1. Abdul Jabbar Mohammad, Said Zaghloul*,
TCP Performance over Multilink PPP in Wireless Networks: Theory and Field Experiences
Abdul Jabbar Mohammad, Said Zaghloul*,
and Victor S. Frost
Information and Telecommunication Technology Center
Electrical Engineering & Computer Science
*Currently working at Sprint in Network Services Gateway
in the Data Network/Network Development department for Brent Scott

2. Outline Motivation Technology Field Experiments
Analytic Prediction of TCP over MLPPP with Call Drops

3. Motivation Polar Radar for Ice Sheet Measurements (PRISM)
The communication requirements of PRISM field experiments in Greenland and Antarctica
Data telemetry from the field to the University
Access to University and web resources from field
Public outreach
Mainstream communication system for polar science expeditions, field camps in Arctic/Antarctic and other research purposes
Government and security use
Solution:
Implement a multi-link point-to-point Iridium communication system to combine multiple links to obtain a single logical channel of sufficient aggregate bandwidth.

4. Technologies-Iridium

5. Technologies-Protocols

6. Technologies-Multi-Link Point-to-Point Protocol
Multilink option are negotiated when establishing the connection.
Packets may be fragmented.
Network Layer
Network Layer
MLPPP PDUs reassembled into the original layer 3 packet
MLPPP
MLPPP
MLPPP PDUs 2 4 5
Link 1
Layer 3 Packet 6 1 6
Link 2
MLPPP fragments layer 3 packets 3 7
Link n
MLPPP fragments have non-decreasing sequence numbers 2
Receiver
Sender

7. Technology- Network Architecture
SUMMIT Camp, Greenland or WAIS camp in Antarctica
ITTC Network, University of Kansas
World Wide Web
User 2
User 3
User 1
ppp0
eth0
PPP Server
PPP Client
P-T-P Satellite link
ITTC Default Router
(Default gateway)
user 4
user 3
user 2
user 1
Camp WI-FI
100 Mbps Ethernet
WAIS=West Antartic Ice Sheet

8. Summer 2003 Field Experiments
Test Location

9. Throughput (bits/sec)
2003 Results – Throughput
Method
1 Modem
2 Modems
3 Modems
4 Modems
Iperf
2.1
4.0
7.0
9.6
1.9
3.9
9.3
1.7
4.5
6.8
9.7
Ttcp
2.29
4.43
6.6
8.9
2.48
4.40
8.78
Average
4.25
6.88
9.26
Tools used – TTCP, IPERF
Throughput varies to some extend due to RTT variation
Efficiency > 90%
Effective throughputs during large
file transfers
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
(K b p s)

10. Applications – Uploads and Downloads
Files were downloaded to support the science and operations of the camp. The importance of each file to the user is noted on a subjective scale of 1-10,10 being the most valuable.
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 4
GPS software
Download
800KB 5
Proposal submission
Upload
600KB 8 6
Access point manager software
Download from Orinoco.com
4.66MB
Drawing of machine spares to order
Upload to University of Copenhagen
1MB
Video of core, datasheet
Upload for press release
2MB
Pictures, press release of longest core in Greenland
Upload to Kangerlussauq for press release

11. Eight Modem Iridium System: 2004/5 Field Experiments
The Modem/Computer box is a 19” rack mount 5U equivalent
The front panel is 8.72’ tall and 19” wide. The sides are 8.34” tall and 24” deep.
Weight approximately 45lbs.
Reproduction cost= ~$18,000
Cost Estimate
KU MULTICHANNEL IRIDIUM COMMUNICATION SYSTEM - 8 Channels (KUMICS-8C)
Item Quantity Unit Price Total Price
1 Enclosure 1 $406 $406
2 Linear Power Supply 1 $ $115
3 Connectors 1 $100 $100
4 Single Board Computer 1 $2,150 $2,150
5 Multi Port serial card 1 $386 $386
6 Mast Antennas 8 $210 $1,680
Iridium Data Modems 8 $1,080 $8,640
8 Labor $4,000
Fully Integrated Unit $17,477
Iridium Modems
Ethernet
USB

12. Field Experiments – System Implementation
8-Channel system in a weather-port at SUMMIT camp in Greenland, July 2004

13. Field Experiments – Antenna Setup
4 ft
10 ft
8 Antenna setup at SUMMIT camp in Greenland, July 2004

14. 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

15. Effective Throughput in Kbps
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 for FTP upload of large files was observed to be Kbps
Due to call drops, the efficiency was reduced to ~80%

