2 VSAT Networks Second Edition Gérard Maral Ecole Nationale Supérieure des Télécommunications, Site de Toulouse France
4 VSAT Networks
6 VSAT Networks Second Edition Gérard Maral Ecole Nationale Supérieure des Télécommunications, Site de Toulouse France
7 Copyright 1995 & 2003 John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex PO19 8SQ, England Telephone (+44) (for orders and customer service enquiries): Visit our Home Page on or All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except under the terms of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London W1T 4LP, UK, without the permission in writing of the Publisher. Requests to the Publisher should be addressed to the Permissions Department, John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex PO19 8SQ, England, or ed to or faxed to (+44) This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the Publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought. Other Wiley Editorial Offices John Wiley & Sons Inc., 111 River Street, Hoboken, NJ 07030, USA Jossey-Bass, 989 Market Street, San Francisco, CA , USA Wiley-VCH Verlag GmbH, Boschstr. 12, D Weinheim, Germany John Wiley & Sons Australia Ltd, 33 Park Road, Milton, Queensland 4064, Australia John Wiley & Sons (Asia) Pte Ltd, 2 Clementi Loop #02-01, Jin Xing Distripark, Singapore John Wiley & Sons Canada Ltd, 22 Worcester Road, Etobicoke, Ontario, Canada M9W 1L1 Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books. Library of Congress Cataloging-in-Publication Data Maral, Gérard. VSAT networks / Gérard Maral. 2nd ed. p. cm. ISBN (Cloth : alk. paper) 1. VSATs (Telecommunication) I. Title. TK V74 M dc British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN Typeset in 11/13pt Palatino by Laserwords Private Limited, Chennai, India Printed and bound in Great Britain by TJ International, Padstow, Cornwall This book is printed on acid-free paper responsibly manufactured from sustainable forestry in which at least two trees are planted for each one used for paper production.
8 Contents Preface ix Acronyms and Abbreviations xiii Notation xvii 1 Introduction VSAT network definition VSAT network configurations User terminal connectivity VSAT network applications and types of traffic Civilian VSAT networks Military VSAT networks VSAT networks: involved parties VSAT network options Star or mesh? Data/voice/video Fixed/demand assignment Frequency bands Hub options VSAT network earth stations VSAT station Hub station Economic aspects Regulatory aspects Licensing Access to the space segment Local regulations Conclusions Advantages Drawbacks 45 2 Use of satellites for VSAT networks Introduction 48
9 vi CONTENTS The relay function Transparent and regenerative payload Coverage Impact of coverage on satellite relay performance Frequency reuse Orbits Newton s universal law of attraction Orbital parameters The geostationary satellite Orbit parameters Launching the satellite Distance to the satellite Propagation delay Conjunction of the sun and the satellite Orbit perturbations Apparent satellite movement Orbit corrections Doppler effect Satellites for VSAT services 77 3 Operational aspects Installation Hub VSAT Antenna pointing The customer s concerns Interfaces to end equipment Independence from vendor Set-up time Access to the service Flexibility Failure and disaster recovery Blocking probability Response time Link quality Availability Maintenance Hazards Cost 97 4 Networking aspects Network functions Some definitions Links and connections Bit rate Protocols Delay Throughput Channel efficiency Channel utilisation Traffic characterisation 105
10 CONTENTS vii Traffic forecasts Traffic measurements Traffic source modelling The OSI reference model for data communications The physical layer The data link layer The network layer The transport layer The upper layers (5 to 7) Application to VSAT networks Physical and protocol configurations of a VSAT network Protocol conversion (emulation) Reasons for protocol conversion Multiple access Basic multiple access protocols Meshed networks Star-shaped networks Fixed assignment versus demand assignment Random time division multiple access Delay analysis Conclusion Network design Principles Guidelines for preliminary dimensioning Example Conclusion Radio frequency link analysis Principles Thermal noise Interference noise Intermodulation noise Carrier power to noise power spectral density ratio Total noise Uplink analysis Power flux density at satellite distance Effective isotropic radiated power of the earth station Uplink path loss Figure of merit of satellite receiving equipment Downlink analysis Effective isotropic radiated power of the satellite Power Flux density at earth surface Downlink path loss Figure of merit of earth station receiving equipment Intermodulation analysis Interference analysis Expressions for carrier-to-interference ratio Types of interference Self-interference External interference Conclusion 225
