MultiLink Iridium Satellite Data Communication System

1
Multi-Link Iridium Satellite Data Communication
System

• Overview, Performance and Reliability from Summer
  2004 SUMMIT, Greenland Field Experiments
• July 14-July 25, 2004
• Abdul Jabbar Mohammad, Said Zaghloul, Graduate
  Research Assistants
• Dr.Victor Frost, Dan F. Servey Distinguished
  Professor
• (August 22, 2003)

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Presentation Outline

• Previous Work
• 4-Channel System
• Conclusions from 2003 Field Experiments
• 8-Channel Iridium System
• Design
• Integrated Unit
• GUI Software
• Analysis
• Network Architecture
• 2004 Field Experiments
• Field Implementation
• Results
• Conclusions and Future Work

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4-Channel Iridium System (Tested in Summer 2003)

• 4 Iridium 4 PSTN data configuration
• Discrete components
• Patch antennas
• Control software on a rugged Laptop

4
Conclusions from 2003 field experiments

• Developed a reliable multi-channel data
  communication system based on Iridium satellites
  that provide round the clock, pole-to-pole
  coverage.
• Developed console based link management software
  that ensures fully autonomous and reliable
  operation
• An end-to-end network architecture providing
  Internet access to science expeditions in Polar
  Regions was demonstrated.
• The system efficiency was observed to be gt90.
  With 4-modems the average end-to-end throughput
  was found to be 9.26 Kbps
• The round trip time of the system in Iridium-PSTN
  configuration was significant 1.8 sec
• The average up-time of the overall connection was
  approx 90. The average time interval between
  primary call drops was 100 minutes
• Mobile tests showed performance very similar to
  that of stationary system up to speeds of 20mph
• 4-Iridium to 4-PSTN configuration was found to be
  stable of autonomous operation

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Conclusions from 2003 field experiments

• The USB-to-serial converter used for multiple
  serial ports was not stable resulting in system
  failures.
• Interaction of PPP level compression with control
  software results in corrupted modem termination,
  resulting in significant packet loss
• Identified areas for additional research
• Evaluate the new data-after-voice (DAV) service
  from Iridium
• Improve the user friendliness of the system
• Research into the spacing and sharing of antennas
  to reduce the antenna footprint
• Increase the the system capacity by scaling the
  system from 4 to 8 channels
• Develop a fully integrated plug and play system
  that can be deployed easily in the field

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8-channel Iridium System Design Elements

• Integrated 8 Iridium modems and all the
  components in an 19 rack mount unit.
• On-board computer to run the control software
• Single board EBX format system ( P-III, 1 GHz,
  512 MB RAM)
• Extended temperature operation (-300 C to 800 C
  )
• PC104 type multi-port serial card with 8 DB9
  ports (extended temp operation)
• Integrated 5x4 LCD screen, front panel flips
  down to hold the keyboard/mouse
• Single linear power supply for the 8 modems and
  on-board computer
• Developed a new GUI based management/control
  software, that configures the unit in all the
  data modes a) Iridium-Iridium DAV mode, b)
  Iridium-Iridium data mode, c) Iridium-PSTN mode
• Replaced the patch antennas with inverted cone
  antennas that can be easily mounted on field and
  do not need a external ground plane.

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8-channel Iridium System Integrated Unit
Bottom View
Top View
19
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Front View

• Dimension 9x19x24 inch
• Weight 50 lbs
• Operating temp -30 to 60 c
• Power input 120 V AC
• Replication Costs 18,000

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8
8-channel Iridium System Client Software
Client Software consists of three modules

• Graphical User Interface
• Easy Configuration and Operation
• Does not require experienced users
• Control Software
• It is the core of the software
• Automatic Modem Control
• XML Database
• Registers all call drops and retrials
• Makes it possible for future analysis of network
  performance data

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8-channel Iridium System Client GUI
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8-channel Iridium System Client GUI
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8-channel Iridium System Analysis
Machine A
Machine B

