbit (s), alsomedium temperature (K)and noise temperature (K)

TA antenna noisetemperature (K)

TD downlink system noisetemperature (K)

TD min minimum value of TD (K)TF feeder temperature (K)TGROUND ground noise temperature

in vicinity of earthstation (K)

NOTATION xxi

TIF intermediate frequencyamplifier effective inputnoise temperature (K)

TLNA low noise amplifiereffective input noisetemperature (K)

Tm average mediumtemperature (K)

TMX mixer effective input noisetemperature (K)

Tp propagation time (s)TR receiver effective input

noise temperature (K)TSKY clear sky noise temperature

(K)TU uplink system noise

temperature (K)THRU throughput (b/s)

W window size

X order of anintermodulation product

XPD cross polar discriminationXPIRX receive antenna cross

polarisation isolationXPITX transmit antenna cross

polarisation isolation

α angular separation betweentwo satellites (degrees)

Γ spectral efficiency (b/s Hz)∆ ratio of co-polar wanted

carrier power to cross-polarinterfering carrier power

η efficiency

ηa antenna efficiency(typically 0.6)

ηc channel efficiencyηcGBN channel efficiency with

go-back-N protocolηcSR channel efficiency with

selective-repeat protocolηcSW channel efficiency with

stop-and-wait protocolθ angle from boresigth of

antenna (degrees)θ3dB half power beamwidth of

an antenna (degrees)θR antenna off-axis of angle

for reception (degrees)θR max maximum value of antenna

off-axis angle for reception(degrees)

θT antenna off-axis angle fortransmission (degrees)

θTmax maximum value of antennaoff-axis angle fortransmission (degrees)

λ wavelength (m) = c/f , alsotraffic generation rate (s−1)

µ product of gravitationalconstant G and mass of theEarthMe : µ = 3.986 × 1014m3/s2

ρ code rateτ packet duration (s)Φ power flux density (W/m2)Φsat power flux density at

saturation (W/m2)Φt total flux density (W/m

2)

1Introduction

This chapter aims to provide the framework of VSAT technologyin the evolving context of satellite communications in terms ofnetwork configuration, services, economics, operational and regu-latory aspects. It can also be considered by the reader as a guideto the following chapters which aim to provide more details onspecific issues.

1.1 VSAT NETWORK DEFINITION

VSAT, now a well established acronym for Very Small ApertureTerminal, was initially a trademark for a small earth station mar-keted in the 1980s by Telcom General in the USA. Its success as ageneric name probably comes from the appealing association of itsfirst letter V, which establishes a ‘victorious’ context, or may be per-ceived as a friendly sign of participation, and SAT which definitelyestablishes some reference to satellite communications.

In this book, the use of the word ‘terminal’ which appears in theclarification of the acronym will be replaced by ‘earth station’, orstation for short, which is the more common designation in the fieldof satellite communications for the equipment assembly allowingreception from or transmission to a satellite. The word terminalwill be used to designate the end user equipment (telephone set,facsimile machine, television set, computer, etc.) which generatesor accepts the traffic that is conveyed within VSAT networks. Thiscomplies with regulatory texts, such as those of the InternationalTelecommunications Union (ITU), where for instance equipmentgenerating data traffic, such as computers, are named ‘Data TerminalEquipment’ (DTE).

VSAT Networks, 2nd Edition. G. Maral 2003 John Wiley & Sons, Ltd ISBN: 0-470-86684-5

2 INTRODUCTION

VSATs are one of the intermediary steps of the general trendin earth station size reduction that has been observed in satellitecommunications since the launch of the first communication satel-lites in the mid 1960s. Indeed, earth stations have evolved from thelarge INTELSAT Standard A earth stations equipped with antennas30 m wide, to today’s receive-only stations with antennas as smallas 60 cm for direct reception of television transmitted by broad-casting satellites, or hand held terminals for radiolocation such asthe Global Postioning System (GPS) receivers. Present day handheld satellite phones (IRIDIUM, GLOBALSTAR) are pocket size.Figure 1.1 illustrates this trend.

Therefore, VSATs are at the lower end of a product line whichoffers a large variety of communication services; at the upper endare large stations (often called trunking stations) which support largecapacity satellite links. They are mainly used within internationalswitching networks to support trunk telephony services betweencountries, possibly on different continents. Figure 1.2 illustrates howsuch stations collect traffic from end users via terrestrial links thatare part of the public switched network of a given country. These sta-tions are quite expensive, with costs in the range of $10 million, andrequire important civil works for their installation. Link capacitiesare in the range of a few thousand telephone channels, or equiv-alently about one hundred Mbs−1. They are owned and operatedby national telecom operators, such as the PTTs, or large privatetelecom companies.