11 viii CONTENTS 5.6 Overall link performance Bit error rate determination Power versus bandwidth exchange Example 231 Appendices 239 Appendix 1: Traffic source models 239 Appendix 2: Automatic repeat request (ARQ) protocols 242 Appendix 3: Interface protocols 245 Appendix 4: Antenna parameters 250 Appendix 5: Emitted and received power 254 Appendix 6: Carrier amplification 257 Appendix 7: VSAT products 260 References 265 Index 267
12 Preface Satellites for communication services have evolved quite significantly in size and power since the launch of the first commercial satellites in This has permitted a consequent reduction in the size of earth stations, and hence their cost, with a consequent increase in number. Small stations, with antennas in the order of rn, have become very popular under the acronym VSAT, which stands for Very Small Aperture Terminals. Such stations can easily be installed at the customer s premises and, considering the inherent capability of a satellite to collect and broadcast signals over large areas, are being widely used to support a large range of services. Examples are broadcast and distribution services for data, image, audio and video, collection and monitoring for data, image and video, two-way interactive services for computer transactions, data base inquiry, internet access and voice communications. The trend towards deregulation, which started in the United States, and progressed in other regions of the world, has triggered the success of VSAT networks for corporate applications. This illustrates that technology is not the only key to success. Indeed, VSAT networks have been installed and operated only in those regions of the world where demand existed for the kind of services that VSAT technology could support in a cost effective way, and also where the regulatory framework was supportive. This book on VSAT networks aims at introducing the reader to the important issues of services, economics and regulatory aspects. It is also intended to give detailed technical insight on networking and radiofrequency link aspects, therefore addressing the specific features of VSAT networks at the three lower layers of the OSI Reference Layer Model for data communications.
13 x PREFACE From my experience in teaching, I felt I should proceed from the general to the particular. Therefore, Chapter 1 can be considered as an introduction to the subject, with rather descriptive contents on VSAT network configurations, services, operational and regulatory aspects. The more intrigued reader can then explore the subsequent chapters. Chapter 2 deals with those aspects of satellite orbit and technology which influence the operation and performance of VSAT networks. Chapter 3 details the operational aspects which are important to the customer. Installation problems are presented, and a list of potential concerns to the customer is explored. Hopefully, this chapter will not be perceived as discouraging, but on the contrary as a friendly guide for avoiding misfortunes, and getting the best from 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 a reference text for network design engineers. Chapter 4 deals with networking. It introduces traffic characterisation, and discusses network and link layers protocols of the OSI Reference Layer Model, as used in VSAT networks. It also presents simple analysis tools for the dimensioning of VSAT networks from traffic demand and user specifications in terms of blocking probability and response time. Chapter 5 covers the physical layer, providing the basic radio frequency link analysis, and presenting the parameters that condition link quality and availability. An important aspect discussed here is interference, as a result of the small size of the VSAT antenna, and its related large beamwidth. Appendices are provided for the benefit of those readers who may lack some background and have no time or opportunity to refer to other sources. The second edition of this book takes into account my experience while using the first edition as a support for my lectures. It incorporates some theoretical developments that were missing in the first edition, which constitute useful tools for the dimensioning and the performance evaluation of VSAT networks. In particular, Chapter 4 provides a more detailed treatment on how to evaluate blocking probability and expands on the information transfer delay analysis of the first edition. This second edition also underplays the regulatory aspects, as during the seven year interval between this second edition and the first, many administrations have simplified and harmonised their regulatory framework. I felt this topic was not perhaps as important as it used to be.
14 PREFACE xi I would like to take this opportunity to thank all the students I have taught, at the Ecole Nationale Supérieure des Télécommunications, the University of Surrey, CEI-Europe and other places, who, by raising questions, asking for details and bringing in their comments, have helped me to organise the material presented here. Gérard Maral, Professor.
