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Multi-Link Iridium Satellite Data
Communication System
Overview, Performance and Reliabilityfrom Summer 2003 NGRIP, Greenland
Field ExperimentsJune 23-July 17, 2003
Abdul Jabbar Mohammad,Graduate Research AssistantDr.Victor Frost,Dan F . Servey Distinguished Professor(September 24, 2003)
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Motivation
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 scientificresearch 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
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Introduction Commercial Satellite Systems
Polar regions do not have conventional communication facilities (dial-up, DSL, CableModem, etc) and are not serviced by most of the major broadband satellite systems.
Intelsat
Inmarsat
Globalstar
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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 limitedvisibility window at Poles typically
10-13 hrs/day.
High satellite altitude and low elevationangles (1-20) result in extremely large
field equipment. May not be readily available
20 m diameter Marisat and GOES antenna at South Pole
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Introduction - Iridium Satellite System
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 largeareas with fewer gateways
Since it was originally designed as a voice only system, it provides a lowdata rate of 2.4Kbps
Not practical to be used as a main stream/ life-line communication system
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Introduction Problem Statement
Problem Statement
A reliable, lightweight, portable and easily scalable data/Internet
access system providing true Polar coverage.
SolutionImplement a multi-link point-to-point Iridium communication system
to combine multiple satellite links to obtain a single logical channel
of aggregate bandwidth.
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Background - Iridium
Satellite Type LEO
Satellite altitude 780 km
Minimum elevation angle 8.20
Average satellite view time 9-10 minutes
Access scheme FDMA and TDMA
Maximum number of located users 80 users in a radius of 318 km
Theoretical throughput 2.4 3.45 Kbps
Type of data services Iridium-to-Iridium, Iridium-to-PSTN
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Background - Inverse Multiplexing
App 3
App 2
App 1
Mux
High Bandwidth Link
Multiplexing
App 1
Inv-Mux
Low Bandwidth Links
Traditional Multiplexing - Data from a multipleapplications/users sent over a single high bandwidth link.
Inverse Multiplexing - Data from a single application isfragmented and or distributed over multiple low
bandwidth links.
Increases the available bandwidth per applicationsignificantly
Multi-link point-to-point protocol (MLPPP), an extensionto the PPP is a packet based inverse multiplexing
solution
Overhead of 12 bytes
Fragmentation of network layer protocol data units(PDUs) into smaller segments depending upon the PDU
size, link MTUs and the number of available links.
Inverse multiplexing
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Background - Multi-link point-to-point protocol
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, packetsize and MTU size
IP Packet
Modem i Modem 1 Modem 2 Modem 3 Modem 4
Not Fragmented Fragmented
IP Packet
IP PacketSH SH SH SH SHPDF PDF PDF PDF
SH-Segment header; PDF- packet data fragment
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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 handlecall drops and system/power failures in remote field deployment.
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Multi-channel Iridium System Protocol Stack
Application
HTTP, FTP, SSH
TCP
IP
PPP/MLPPP
Physical Modems
Application
HTTP, FTP, SSH
TCP
IP
PPP/MLPPP
Physical Modems
Remote System Local System
point-to-point satellite links
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Multi-channel Iridium System Network Architecture
NGRIP Camp, Greenland
ITTC Network, University of Kansas
World Wide Web
User 2
User 3
User 1
ppp0eth0
PPP Server
ppp0 eth0
PPP Client
P-T-P Satellite link
ITTC Default Router
(Default gateway)(Default gateway)Guser 4
Guser 3
Guser 2
G`user 1Camp
WI-FI
100 Mbps
Ethernet
100 Mbps
Ethernet
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4-Channel Iridium System Implementation
Iridium
Gateway
PSTN
USB-SERIAL
I. Modem 3
I. Modem 4
I. Modem 2
I. Modem 1 Antenna
Grid
Multi-port
PCI
card
Remote
System
PPP client
LocalSystem
PPP Server
Modem
Pool
Remote Subsystem
Local Subsystem
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4-Channel System Implemented at KU
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4-Channel System Implemented at KU
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4-Channel System Software Overview
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, detectingfailures, reconnections, handling power failures/system resets, generating
status information (text)
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4-Channel System Modem Flow Control
Check if any
modem isalive
MonitorStatus
OKFAILED
FAILConnectModem 2
OK
MonitorStatus
OKFAILED
FAILConnectModem 3
OK
MonitorStatus
OKFAILED
FAILConnectModem 4
OK
STARTConnectModem 1
Terminate
All
Monitor
Status
OK
FAIL
OK
FAIL
YES
NO
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Field Tests and Results Field implementation
Dimensions: 24x19x5
Antenna
Connectors
USB Out
USB In
System withcontrol software
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Field Tests and Results Antenna Setup
10 FT
3 FT
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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 [email protected] = 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
Packets
sent
Packets
received
%
Loss
RTT (sec)
Avg Min Max mdev
50 50 0 1.835 1.347 4.127 0.798
100 100 0 1.785 1.448 4.056 0.573
100 100 0 2.067 1.313 6.255 1.272
200 200 0 1.815 1.333 6.228 0.809
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Results Delay Measurement
Random variation of delay
High mean deviation
Delay increases linearly withpacket size
Normalized delay is almost
constant for MTU sizes > 800
bytes
Changing distance between theuser 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
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Results Throughput
Method 1 Modem 2 Modems 3 Modems 4 Modems
Iperf 2.1 4.0 7.0 9.6
Iperf 1.9 3.9 7.0 9.3
Iperf 1.7 4.5 6.8 9.7
Ttcp 2.29 4.43 6.6 8.9
Ttcp 2.48 4.40 7.0 8.78
Average 2.1 4.25 6.88 9.26
Tools used TTCP, IPERF
Throughput varies to someextend due to RTT variation
Efficiency > 90%
File Size (MB)Upload Time
(min)Throughput
(bits/sec)
0.75 11 9091
3.2 60 7111
1.6 23 9275
2.3 45 6815
1.5 28 7143
2.5 35 9524
Effective throughputs during large file transfers
(Kb
ps)
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Results Reliability: 10th July 24 hr test
Time intervalbetween call drops
(minutes)
146 106 114 50 25 84 89 8 7 7 17 11 137 618
Total :
13 Call drops
80.6
91.8
94.7
96.8
Uptime %
Call drops on the first modem
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Results Reliability: 12th July 24 hr test
Time interval
between call drops(minutes)
135 248 93 40 26 16 8 211 108 91 8 5 6 5 8 7 386
80
92
95
96
Uptime %
Total :
16 Call drops
Call drops on the first modem
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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 betweenthe 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 is2.45Kbps.
