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)
University of Kansas
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
PPP client
I. Modem 2
I. Modem 3
I. Modem 4
Antenna
Grid
I. Modem 1
USB-SERIAL
Remote
System
(Tested in Summer 2003)
Iridium
Gateway
Remote Subsystem
PPP Server
Multi-port
PCI card
Local
System
Modem
Pool
PSTN
Local Subsystem

4 Iridium – 4 PSTN data configuration

Discrete components

Patch antennas

Control software on a rugged Laptop
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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 >90%. 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 5”x4” 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”
24”

Dimension
: 9x19x24 inch

Weight
: 50 lbs

Operating temp
: -30 to 60 c

Power input
: 120 V AC

Replication Costs : ~$18,000
Front View
9”
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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
HTTP
App
App
FTP
TCP
Agent
Agent
UDP
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
World Wide Web
SUMMIT Camp, Greenland
100 Mbps
Ethernet
(Default gateway)
(Default gateway)
PPP Server
user 4
PPP Client
ITTC Default
Router
eth0
User 2
100 Mbps
Ethernet
ppp0
P-T-P Satellite link
ppp0
eth0
User 1
user 3
User 3
Camp
WI-FI
user 2
user 1
ITTC Network, University of Kansas
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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
Throughput (Kbps)
Variation of throughput with number of modems
20
18
16
14
12
10
8
6
4
2
0
18.60
16.43
13.90
12.08
8.98
6.93
4.97
2.49
1
2
3
4
5
6
7
8
Number of modems

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
Size of file in MB
Approx. Upload Time
Effective Throughput in Kbps
1.38
0:11:24
16.53
3.77
0:35:42
14.42
5.62
0:46:12
16.61
15.52
2:30:00
14.12
20.6
3:00:00
15.62
35.7
5:15:00
15.47
55.23
9:00:00
13.96

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
Round trip tim e during different tim es of the day
Variation of RTT
2500
4000
min
avg
mdev
3000
1952
2000
2500
RTT in msec
RTT in msec
3500
2000
1500
1000
500
0
1
6
11 16
21 26
31 36 41
46 51 56
Time in sec
61 66 71
76 81 86
1495
1500
1291
1075
930
1000
710
1436
1304
1232
801
608
681
1244
1020
891
748
995
820
920
760
587
500
91 96
0
8:40
9:02
10:34
10:34
11:45
Time
11:56
12:45
Variation of RTT
7000
RTT in msec
6000

Average RTT = 1.4 sec

Minimum observed RTT = 608 msec

Mean deviation = 800 msec

Detailed analysis in progress
5000
4000
3000
2000
1000
0
1
6
11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 91 96
Time in sec
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Results – Reliability: 14th July 12-hr test
Uptime %
89
95
96
97
97
97
97
98

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 %
85
92
93
93
94
94
94
96

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
Uptime %
67
81
85
85
85
85
85
90

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
Uptime %
65
79
82
84
84
85
87
92

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 > 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 performancechallenged 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?
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