16 Acronyms and Abbreviations ABCS ACI ACK AMP ARQ ARQ-GB(N) ARQ-SR ARQ-SW ASYNC BEP BER BITE BPF BPSK BSC BSS CCI CCIR CCITT CCU CDMA Advanced Business Communications via Satellite Adjacent Channel Interference ACKnowledgement Amplifier Automatic repeat ReQuest Automatic repeat ReQuest-Go Back N Automatic repeat ReQuest-Selective Repeat Automatic repeat ReQuest-Stop and Wait ASYNChronous data transfer Bit Error Probability Bit Error Rate Built-In Test Equipment Band Pass Filter Binary Phase Shift Keying Binary Synchronous Communications (bisync) Broadcasting Satellite Service Co-Channel Interference Comité Consultatif International des Radiocommunications (International Radio Consultative Committee) Comité Consultatif International du Télégraphe et du Téléphone (The International Telegraph and Telephone Consultative Committee) Cluster Control Unit Code Division Multiple Access
17 xiv ACRONYMS AND ABBREVIATIONS CFDMA CFRA COST DA DAMA db D/C DCE DEMOD DTE DVB-S EIA EIRP EIRP ES EIRP SL ES ETR ETS ETSI EUTELSAT FA FCC FDM FDMA FEC FET FIFO FODA FSK FSS GBN GVF HDLC HEMT HPA IAT Combined Free/Demand Assignment Multiple Access Combined Fixed/Reservation Assignment European COoperation in the field of Scientific and Technical research Demand Assignment Demand Assignment Multiple Access decibel Down-Converter Data Circuit Terminating Equipment DEMODulator Data Terminal Equipment Digital Video Broadcasting by Satellite Electronic Industries Association Effective Isotropic Radiated Power Effective Isotropic Radiated Power of earth station (ES) Effective Isotropic Radiated Power of satellite (SL) Earth Station ETSI Technical Report European Telecommunications Standard, created within ETSI European Telecommunications Standards Institute European Telecommunications Satellite Organisation Fixed Assignment Federal Communications Commission, in the USA Frequency Division Multiplex Frequency Division Multiple Access Forward Error Correction Field Effect Transistor First In First Out FIFO Ordered Demand Assignment Frequency Shift Keying Fixed Satellite Service Go Back N Global VSAT Forum High level Data Link Control High Electron Mobility Transistor High Power Amplifier InterArrival Time
18 ACRONYMS AND ABBREVIATIONS xv IBO IDU IF IM IMUX IP IPE ISDN ISO ITU LAN LAP LNA LO MAC MCPC MIFR MOD MTBF MUX MX NACK NMS OBO ODU OMUX OSI PABX PAD PBX PC PDF PDU POL PSD PSK QPSK RCVO Rec Rep RF RX S-ALOHA SCADA Input Back-Off InDoor Unit Intermediate Frequency InterModulation Input Multiplexer Internet Protocol Initial Pointing Error Integrated Services Digital Network International Organisation for Standardisation International Telecommunication Union Local Area Network Link Access Protocol Low Noise Amplifier Local Oscillator Medium Access Control Multiple Channels Per Carrier Master International Frequency Register MODulator Mean Time Between Failures MUltipleXer MiXer Negative ACKnowledgement Network Management System Output Back-Off OutDoor Unit Output MUltipleXer Open System Interconnection Private Automatic Branch exchange Packet Assembler/Disassembler Private (automatic) Branch exchange Personal Computer Probability Density Function Protocol Data Unit POLarisation Power Spectral Density Phase Shift Keying Quaternary Phase Shift Keying ReCeiVe-Only Recommendation Report Radio Frequency Receiver Slotted ALOHA protocol Supervisory Control and Data Acquisition
19 xvi ACRONYMS AND ABBREVIATIONS SCPC SDLC SKW SL SNA SNG SR SSPA SW TCP TDM TDMA TTC TV TWT TX VSAT XPD XPI Single Channel Per Carrier Synchronous Data Link Control Satellite-Keeping Window SateLlite Systems Network Architecture (IBM) Satellite News Gathering Selective Repeat Solid State Power Amplifier Stop and Wait Transmission Control Protocol Time Division Multiplex Time Division Multiple Access Telemetry, Tracking and Command TeleVision Travelling Wave Tube Transmitter Very Small Aperture Terminal Cross Polarisation Discrimination Cross Polarisation Isolation