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.
-------- avg. of last 10 segments
-------- avg. of all the segments upto that point
Throughput vs. Time
RTT vs. Time
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Results TCP performance of a 4-channel system
-------- avg. of last 10 segments
-------- avg. of all the segments upto that point
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
------- Received Acknowledgements
------- TCP segments transmitted
------- Received window advertisement
RTT vs. Time
Throughput vs. Time
Time Sequence Graph
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Results TCP performance of a 4-channel system
------- Instantaneous outstanding data samples
------- Average outstanding data
------- Weighted average of outstanding data
------- Received Acknowledgements
------- TCP segments transmitted
------- Received window advertisement
Outstanding Unacknowledgeddata and Congestion window
Time Sequence Graph
Time Sequence Graph
Outstanding Unacknowledged Data
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Results TCP performance degradation due to packet loss
Effect on the TCP performance due to packetloss
Decrease in the throughput following the packetloss
RTT peaks around the packet loss
The average throughput of the connection isobserved to be 7.44 Kbps.
Outstanding Unacknowledged DataThroughput vs. Time
RTT vs. Time
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Results TCP performance degradation due to call drop
Packet loss due to a call drop on one links ofthe multilink bundle
A finite amount of time for the data link layerrealize 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
------- Instantaneous outstanding data samples
------- Average outstanding data
------- Weighted average of outstanding data
Time Sequence Graph
Time Sequence Graph
Outstanding Unacknowledged Data
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Results Mobile tests
Test Location
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Results Mobile Performance
STOP
START
Avg. Throughput = 9 Kbps
Throughput vs. Time
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Results Mobile Performance (cont.d)
START
STOP
Avg. Throughput = 9.34 Kbps
Throughput vs. Time
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Applications Uploads and Downloads
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
Title Downloaded/uploaded Size Imp
1 Spectrum Analyzer programmers
Manual
Download from Agilent.com 7.2MB 9
2 Matlab Programs Download from ITTC 500KB 7
3 Voltage regulator data sheet Download from Fairchild.com 226KB 9
4 GPS software Download 800KB 9
5 Proposal submission Upload 600KB 8
6 Access point manager software Download from Orinoco.com 4.66MB 7
7 Drawing of machine spares to
order
Upload to University of Copenhagen 1MB 9
8 Video of core, datasheet Upload for press release 2MB 8
9 Pictures, press release of longest
core in Greenland
Upload to Kangerlussauq for press
release
500KB 6
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Applications Internet at camp
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Applications WI-FI setup integrated with Iridium
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Applications - Wireless Internet
Wireless Internet access was usedby many NGRIP researchers
Email access put the researchers intouch with their home institutions
Scientist at NGRIP were able tosend information back to their home
institutions
The system was also useful for general camp purpose: sending drawings to orderspares for a broken caterpillar, excel spreadsheet for food order, general pressreleases, downloading weather reports for planning C-130 landings
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Applications Outreach
Daily journal logs were uploaded to the Universityof Kansas and posted on www.ku-prism.org
Two way video conference was held twice to testthe real time audio/video
The system not only helped PRISM group todownload critical software, but also made
it possible to obtain faculty guidance and expertise
on the field
It also helped the researchers back at theUniversity 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
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Lessons Learned
Multi-link PPP is relatively efficient over Iridium system. With 4-modems efficiency was observed tobe >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 PPPnetworking 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 littledisturbance 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
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Conclusions
In order to provide data and Internet access to Polar Regions, we have developed areliable, 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 reliableoperation
An end-to-end network architecture providing Internet access to science expeditions inPolar Regions is demonstrated.
This system provided for the first time, Internet access to NGRIP camp at Greenland andobtained the call drop pattern.
The TCP performance and the reliability of the system is characterized
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Future Work
Modify the MLPPP code so that the interface is attached to the bundle and not to theprimary link
Additional research to determine the cause of delay and develop methods toovercome it.
Evaluate the new data-after-voice (DAV) service from Iridium
Evaluate different versions of TCP to determine the enhancements that can handlethe 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
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