University of Kansas 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)
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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 - PowerPoint PPT Presentation
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University of Kansas
Multi-Link Iridium Satellite Data Communication System
Overview, Performance and Reliability from Summer 2003
NGRIP, Greenland Field Experiments
June 23-July 17, 2003Abdul Jabbar Mohammad, Graduate Research AssistantDr.Victor Frost, Dan F. Servey Distinguished Professor
(September 24, 2003)
University of Kansas 2
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 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
University of Kansas 3
Introduction – Commercial Satellite Systems
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.
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 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
University of Kansas 5
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 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
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Introduction – Problem Statement
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.
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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
University of Kansas 8
Background - Inverse Multiplexing
App 3
App 2
App 1M
uxHigh Bandwidth Link
Multiplexing
App 1
Inv-Mux
Low Bandwidth Links
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
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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, packet size 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
University of Kansas 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.
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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
ppp0 eth0
PPP Server
ppp0 eth0
PPP Client
P-T-P Satellite link
ITTC Default Router
(Default gateway)(Default gateway)Guser 4
Guser 3
Guser 2G`user 1
Camp WI-FI
100 Mbps Ethernet
100 Mbps Ethernet
University of Kansas 13
4-Channel Iridium System Implementation
Iridium Gateway
PSTN
US
B-S
ER
IAL
I. Modem 3
I. Modem 4
I. Modem 2
I. Modem 1 Antenna G
ridM
ulti-port P
CI card
Remote System
PPP client
Local System
PPP Server
Modem Pool
Remote Subsystem
Local Subsystem
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4-Channel System – Implemented at KU
University of Kansas 15
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, detecting
failures, reconnections, handling power failures/system resets, generating
status information (text)
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4-Channel System – Modem Flow Control
Check if any modem is
alive
Monitor Status
OKFAILED
FAIL Connect Modem 2
OK
Monitor Status
OKFAILED
FAIL Connect Modem 3
OK
Monitor Status
OKFAILED
FAIL Connect Modem 4
OK
STARTConnect Modem 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 with control 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
-------- avg. of all the segments up to 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 up to 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
University of Kansas 27
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 Unacknowledged
data and Congestion window
Time Sequence Graph
Time Sequence Graph
Outstanding Unacknowledged Data
University of Kansas 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
Retransmitted packetRetransmitted packet
Time Sequence Graph Time Sequence Graph
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Results – TCP performance degradation due to packet loss
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.Outstanding Unacknowledged Data
Throughput vs. Time
RTT vs. Time
University of Kansas 30
Results – TCP performance degradation due to call drop
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
------- 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
University of Kansas 36
Applications –WI-FI setup integrated with Iridium
University of Kansas 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
University of Kansas 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