20 Notation A A RAIN Az a B B i B inb B N B outb B Xpond BU c C C D C U C x C y C/N (C/N) D attenuation (larger than one in absolute value, therefore positive value in db), also length of acknowledgement frame (bits) attenuation due to rain azimuth angle (degree) semi-major axis (m) bandwidth (Hz) interfering carrier bandwidth (Hz) inbound carrier bandwidth (Hz) receiver equivalent noise bandwidth (Hz) outbound carrier bandwidth (Hz) transponder bandwidth (Hz) burstiness speed of light: c = m/s carrier power (W) carrier power at earth station receiver input (W) carrier power at satellite transponder input (W) received carrier power on X-polarisation (W) received carrier power on Y-polarisation (W) carrier to noise power ratio downlink carrier to noise power ratio (C/N) Dsat (C/N) IM (C/N) U (C/N) Usat (C/N) T C/N i (C/N i ) D (C/N i ) U (C/N i ) T C/N 0 (C/N 0 ) D (C/N 0 ) Dsat (C/N 0 ) IM (C/N 0 ) U (C/N 0 ) Usat same as above, at saturation carrier to intermodulation noise power ratio (Hz) uplink carrier power to noise power ratio same as above, at saturation overall link (from station to station) carrier to total noise power ratio carrier to interference power ratio downlink carrier to interference power ratio uplink carrier to interference power ratio overall link (from station to station) carrier to interference power ratio carrier power to noise power spectral density ratio (Hz) downlink carrier power to noise power spectral density ratio (Hz) same as above, at saturation (Hz) carrier power to intermodulation noise power spectral density ratio (Hz) uplink carrier power to noise power spectral density ratio (Hz) same as above, at saturation (Hz)
21 xviii NOTATION (C/N 0 ) T C/N 0i (C/N 0i ) D (C/N 0i ) U (C/N 0i ) T D dbx E E b E c e EIRP EIRP ES EIRP ESmax EIRP ESsat EIRP ESi EIRP ESi,max EIRP ESw EIRP SL EIRP SLsat overall link (from station to station) carrier power to total noise power spectral density ratio (W/Hz) carrier power to interference noise power spectral density ratio (Hz) downlink carrier power to interference noise power spectral density ratio (Hz) uplink carrier power to interference noise power spectral density ratio (Hz) overall link (from station to station) carrier power to total interference noise power spectral density ratio (W/Hz) antenna diameter (m), also number of data bits per frame to be conveyed from source to destination value in db relative to x elevation angle (degree), also energy per bit (J) energy per information bit (J) energy per channel bit (J) eccentricity equivalent isotropic radiated power of transmitting equipment (W) EIRP of earth station (W) maximum value of EIRP ES (W) value of EIRP ES,at transponder saturation (W) EIRP of interfering earth station (W) maximum value of earth station EIRP allocated to interfering carrier (W) EIRP of wanted earth station (W) EIRP of satellite transponder (W) EIRP of satellite transponder at saturation (W) EIRP SL1sat EIRP SL2sat EIRP SLi,max EIRP SLw,max EIRP SLww EIRP SLiw EIRP SL1ww EIRP SL2iw EIRP SL1wsat EIRP SL2wsat f f D f IM f LO f U G EIRP of satellite transponder in beam 1 at saturation (W) EIRP of satellite transponder in beam 2 at saturation (W) maximum value of interfering satellite EIRP allocated to interfering carrier (W) maximum value of wanted satellite EIRP for wanted carrier (W) wanted satellite EIRP for wanted carrier in direction of wanted station (W) interfering satellite EIRP for interfering carrier in direction of wanted station (W) EIRP of satellite transponder in beam 1 for wanted carrier in direction of wanted station (W) EIRP of satellite transponder in beam 2 for interfering carrier in direction of wanted station (W) EIRP of satellite transponder in beam 1 in direction of wanted station at saturation (W) EIRP of satellite transponder in beam 2 in direction of wanted station at saturation (W) frequency (Hz): f = c/λ downlink frequency (Hz) frequency of an intermodulation product (Hz) local oscillator frequency (Hz) uplink frequency (Hz) power gain (larger than one in absolute value, therefore positive value in db), also normalised offered traffic, also gravitational constant: G = m 3 /kg s 2
22 NOTATION xix G cod G D G IF G LNA G max G MX G R G Rmax G RX G RX max G RXi G RXw G T G Tmax G Ti,max G T1w G T2w G TE G Xpond G 1 G/T (G/T) ES (G/T) ESmax (G/T) SL coding gain (db) power gain from transponder output to earth station receiver input intermediate frequency amplifier power gain low noise amplifier power gain maximum gain mixer power gain antenna receive gain in direction of transmitting equipment antenna receive gain at boresight receiving equipment composite receive gain: G RX = G Rmax /L R L pol L FRX maximum value of G RX receiving equipment composite receive gain for interfering carrier receiving equipment composite receive gain for wanted carrier antenna transmit gain in direction of receiving equipment antenna transmit gain at boresight antenna transmit gain at boresight for interfering carrier satellite beam 1 transmit antenna gain in