Multi-Link Iridium Satellite Data Communication System by Mohammad Abdul Jabbar Bachelor of Engineering, Electrical Engineering Osmania University, Hyderabad, India, 2001 Submitted to the Department of Electrical Engineering and Computer Science and the Faculty of the Graduate School of the University of Kansas in partial fulfillment of the requirements for the degree of Master of Science. Thesis Committee Chairperson: Dr. Victor S. Frost Dr. Glenn Prescott Dr. Christopher Allen Date Accepted: __________________ © Copyright 2004 by Mohammad Abdul Jabbar. All rights reserved. i i To my caring parents who are my inspiration To my graduate advisor who is a remarkable mentor ii i Acknowledgements I would like to extend my sincere and utmost gratitude to Dr. Victor Frost, my advisor and committee chair for his invaluable guidance and inspiration throughout my research. I would like to thank him for giving me the opportunity to work on this project, which is his brainchild. I would also like to thank Dr. Glenn Prescott and Dr. Chris Allen for being on my committee and helping me solve some of the technical problems faced during my research. I would like to take this opportunity to thank Tory Akins and the entire Greenland field team for their support during the field experiments, without which this work would not have been possible. Thanks are due to Dennis Sundermeyer who helped me with the packaging of the system and carrying out field experiments. I would like to thank Brett Becker for his help with the network design and his support throughout my research. Thanks are also due to Dan Depardo, who helped me with the setup in the wireless lab and provided valuable suggestions. I extend a special note of thanks to my friends, Nandish Chalishazar and Radha Krishna Mukkai for providing the much needed help during the Greenland experiments. I would like to express my gratitude to all my friends at KU for their moral support. iv Abstract Data and Internet access are integral for the success of any field research. The current state-of-the-art communication facilities, besides their wide penetration are still not available in many geographically remote regions including the Arctic and Antarctic regions. Research in the Polar Region involving data collection and telemetry has grown significantly over past few years. The commercial broadband satellite systems do not provide coverage at higher latitudes. The current geo-synchronous NASA satellites providing broadband access to Polar research have a limited visibility window at poles and need extremely large field equipment. Hence the need for a reliable, portable and easily available data/Internet access system is clearly evident. In this thesis, we present an Iridium based data communication system that can provide round the clock coverage from pole-to-pole. Since the Iridium satellite system provides a low bandwidth that is not sufficient to support most of the data applications, inverse multiplexing technique is used to combine multiple satellite links into a single logical channel of aggregate bandwidth. Multi-link point-to-point protocol is used to implement packet level inverse multiplexing. This technique effectively increases the available bandwidth per application to useable limits. Field experiments conducted at NGRIP, Greenland showed that the system is highly efficient and reliable. TCP performance analysis showed that the system has high throughput efficiency and high round trip time. An analysis of the observed TCP behavior is presented. v Table of Contents 1. INTRODUCTION……………….……………………………………………....1 1.1. Motivation…………………………………………………………...……..…2 1.2. Satellite data communication………..…………………………......................4 1.2.1. Commercial satellite systems……………………………………….....4 1.2.2. Special purpose satellite systems..…………………………………….7 1.2.3. Iridium satellite system…………….……………………………….....8 1.3. Project goals……………………………….………………………………...10 1.4. Accomplishments…………………………………………………..……….10 1.5. Thesis organization……………………………………………………...…..11 2. BACKGROUND………………………………………………………..………13 2.1. Iridium Satellite System………………………………………………….....13 2.1.1. Architecture……………………………………………..……………14 2.1.2. Constellation design………………………………………………….16 2.1.3. Multi-access scheme………………………………………………....19 2.1.4. System capacity…………………………………………………..….22 2.1.5. Network performance……………………………………………..…23 2.1.6. Hardware……………………………………………………..………26 2.1.7. Types of data services………………………………………………..27 2.1.8. Summary of Iridium………………………………………………….29 2.2. Supporting Technologies…………...……………………………………….30 2.2.1. Inverse Multiplexing………………...……………………………….31 2.2.2. Point-to-Point protocol……………………………...………………..33 2.2.3. Multi-link Point-to-Point protocol……………………………..…….35 3. SYSTEM DESIGN…………………………………………………………...…38 3.1. System design requirements………………………………………………...38 3.2. System design…………………………………………………………….....38 3.3. Protocol Stack…………………………………………………………….....40 v i 3.4. Network architecture………..…………………………………………..…..41 4. SYSTEM IMPLEMENTATION……………..……….…………………..…...44 4.1. Four channel Iridium communication system………………………….…...44 4.2. Modem control……….……………………………………………….….….46 4.3. PPP link parameters and user authentication………………………..………47 4.4. Client-Server configuration………………………………………………....49 4.5. Autonomous system operation………………………………………..…….51 5. FIELD TESTS AND RESULTS……..…………………….…………………..54 5.1. Overview of field experiments…………………..………………………….54 5.2. General system performance…………………………………..……………57 5.2.1. Delay and loss.…………………………………………………..…...57 5.2.2. Throughput…………………………………………………..……….61 5.3. System reliability………………………………………………………..…..64 5.4. TCP performance over Iridium…………………………………………..….68 5.4.1. Related work………………………………………………………....68 5.4.2. Overview of TCP performance tests………………………………....70 5.4.3. TCP performance over a single Iridium link………………………...71 5.4.4. TCP performance over a multi-channel Iridium system………..……73 5.4.5. Performance degradation due to packet losses………………….…...79 5.4.6. Performance degradation due to call drops and link termination…....82 5.5. Mobile system performance…………………………………………..…….87 5.6. Applications………………………………………………………..………..89 6. CONCLUSIONS AND FUTURE WORK……………………………..