• System Model
• Application FTP, HTTP
• Agent TCP, UDP
• MLPPP
• 8 Modem Links

App
App
Agent
Agent
Iridium Network

• Modems Model
• Each link has a dropping probability
• Each link has a probability of error

8 Modem Links
MLPPP
MLPPP
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8-channel Iridium System Network Architecture
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Field Experiments System Implementation
8-Channel system in a weather-port at SUMMIT camp
in Greenland, July 2004
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Field Experiments Antenna Setup
4 ft
10 ft
8 Antenna setup at SUMMIT camp in Greenland, July
2004
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Results Throughput

• Average throughput efficiency was observed to be
  95
• The above results are from the test cases where
  no call drops were experienced
• In event of call drops the effective throughput
  of the system will be less than the above values

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Results Throughput
FTP throughput observed during data transfer
between the field camp and KU

• Average throughput during the FTP upload of large
  files was observed to be 15.38 Kbps
• Due to call drops, the efficiency was reduced to
  80
• Detailed TCP analysis based on IPERF and FTP data
  is in progress

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Results Round Trip Time

• Average RTT 1.4 sec
• Minimum observed RTT 608 msec
• Mean deviation 800 msec
• Detailed analysis in progress

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Results Reliability 14th July 12-hr test

• Call drop pattern during 8 Iridium 8 Iridium
  DAV mode test for 12 hrs
• Percentage uptime with full capacity (8 channels)
  is 89 and with at least one modem is 98
• Total number of primary call drops during 12 hrs
  4
• Average time interval between call drops is 180
  mins

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Results Reliability 22nd July 32-hr test
Uptime

• Call drop pattern during 8 Iridium 8 Iridium
  DAV mode test for 32 hrs
• Percentage uptime with full capacity (8 channels)
  is 85 and with at least one modem is 96
• Total number of primary call drops during 32 hrs
  24
• Average time interval between call drops is 72
  mins

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Results Reliability 19th July 6-hr test

• Call drop pattern during 8 Iridium 8 PSTN data
  mode test for 32 hrs
• Percentage uptime with full capacity (8 channels)
  is 67 and with at least one modem is 90
• Total number of primary call drops during 6 hrs
  9
• Average time interval between call drops is 35
  mins

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Results Mobile tests
Iridium antennas
Iridium system mounted in an autonomous vehicle
(MARVIN)
Experiments monitored from another vehicle
through 802.11b link
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Results Mobile tests

• Call drop pattern during 8 Iridium 8 Iridium
  DAV mode test for 2 hrs
• Percentage uptime with full capacity (8 channels)
  is 65 and with at least one modem is 92
• Average time interval between call drops is 45
  mins
• Average throughput 18.6 Kbps, Average RTT 2
  sec

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Applications

• Summer 2004 field experiments
• Communications data upload up to 40 MB files
• Radar data uploads up to 55 MB files
• Text chat with PRISM group at KU
• Video conference - real time audio/video
• Individual audio or video conference works with
  moderate quality with the commonly available
  codecs
• Outreach Use
• Daily Journal logs uploaded
• Daily Pictures uploaded
• Video clips uploaded
• Held video conference with science teachers/
  virtual camp tour
• Wireless Internet access

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Conclusions

• Integrated 8-channel system
• Works out of the box
• Reliable and fully autonomous operation
• The newly developed GUI based control software
• Reduced the field setup time, increased the ease
  of operation
• Suitable for operation by non-technical users
• System performance based on field experiments
• Average throughput with 8 channels is 18.6 Kbps,
  efficiency gt 90
• Average round trip time using DAV modes is 1.4
  sec, significantly less than 1.8 sec of
  Iridium-PSTN configuration
• Average uptime with full capacity using DAV mode
  was 85 better than both non-DAV mode and PSTN
  mode
• Percentage system uptime (at least one mode) was
  95 for all the modes
• Average time interval between call drops is 60
  mins and varies a lot.
• In conclusion, the throughput and delay
  performance of the system using Iridium-Iridium
  DAV mode is better than other data modes.