direction of wanted station satellite beam 2 transmit antenna gain in direction of wanted station power gain from satellite transponder input to earth station receiver input transponder power gain gain of an ideal antenna with area equal to 1m 2 : G 1 = 4π/λ 2 figure of merit of receiving equipment (K 1 ) figure of merit of earth station receiving equipment (K 1 ) maximum value of (G/T) ES figure of merit of satellite receiving equipment (K 1 ) H i I x IBO IBO inb IBO outb IBO 1 IBO t J x k l L L a L w L D L FRX L FTX total number of bits in the frame header (and trailer if any) orbit inclination received cross polar interference on X-polarisation (W) input back-off input back-off for inbound carrier input back-off for outbound carrier input back-off per carrier with multicarrier operation mode total input back-off with multicarrier operation mode cross polar interference on X-polarisation generated by receive antenna (W) Boltzmann constant: k = J/K; k(dbj/k) = 10 log k = dbj/k Earth station latitude with respect to the satellite latitude (degrees) loss (larger than one in absolute value, therefore positive value in db), also earth station relative longitude with respect to a geostationary satellite (degrees), also length of a frame (bits), also length of a message (bits) Earth station relative longitude with respect to the adjacent satellite (degrees) Earth station relative longitude with respect to the wanted satellite (degrees) downlink path loss feeder loss from antenna to receiver input feeder loss from transmitter output to antenna
23 xx NOTATION L pol L R L Rmax L U L Ui L Uw M e N N i N IM N 0D N 0U N 0iD N 0IM N 0iU N 0T OBO OBO 1 OBO i OBO t OBO w P P f P R P T P TX antenna gain loss as a result of antenna polarisation mismatch off-axis receive gain loss maximum value of L R uplink path loss Uplink path loss for interfering carrier Uplink path loss for wanted carrier mass of the Earth: M e = kg noise power (W) interference power (W) intermodulation noise power (W) downlink thermal noise power spectral density (W/Hz) uplink thermal noise power spectral density (W/Hz) downlink interference power spectral density (W/Hz) intermodulation noise power spectral density (W/Hz) uplink interference power spectral density (W/Hz) total noise power spectral density at the earth station receiver input (W/Hz) output back-off output back-off per carrier with multicarrier operation mode output back-off for interfering carrier total output back-off with multicarrier operation mode output back-off for wanted carrier power (W) probability for a frame to be in error received power at antenna output (W) power fed to transmitting antenna (W) transmitter output power (W) P TX max P x P y PSD PSD i PSD w Q x r R R a R b R binb R boutb R c R cinb R coutb R e R 0 R w S SKW T T A T D T Dmin T F T GROUND transmitter output power at saturation (W) transmitted carrier power on X-polarisation (W) transmitted carrier power on Y-polarisation (W) power spectral density (W/Hz) interfering carrier power spectral density (W/Hz) wanted carrier power spectral density (W/Hz) cross polar interference on X-polarisation generated by transmit antenna (W) distance from centre of earth to satellite range, also bit rate slant range from earth station to adjacent satellite information bit rate (b/s) information bit rate on inbound carrier (b/s) information bit rate on outbound carrier (b/s) transmission bit rate (b/s) transmission bit rate on inbound carrier (b/s) transmission bit rate on outbound carrier (b/s) earth radius: R e = 6378 km geostationary satellite altitude: R 0 = km slant range from earth station to wanted satellite normalised throughput satellite station keeping window halfwidth (degrees) interval of time (s), also period of orbit (s), also medium temperature (K) and noise temperature (K) antenna noise temperature (K) downlink system noise temperature (K) minimum value of T D (K) feeder temperature (K) ground noise temperature in vicinity of earth station (K)
