Multi-Link Iridium Satellite Data Communication System
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University of Kansas
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 Kansas2
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
University of Kansas3
4-Channel Iridium System (Tested in Summer
2003)
Iridium Gateway
PSTN
US
B-
SE
RIA
L I. Modem 3
I. Modem 4
I. Modem 2
I. Modem 1 An
ten
na
G
ridM
ulti-
po
rt PC
I ca
rd
Remote System
PPP client
Local System
PPP Server
Modem Pool
Remote Subsystem
Local Subsystem
4 Iridium – 4 PSTN data configuration
Discrete components
Patch antennas
Control software on a rugged Laptop
University of Kansas4
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
University of Kansas5
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
Conclusions from 2003 field experiments
University of Kansas6
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.
University of Kansas7
8-channel Iridium System – Integrated Unit
9”
19”
24”
Bottom View Top View
Front View
Dimension : 9x19x24 inch
Weight : 50 lbs
Operating temp : -30 to 60
c
Power input : 120 V
AC
Replication Costs :
~$18,000
University of Kansas8
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
University of Kansas9
8-channel Iridium System – Client GUI
University of Kansas10
8-channel Iridium System – Client GUI
University of Kansas11
8-channel Iridium System – Analysis
App
Agent
8 Modem Links
MLPPP MLPPP
Iridium Network
System Model
Application: FTP, HTTP
Agent: TCP, UDP
MLPPP
8 Modem Links
Modems Model
Each link has a dropping probability
Each link has a probability of error
App
Agent
Machine A Machine B
HTTP
FTP
TCP
UDP
University of Kansas12
8-channel Iridium System –Network Architecture
SUMMIT Camp, Greenland
ITTC Network, University of Kansas
World Wide Web
User 2
User 3
User 1
ppp0 eth0
PPP Server
ppp0 eth0
PPP Client
P-T-P Satellite link
ITTC Default Router
(Default gateway)(Default gateway) user 4
user 3
user 2user 1
Camp WI-FI
100 Mbps Ethernet
100 Mbps Ethernet
University of Kansas13
Field Experiments – System Implementation
8-Channel system in a weather-port at
SUMMIT camp in Greenland, July 2004
University of Kansas14
Field Experiments – Antenna Setup
4 ft
10 ft
8 Antenna setup at SUMMIT camp in Greenland, July 2004
University of Kansas15
Results – Throughput
Variation of throughput with number of modems
2.49
4.97
6.93
8.98
12.08
13.90
16.43
18.60
02468
101214161820
1 2 3 4 5 6 7 8Number of modems
Thr
ough
put (
Kbp
s)
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
University of Kansas16
Results – Throughput
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
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
University of Kansas17
Results – Round Trip TimeVariation of RTT
0
500
1000
1500
2000
2500
3000
3500
4000
1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 91 96Time in sec
RT
T in
mse
c
Variation of RTT
0
1000
2000
3000
4000
5000
6000
7000
1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 91 96
Time in sec
RTT
in m
sec
Round trip time during different times of the day
710608
748
1020
820 760920
1291 1232
1495
1952
14361244
801681
891
1304
995
587
1075930
0
500
1000
1500
2000
2500
8:40 9:02 10:34 10:34 11:45 11:56 12:45Time
RT
T in
mse
c
min avg mdev
Average RTT = 1.4 sec
Minimum observed RTT = 608 msec
Mean deviation = 800 msec
Detailed analysis in progress
University of Kansas18
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
University of Kansas19
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
University of Kansas20
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
University of Kansas21
Results – Mobile tests
Iridium system mounted in an autonomous vehicle (MARVIN)
Experiments monitored from another vehicle through 802.11b link
Iridium antennas
University of Kansas22
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
University of Kansas23
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
University of Kansas24
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.
University of Kansas25
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.
University of Kansas26
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
University of Kansas27
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.
University of Kansas28
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.
University of Kansas29
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.
University of Kansas30
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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