……...92 6.1. Conclusions…………………………………………………….…………....92 6.2. Future work……………………………………………………….………....93 REFERENCES……………….……………………………………….…………….93 APPENDIX A……………….…………………………………………………....…98 APPENDIX B………………..………………………………………………….....100 APPENDIX C...………………………..…………………………………………..101 vi i List of Figures Figure 1.1: Coverage map of Inmarsat……………………………..………................5 Figure 1.2: Coverage map of Globalstar………………………………........................6 Figure 1.3: Typical South Pole satellite visibility………………………………..........7 Figure 2.1: Iridium satellite………………………………..........................................13 Figure 2.2: An Iridium gateway ………………………………..................................15 Figure 2.3: Call hand-off between two satellites………………………………..…...17 Figure 2.4: Coverage area of a satellite…………………...........................................18 Figure 2.5: Iridium FDMA scheme………………….................................................19 Figure 2.6: The three panels of an Iridium phased array antenna…………..…….….20 Figure 2.7: Frequency reuse in Iridium…………………………................................21 Figure 2.8: Iridium TDMA scheme………………….................................................22 Figure 2.9: Iridium data modem and antenna…………………..................................26 Figure 2.10: Iridium-to-PSTN data connection…………………...............................27 Figure 2.11: Iridium-to-Iridium data call………………….........................................28 Figure 2.12: Traditional multiplexing…………………..............................................30 Figure 2.13: Inverse multiplexing…………………....................................................31 Figure 2.14: PPP phase diagram…………………......................................................34 Figure 2.15: Multi-link PPP operation………………….............................................36 Figure 3.1: Multi-channel data communication system…...……………....................39 Figure 3.2: Protocol stack of the remote and local system…………..........................40 Figure 3.3: Network architecture………….................................................................41 Figure 4.1: Four-channel Iridium communication system………….....................…..44 Figure 4.2: Four-channel Iridium communication unit and its antenna array….....…45 Figure 4.3: Software flow control diagram of the remote system………...…………50 Figure 4.4: Software flow control of the local system (server)…………..………….52 Figure 4.5: Modem flow control diagram…………....................................................53 Figure 5.1: Field implementation of multi-channel Iridium communication system..55 Figure 5.2: Field antenna setup……………………………........................................56 vi ii Figure 5.3: Mobile performance tests…………..........................................................56 Figure 5.4: Round trip time vs. time of the day…………...........................................59 Figure 5.5: Variation of RTT with respect to packet size…………............................60 Figure 5.6: Variation of normalized RTT with respect to packet size…………….....61 Figure 5.7: Plot of Throughput vs. Number of modems…………..............................63 Figure 5.8: Call drop pattern of the first modem during the 10th July 24-hour test….65 Figure 5.9: Availability of modems during the 10th July 24-hour tests…………..….65 Figure 5.10: Call drop pattern of the first modem during the 12th July 24-hour test...67 Figure 5.11: Availability of modems during the 12th July 24-hour tests…………….67 Figure 5.12: Throughput of a single Iridium link………………………………..…..72 Figure 5.13: Measured RTT of a single Iridium Link………………………..……...73 Figure 5.14: Throughput of a 4-channel Iridium communication system…………...74 Figure 5.15: RTT of the 4-channel Iridium communication system………………...74 Figure 5.16: Time sequence graph of the 4-channel Iridium system…………...……77 Figure 5.17: Closer look at time sequence graph of a 4-channel Iridium system……77 Figure 5.18: Outstanding unacknowledged data during a TCP connection using 4-channel Iridium system……………..……………………………………….78 Figure 5.19: Time sequence graph of a video upload…………………………..……79 Figure 5.20: Closer look at the time sequence graph of a video upload …………….80 Figure 5.21: Outstanding unacknowledged data of a video upload……………….....81 Figure 5.22: Measured RTT during video upload……………………………..…….81 Figure 5.23: Throughput observed during video upload………………………….....82 Figure 5.24: Time sequence graph of a TCP connection with a single call drop...….83 Figure 5.25: Outstanding data window of TCP connection with a single call drop…83 Figure 5.26: Time sequence graph a TCP connection with a single call drop….……85 Figure 5.27: Throughput of the TCP connection with a single call drop…………....85 Figure 5.28: Time sequence graph of a TCP connection with a single link termination…………………………………………………………………………...86 ix Figure 5.29: Closer look at the time sequence graph of a TCP connection with a single link termination……………………………………………………………….87 Figure 5.30: Path taken by test vehicle-first test………………………………..……87 Figure 5.31: Speed of the vehicle-first test……………………………………..……87 Figure 5.32: Throughput of the mobile 4-channel system-first test…………...……..88 Figure 5.33: Path taken by test vehicle-second test……………………..…………...88 Figure 5.34: Speed of the vehicle-second test………………………………….……88 Figure 5.35: Throughput of the mobile 4-channel system during second test….……89 Figure 5.36: Applications: (a) Video conferencing and (b) Wireless Internet access.90 List of Tables Table 2.1: Summary of Iridium system………………………………..…………….29 Table 5.1: Round trip time of Iridium data link…………………...………………....58 Table 5.2: Variation of throughput with number of modems…………………..……62 Table 5.3: Effective throughput for large file transfers…………..………………….63 Table 5.4: Time interval between successive call drops during the10th July 24hr test…………………………………………………………….……………………...65 Table 5.5: Statistics of the 10th July 24 hour test……………..……………………...66 Table 5.6: Time interval between successive call drops during the 12th July 24hr test……………………………………………………………………………………67 Table 5.7: Statistics of the 12th July 24 hour test…………………………………….68 Table 5.8: Summary of file downloads from Greenland using the Iridium system.....89 x