16. Results – Round Trip Time
Average RTT = 1.4 sec
Minimum observed RTT = 608 msec
Mean deviation = 800 msec

17. Results – Reliability: 14th July 12-hr test
Uptime % 89 95 96 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

18. Results – Reliability: 22nd July 32-hr test
Uptime % 85 92 93 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

19. Results – Reliability: 19th July 6-hr test
Uptime % 67 81 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

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

21. Results – Mobile tests
Uptime % 65 79 82 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

22. 2004 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

23. 2005 WAIS Field Experiments
West
Antarctica
Ice Sheet

24. 2005 WAIS Field Experiments
Item
Size
Importance
Component data sheets
2.2 MB 8
Oscilloscope lab measurements
2 MB 9
Modified code for SAR measurements
500 KB 10
C++ IDE
10 MB 5
GIS scripts 7
GPS troubleshooting manual
PICO editor
4 MB 4
Outreach pictures, journal and weather data upload
500 KB/day
Video conference
Variable 6
Virtual dashboard application
50 MB
Critical data internet search by the drilling team
Internet/ access to all field personnel at WAIS camp
Remote ssh access to field programs from KU
Applications – Uploads and Download
Files were downloaded to support the science and operations of the camp. The importance of each file to the user is noted on a subjective scale of 1-10,10 being the most valuable.

25. Analytic Prediction of TCP over MLPPP with Call Drops
A call drop is the event of losing an established connection suddenly
Connections are automatically re-established
It was observed that a call drop results in TCP timeouts
Various reasons that might lead to call drops,
Low signal level
Failure of the inter-satellite handovers
Goal: Predict the throughput as a function of drop rate and other system parameters
First step: Call Drop model

26. Call Drops - Distribution
394 call-drop measurements were collected in the field
Call drop pdf~exponential
The single link ICTD is a Poisson random process with a rate b
A N Link bundle’s ICTD is a Poisson process with a dropping rate of:
ICTD PDF based on Greenland–Kansas measurements. Estimated exponential distribution (0.02exp(-0.02t)) passes the chi-square goodness-of-fit test (5% significance level and 14 bins)
l=N b

27. Call Drops - Distribution
KS-Greenland call dropping rate per link is 1/50 min-1
KS-KS call dropping rate per link is 1/52 min-1

28. TCP Performance Model
TCP transfer latency for fs bytes given the MSS is
To estimate TCP throughput (B) in packets/sec:
Evaluate the throughput if no timeouts take place
Extend the no timeout throughput using the empirical call drops PDF to include timeouts
Main Assumptions
Packet losses are due to ARQ failures (no timeouts)
Timeouts are caused by call drops only

29. Methodology
Modify exiting results to account for call drops: J. Padhye, V. Firoiu, D. Towsley, and J. Kurose, “Modeling TCP Reno performance: a simple model and its empirical validation,” IEEE/ACM Trans. Networking, vol. 8, pp , Apr
For details of the modification see: Modeling TCP Long File Transfer Latency over Long Delay Wireless Multilink PPP, Said Zaghloul, Victor Frost, Abdul Jabbar Mohammad; IEEE Communications Letters, Vol. 9, No. 11; November 2005, pp

30. Model Validation with Experiential data
File Transfers from Greenland to the University of Kansas (Summer 2004), T0 = 60s, p = 5E-4,  = 1/50 min-1, MSS = 1448, RTT = 19s, Wmax= 47.9KB

31. Case Study: Increased Dropping Rates
A software module was built and added to the developed link management software to increase “real” drop rate
The added module generates call drops according to a Poisson process for any given dropping rate
Results
Observed average is based on eight measurements at each call dropping rate
Error bars correspond to the 25% and the 75% percentiles
RTT = 18.2 sec, p = 5E-4

32. Case Study: Effect of Wireless Errors
A wireless error refers to the errors that the physical layer ARQ could not handle
Effect is amplified for low bandwidth-long delay connections (ex. Iridium)
An efficient ARQ mechanism minimizes wireless errors
Inmarsat GEO, bundle =4
RTT = 0.61 s, BW = 128 kbps
MSS=1 KB and Wmax=40 KB
A slight increase of the packet loss probability results in approximately 25 min increase in the transfer time

33. Information and Telecommunication Technology Center