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Lessons Learned

• The average time interval between call drops
  reduced from 100 minutes in case of 4 Iridium-4
  PSTN system to 60 minutes in case of 8 Iridium
  8 Iridium DAV system.
• The call drop pattern as seen in number of
  online modems vs. time characteristics varies
  over time. (detailed study in progress)
• Modem firmware failures were experience for the
  first time. Modem locks up randomly and needs
  power cycling. This problem is not very severe
  and occurred less than 5 times during the field
  experiments . Further, this issues has been
  noticed by the other researchers using Iridium
  for field work.
• Mounting of antennas on the mobile vehicle could
  be improved to increase stability for long
  duration experiments. While the current mounting
  works for short duration tests, it is not stable
  for permanent field operation
• Due to a bug in linux pppd software, a call drop
  on the primary modem still causes the entire
  bundle to drop.

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Future Work to Understand and Enhance the MLPPP
Iridium System

• The performance of network using the
  Iridium/MLPPP needs to be evaluated
• A system model is needed in order to explain the
  network behavior and to develop enhancements to
  the system
• Call drops needs to be categorized and reasons
  for call drops need to be studied
• Due to poor signal strength (Low SNR)
• Due to Handovers (inter-satellite and
  intra-cell)
• Other reasons
• The performance of the TCP RTT measurement
  algorithm needs to be evaluated over the MLPPP
  Iridium link

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Future Work to Understand and Enhance the MLPPP
Iridium System

• Analyze call drop pattern. Experiments at ITTC to
  validate the number of call drops.
• Upgrade modem firmware (as it becomes available)
  to solve the problem of failures. Else the
  control software should be modified so that it
  can recognize modem failures and cycle power to
  that modem.
• Develop user-friendly GUI based server software
  (similar to the client software) to increase the
  functionality and ease of operation
• Research the pppd bug that causes the entire
  bundle to drop on the event of a primary modem
  call drop. Modification of PPP networking code
  could be one solution.
• While detailed TCP analysis is in progress, it is
  evident that a call drop results in a degradation
  in the system performance. This effect could
  increase as the propagation distance/delay (e.g.
  data transfer between Kansas and Antarctica),
  understanding and then being able to predict such
  degradations is needed.

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Future Work-Research

• Delay Tolerant Networking (DTN) Research of new
  network protocols and methods for reliable data
  communication among extreme and
  performance-challenged environments. The
  efficiency of the standard internet protocols
  decreases considerably with propagation distance
  and intermittent connectivity, making them
  unsuitable for very long distance/intermittent
  communication.
• Communications from Polar Regions involves
  similar problems as addressed by DTN, e.g.,
  connectivity over low speed links and
  intermittent connectivity over high speed links.
• Methods developed for networks with intermittent
  connectivity would be suitable for communication
  over satellite links with frequent call drops as
  experienced with Iridium.

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Future Work-Research

• Typical DTN applications involve low bandwidth
  intermittent (satellite) link and high bandwidth
  conventional (Internet) links as parts of the
  same network. Hence, interoperability is a major
  issue.
• A new suite of communication protocols is being
  researched by the Consultative Committee for
  Space Data Systems (CCSDS) and Delay Tolerant
  Networking Research Group (DTNRG). The CCSDS File
  Delivery Protocol concentrates on a tiered
  architecture building over the existing regional
  protocols wherever possible. Adapting the
  protocols being developed by CCSDS and DTNRG for
  polar research in needed.

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Future Work-Research Issues

• Can the evolving DTN technologies be adapted to
  enhance communications in polar regions, if so
  how?
• How can optimum DTN system parameters be
  determined?
• What is reliability vs. efficiency of the
  developed protocols?
• Can the Iridium be used to evaluate the new DTN
  protocols?
• Are existing protocols (like CFDP) over satellite
  networks (Iridium) suitable for polar
  communications?