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 frost@eecs.ku.edu, 785-864-4833 *Currently working at Sprint in Network Services Gateway in the Data Network/Network Development department for Brent Scott

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

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.

Technologies-Iridium

Technologies-Protocols

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

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

Summer 2003 Field Experiments Test Location

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)

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

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 $115.00 $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 7 9505 Iridium Data Modems 8 $1,080 $8,640 8 Labor $4,000 Fully Integrated Unit $17,477 Iridium Modems Ethernet USB

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

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

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

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 15.38 Kbps Due to call drops, the efficiency was reduced to ~80%

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

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

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

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

Results – Mobile tests Iridium antennas Experiments monitored from another vehicle through 802.11b link Iridium system mounted in an autonomous vehicle (MARVIN)

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

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

2005 WAIS Field Experiments West Antarctica Ice Sheet

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/email 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.

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

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

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

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

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. 133-145, Apr. 2000. 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. 988-990.

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

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

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

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