24 NOTATION xxi T IF T LNA T m T MX T p T R T SKY T U THRU W X XPD XPI RX XPI TX α Γ η intermediate frequency amplifier effective input noise temperature (K) low noise amplifier effective input noise temperature (K) average medium temperature (K) mixer effective input noise temperature (K) propagation time (s) receiver effective input noise temperature (K) clear sky noise temperature (K) uplink system noise temperature (K) throughput (b/s) window size order of an intermodulation product cross polar discrimination receive antenna cross polarisation isolation transmit antenna cross polarisation isolation angular separation between two satellites (degrees) spectral efficiency (b/s Hz) ratio of co-polar wanted carrier power to cross-polar interfering carrier power efficiency η a antenna efficiency (typically 0.6) η c channel efficiency η cgbn channel efficiency with go-back-n protocol η csr channel efficiency with selective-repeat protocol η csw channel efficiency with stop-and-wait protocol θ angle from boresigth of antenna (degrees) θ 3dB half power beamwidth of an antenna (degrees) θ R antenna off-axis of angle for reception (degrees) θ Rmax maximum value of antenna off-axis angle for reception (degrees) θ T antenna off-axis angle for transmission (degrees) θ Tmax maximum value of antenna off-axis angle for transmission (degrees) λ wavelength (m) = c/f,also traffic generation rate (s 1 ) µ product of gravitational constant G and mass of the Earth M e : µ = m 3 /s 2 ρ code rate τ packet duration (s) Φ power flux density (W/m 2 ) Φ sat power flux density at saturation (W/m 2 ) Φ t total flux density (W/m 2 )
26 1 Introduction This chapter aims to provide the framework of VSAT technology in the evolving context of satellite communications in terms of network configuration, services, economics, operational and regulatory aspects. It can also be considered by the reader as a guide to the following chapters which aim to provide more details on specific issues. 1.1 VSAT NETWORK DEFINITION VSAT, now a well established acronym for Very Small Aperture Terminal, was initially a trademark for a small earth station marketed in the 1980s by Telcom General in the USA. Its success as a generic name probably comes from the appealing association of its first letter V, which establishes a victorious context, or may be perceived as a friendly sign of participation, and SAT which definitely establishes some reference to satellite communications. In this book, the use of the word terminal which appears in the clarification of the acronym will be replaced by earth station, or station for short, which is the more common designation in the field of satellite communications for the equipment assembly allowing reception from or transmission to a satellite. The word terminal will be used to designate the end user equipment (telephone set, facsimile machine, television set, computer, etc.) which generates or accepts the traffic that is conveyed within VSAT networks. This complies with regulatory texts, such as those of the International Telecommunications Union (ITU), where for instance equipment generating data traffic, such as computers, are named Data Terminal Equipment (DTE). VSAT Networks, 2 nd Edition. G. Maral 2003 John Wiley & Sons, Ltd ISBN:
27 2 INTRODUCTION VSATs are one of the intermediary steps of the general trend in earth station size reduction that has been observed in satellite communications since the launch of the first communication satellites in the mid 1960s. Indeed, earth stations have evolved from the large INTELSAT Standard A earth stations equipped with antennas 30 m wide, to today s receive-only stations with antennas as small as 60 cm for direct reception of television transmitted by broadcasting satellites, or hand held terminals for radiolocation such as the Global Postioning System (GPS) receivers. Present day hand held satellite phones (IRIDIUM, GLOBALSTAR) are pocket size. Figure 1.1 illustrates this trend. Therefore, VSATs are at the lower end of a product line which offers a large variety of communication services; at the upper end are large stations (often called trunking stations) which support large capacity satellite links. They are mainly used within international switching networks to support trunk telephony services between countries, possibly on different continents. Figure 1.2 illustrates how such stations collect traffic from end users via terrestrial links that are part of the public switched network of a given country. These stations are quite expensive, with costs in the range of $10 million, and require important civil works for their installation. Link capacities are in the range of a few thousand telephone channels, or equivalently about one hundred Mbs 1. They are owned and operated by national telecom operators, such as the PTTs, or large private telecom companies TRUNKING STATIONS THIN ROUTE STATIONS VSATS MOBILE and PERSONAL STATIONS 2000 Figure 1.1 VSAT: a step towards earth station size reduction