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Multi-Link Iridium Satellite Data Communication System

Multi-Link Iridium Satellite Data Communication System. Overview, Performance and Reliability from Summer 2003 NGRIP, Greenland Field Experiments June 23-July 17, 2003 Abdul Jabbar Mohammad, Graduate Research Assistant Dr.Victor Frost, Dan F. Servey Distinguished Professor

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Multi-Link Iridium Satellite Data Communication System

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  1. Multi-Link Iridium Satellite Data Communication System Overview, Performance and Reliability from Summer 2003 NGRIP, Greenland Field Experiments June 23-July 17, 2003 Abdul Jabbar Mohammad, Graduate Research Assistant Dr.Victor Frost, Dan F. Servey Distinguished Professor (September 24, 2003)

  2. Polar Radar for Ice Sheet Measurements (PRISM) The communication requirements of PRISM field experiments in Greenland and Antarctica include Data telemetry from the field to the University Access to University and web resources from field Public outreach to increase the interest of student community (K-12) in scientific research and enable the science community to virtually participate in polar expeditions Generic data communication for Remote field research Mainstream communication system for polar science expeditions, field camps in Arctic/Antarctic and other research purposes Government and security use Motivation

  3. Polar regions do not have conventional communication facilities (dial-up, DSL, Cable Modem, etc) and are not serviced by most of the major broadband satellite systems. Introduction – Commercial Satellite Systems Inmarsat Intelsat Globalstar

  4. Introduction – Special Purpose Satellite Systems • NASA satellites like ATS3, LES9, GOES, TDRS 1,and MARISAT2 provide broadband access to Polar Regions • Geo-synchronous, they have a limited visibility window at Poles – typically 10-13 hrs/day. • High satellite altitude and low elevation angles (1-20) result in extremely large field equipment. • May not be readily available 20 m diameter Marisat and GOES antenna at South Pole

  5. The only satellite system with true pole-to-pole coverage 66 low earth orbiting (LEO) satellites with 14 spares It has onboard satellite switching technology which allows it to service large areas with fewer gateways Since it was originally designed as a voice only system, it provides a low data rate of 2.4Kbps Not practical to be used as a main stream/ life-line communication system Introduction - Iridium Satellite System

  6. Problem Statement A reliable, lightweight, portable and easily scalable data/Internet access system providing true Polar coverage. Solution Implement a multi-link point-to-point Iridium communication system to combine multiple satellite links to obtain a single logical channel of aggregate bandwidth. Introduction – Problem Statement

  7. Background - Iridium

  8. Background - Inverse Multiplexing Mux App 1 High Bandwidth Link App 2 App 3 Multiplexing Low Bandwidth Links Inv-Mux App 1 Traditional Multiplexing - Data from a multiple applications/users sent over a single high bandwidth link. Inverse Multiplexing - Data from a single application is fragmented and or distributed over multiple low bandwidth links. Increases the available bandwidth per application significantly Multi-link point-to-point protocol (MLPPP), an extension to the PPP is a packet based inverse multiplexing solution Overhead of 12 bytes Fragmentation of network layer protocol data units (PDU’s) into smaller segments depending upon the PDU size, link MTUs and the number of available links. Inverse multiplexing

  9. Layer 2 protocol that implements inverse multiplexing on a packet by packet basis IP packets are encapsulated in to PPP frames with segment numbers – 12 byte overhead Packet fragmentation depends on the number of available links and their capacity, packet size and MTU size Background - Multi-link point-to-point protocol Not Fragmented Fragmented IP Packet IP Packet SH SH PDF SH PDF SH PDF SH PDF IP Packet Modem 1 Modem 2 Modem 3 Modem 4 Modem i SH-Segment header; PDF- packet data fragment

  10. Multi-channel Iridium System – Design The design requirements of the system are as follows. • The multi-channel implementation should maximize the throughput. • To support real-time interactions, the system should minimize the end-to-end delay. • The overall system should be reliable and have autonomous operation so as to handle call drops and system/power failures in remote field deployment.