TRUNKING STATIONS

THIN ROUTE STATIONS

VSATS

MOBILE and PERSONAL STATIONS

1960

2000

Figure 1.1 VSAT: a step towards earth station size reduction

INTRODUCTION 3

satellite

internationaltrunk exchange

internationaltrunk exchange

nationaltrunk exchange

localexchange

localexchange

nationaltrunk exchange

TRUNKINGSTATION

TRUNKINGSTATION

COUNTRY A COUNTRY B

terrestrial link

regionaltrunk

exchange

regionaltrunk

exchange

subscribers subscribers

Figure 1.2 Trunking stations

At the lower end are VSATs. These are small stations with antennadiameters less than 2.4 m, hence the name ‘small aperture’ whichrefers to the area of the antenna. Such stations cannot supportsatellite links with large capacities, but they are cheap, with manu-facturing costs in the range of $1000 to $5000, and easy to install anywhere, on the roof of a building or on a parking lot. Installation costsare usually less than $2000. Therefore, VSATs are within the finan-cial capabilities of small corporate companies, and can be used to setup rapidly small capacity satellite links in a flexible way. Capacitiesare of the order of a few tens of kbs−1, typically 56 or 64 kbs−1.

The low cost of VSATs has made these very popular, with a marketgrowth of the order of 20–25% per year in the nineties. There were

4 INTRODUCTION

about 50 000 VSATs in operation worldwide in 1990, and more than600 000 twelve years later. This trend is likely to continue.

Referring to transportation, VSATs are for information transport,the equivalent of personal cars for human transport, while the largeearth stations mentioned earlier are like public buses or trains.

At this point it is worth noting that VSATs, like personal cars, areavailable at one’s premises. This avoids the need for using any publicnetwork links to access the earth station. Indeed, the user can directlyplug into the VSAT equipment his own communication terminalssuch as a telephone or video set, personal computer, printer, etc.Therefore, VSATs appear as natural means to bypass public networkoperators by directly accessing satellite capacity. They are flexibletools for establishing private networks, for instance between thedifferent sites of a company. Figure 1.3 illustrates this aspect by

satellite

internationaltrunk exchange

internationaltrunk exchange

nationaltrunk exchange

nationaltrunk exchange

TRUNKINGSTATION

TRUNKINGSTATION

COUNTRY ACOUNTRY B

terrestrial link

regionaltrunk

exchange

regionaltrunk

exchange

subscribersVSATs VSATssubscribers

localexchange

localexchange

Figure 1.3 From trunking stations to VSATs

INTRODUCTION 5

emphasising the positioning of VSATs near the user compared totrunking stations, which are located at the top level of the switchinghierarchy of a switched public network.

The bypass opportunity offered by VSAT networks has not alwaysbeen well accepted by national telecom operators as it could meanloss of revenue, as a result of business traffic being diverted from thepublic network. This has initiated conservative policies by nationaltelecom operators opposing the deregulation of the communicationssector. In some regions of the world, and particularly in Europe, thishas been a strong restraint to the development of VSAT networks.

1.2 VSAT NETWORK CONFIGURATIONS

As illustrated in Figure 1.3, VSATs are connected by radio frequency(RF) links via a satellite, with a so-called uplink from the station tothe satellite and a so-called downlink from the satellite to the sta-tion (Figure 1.4). The overall link from station to station, sometimescalled hop, consists of an uplink and a downlink. A radio frequencylink is a modulated carrier conveying information. Basically thesatellite receives the uplinked carriers from the transmitting earthstations within the field of view of its receiving antenna, amplifiesthose carriers, translates their frequency to a lower band in order toavoid possible output/input interference, and transmits the ampli-fied carriers to the stations located within the field of view of itstransmitting antenna. A more detailed description of the satellitearchitecture is given in Chapter 2 (section 2.1).

Present VSAT networks use geostationary satellites, which aresatellites orbiting in the equatorial plane of the earth at an altitudeabove the earth surface of 35 786 km. It will be shown in Chapter 2

satellite

UPLINK DOWNLINK

Figure 1.4 Definition of uplink and downlink

6 INTRODUCTION

Figure 1.5 Geostationary satellite

that the orbit period at this altitude is equal to that of the rotationof the earth. As the satellite moves in its circular orbit in the samedirection as the earth rotates, the satellite appears from any stationon the ground as a fixed relay in the sky. Figure 1.5 illustrates thisgeometry. It should be noted that the distance from an earth stationto the geostationary satellite induces a radio frequency carrier powerattenuation of typically 200 dB on both uplink and downlink, and apropagation delay from earth station to earth station (hop delay) ofabout 0.25 s (see Chapter 2, section 2.3).

As a result of its apparent fixed position in the sky, the satellitecan be used 24 hours a day as a permanent relay for the uplinkedradio frequency carriers. Those carriers are downlinked to all earthstations visible from the satellite (shaded area on the earth inFigure 1.5). Thanks to its apparent fixed position in the sky, there isno need for tracking the satellite. This simplifies VSAT equipmentand installation.

As all VSATs are visible from the satellite, carriers can be relayedby the satellite from any VSAT to any other VSAT in the network,as illustrated by Figure 1.6.

Regarding meshed VSAT networks, as shown in Figure 1.6, onemust take into account the following limitations:

– typically 200 dB carrier power attenuation on the uplink and thedownlink as a result of the distance to and from a geostation-ary satellite;

– limited satellite transponder radio frequency power, typically afew tens of watts;

– small size of the VSAT, which limits its transmitted power andits receiving sensitivity.

As a result of the above, it may well be that the demodulatedsignals at the receiving VSAT do not match the quality requested bythe user terminals. Therefore direct links from VSAT to VSAT maynot be acceptable.