  11. Multi-channel Iridium System – Protocol Stack Remote System Local System point-to-point satellite links

  12. Multi-channel Iridium System – Network Architecture NGRIP Camp, Greenland 100 Mbps Ethernet (Default gateway) (Default gateway) Guser 4 PPP Client ITTC Default Router P-T-P Satellite link eth0 ppp0 100 Mbps Ethernet User 2 User 1 Guser 3 G`user 1 User 3 Guser 2 Camp WI-FI World Wide Web ITTC Network, University of Kansas PPP Server eth0 ppp0

  13. 4-Channel Iridium System Implementation Remote System PPP client Iridium Gateway USB-SERIAL I. Modem 1 Antenna Grid I. Modem 2 I. Modem 3 I. Modem 4 Remote Subsystem Local System PPP Server Multi-port PCI card PSTN Modem Pool Local Subsystem

  14. 4-Channel System – Implemented at KU

  15. 4-Channel System – Implemented at KU

  16. Linux based system Open source PPP package Custom software developed at University of Kansas Chat scripts for Iridium modems PPP script to tune link parameters for Iridium satellite system Client-Server configuration Autonomous operation-connection setup, user authentication, detecting failures, reconnections, handling power failures/system resets, generating status information (text) 4-Channel System – Software Overview

  17. 4-Channel System – Modem Flow Control Check if any modem is alive Connect Modem 1 NO START FAIL OK YES Terminate All OK Monitor Status FAIL FAIL FAIL FAIL Connect Modem 4 Connect Modem 3 Connect Modem 2 OK OK OK FAILED FAILED FAILED OK OK OK Monitor Status Monitor Status Monitor Status

  18. Field Tests and Results – Field implementation System with control software USB In USB Out Antenna Connectors Dimensions: 24x19x5

  19. Field Tests and Results – Antenna Setup 3 FT 10 FT

  20. Results – Delay and Loss Measurement • Ping tests between the two machines at the end of the of satellite link • One way propagation delay = (800+8000+6000)Km / (3e6)Km/sec= 49msec • Transmission time for 64 bytes@2.4Kbps = 64*8/2400=213msec • Theoretically, the RTT delay =2*(49+213)= 524msec+delay at the gateway • Test results show an average RTT delay of 1.8 sec, which may be attributed to the inter-satellite switching and delay at the gateway

  21. Results – Delay Measurement Random variation of delay High mean deviation Delay increases linearly with packet size Normalized delay is almost constant for MTU sizes > 800 bytes Changing distance between the user and satellite as well as between satellites themselves (ISLs) Non-uniform traffic distribution, varying delays on different routes 56 100 200 300 500 800 1000 1200 1500

  22. Results – Throughput Tools used – TTCP, IPERF Throughput varies to some extend due to RTT variation Efficiency > 90% Effective throughputs during large file transfers (K b p s)

  23. Results – Reliability: 10th July 24 hr test Call drops on the first modem Total : 13 Call drops Uptime % 80.6 91.8 94.7 96.8

  24. Results – Reliability: 12th July 24 hr test Call drops on the first modem Total : 16 Call drops Uptime % 80 92 95 96

  25. Results – TCP performance of a single link TCP Version – TCP SACK Throughput of a segment is defined as the size of the segment seen divided by the time since the last segment was seen (in this direction). RTT is defined as the time difference between the instance a TCP segment is transmitted (by the TCP layer) and the instance an acknowledgement of that segment is received. The average throughput of the connection is 2.45Kbps. The average RTT was found to be 20 seconds Bandwidth Delay Product (BDP) = 2400*20/8 = 6000 bytes = 4 segments. Due to low throughput, the BDP is small. Throughput vs. Time -------- avg. of last 10 segments -------- avg. of all the segments up to that point RTT vs. Time

  26. Results – TCP performance of a 4-channel system • The average throughput obtained - 9.4 Kbps • The average RTT observed - 16.6 seconds • Factors affecting throughput and RTT • TCP Slow start • MLPPP fragmentation • Random delay variation • TCP cumulative acknowledgments Throughput vs. Time -------- avg. of last 10 segments -------- avg. of all the segments up to that point Time Sequence Graph RTT vs. Time ------- Received Acknowledgements ------- TCP segments transmitted ------- Received window advertisement

  27. Results – TCP performance of a 4-channel system Time Sequence Graph Outstanding Unacknowledged data and Congestion window Time Sequence Graph Outstanding Unacknowledged Data ------- Received Acknowledgements ------- TCP segments transmitted ------- Received window advertisement ------- Instantaneous outstanding data samples ------- Average outstanding data ------- Weighted average of outstanding data

  28. Results – TCP performance degradation due to packet loss Low packet loss, long time experiments needed to determine the performance degradation Two packet losses were observed in the FTP video upload resulting in packet retransmissions Packet losses usually occur during call hand-offs Retransmission time outs (RTO) is very large due to high RTT and high mean deviation Time Sequence Graph Time Sequence Graph Retransmitted packet Retransmitted packet

  29. Results – TCP performance degradation due to packet loss RTT vs. Time Effect on the TCP performance due to packet loss Decrease in the throughput following the packet loss RTT peaks around the packet loss The average throughput of the connection is observed to be 7.44 Kbps. Throughput vs. Time Outstanding Unacknowledged Data

  30. Results – TCP performance degradation due to call drop Time Sequence Graph Packet loss due to a call drop on one links of the multilink bundle A finite amount of time for the data link layer realize the link has failed Large RTO timer The entire window of packets (12 in this case) and acknowledgements that are in flight on that particular link are dropped. Throughput of the connection – 7.6 Kbps. ------- Received Acknowledgements ------- TCP segments transmitted ------- Received window advertisement Outstanding Unacknowledged Data Time Sequence Graph ------- Instantaneous outstanding data samples ------- Average outstanding data ------- Weighted average of outstanding data

  31. Results – Mobile tests Test Location

  32. Results – Mobile Performance START STOP Throughput vs. Time Avg. Throughput = 9 Kbps

  33. Results – Mobile Performance (cont.d) START STOP Throughput vs. Time Avg. Throughput = 9.34 Kbps

  34. The following files were downloaded from WEB and ITTC network. The size of these files, their importance (on a scale of 1-10, based on user survey) are shown Applications – Uploads and Downloads

  35. Applications – Internet at camp

  36. Applications –WI-FI setup integrated with Iridium

  37. Applications - Wireless Internet • Wireless Internet access was used by many NGRIP researchers • Email access put the researchers in touch with their home institutions • Scientist at NGRIP were able to send information back to their home institutions • The system was also useful for general camp purpose: sending drawings to order spares for a broken caterpillar, excel spreadsheet for food order, general press releases, downloading weather reports for planning C-130 landings

  38. Applications – Outreach • Daily journal logs were uploaded to the University of Kansas and posted on www.ku-prism.org • Two way video conference was held twice to test the real time audio/video • The system not only helped PRISM group to download critical software, but also made it possible to obtain faculty guidance and expertise on the field • It also helped the researchers back at the University of Kansas to virtually view the field experiments since the video clips of experiments being carried out were also uploaded to the University and posted on project website Testing Video conference between the camp and the University Uploading daily field logs

  39. Multi-link PPP is relatively efficient over Iridium system. With 4-modems efficiency was observed to be >90% The RTT the system is significant ~ 1.8 seconds, which is an impairment to real time interactions There is a large (random) variation in delay of the system The average up-time of the overall connection is good > 95% Mobile tests showed performance very similar to that of stationary system up to speeds of 20mph A call drop on the first modem, results in a complete loss of connection, a potential bug in the PPP networking code; could be fixed in newer version of the PPP software Strong interference from a nearby source on the same frequency (1.6GHz) could cause little disturbance in the system leading to either connection failures or call drops. This was observed when a radar sweeping between 500MHz-2GHz with an output of 20dBm was placed at distance less than 15 meters Lessons Learned

  40. Conclusions In order to provide data and Internet access to Polar Regions, we have developed a reliable, easily scalable, lightweight, and readily available multi-channel data communication system based on Iridium satellites that provide round the clock, pole-to-pole coverage. A link management software is developed that ensures fully autonomous and reliable operation An end-to-end network architecture providing Internet access to science expeditions in Polar Regions is demonstrated. This system provided for the first time, Internet access to NGRIP camp at Greenland and obtained the call drop pattern. The TCP performance and the reliability of the system is characterized

  41. Modify the MLPPP code so that the interface is attached to the bundle and not to the primary link Additional research to determine the cause of delay and develop methods to overcome it. Evaluate the new data-after-voice (DAV) service from Iridium Evaluate different versions of TCP to determine the enhancements that can handle the random variation and value of RTT Improve the user friendliness of the system GUI for connection setup Self-test / diagnostic tools for troubleshooting Research into the spacing and sharing of antennas to reduce the antenna footprint Future Work

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