MIT Roofnet Performance MSR Mesh Summit, June 2004 Robert Morris - - PDF document

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Robert Morris

Daniel Aguayo, John Bicket, Sanjit Biswas, Douglas De Couto, Glenn Judd http://pdos.lcs.mit.edu/roofnet

MIT Roofnet Performance

MSR Mesh Summit, June 2004

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Roofnet node map

1 kilometer

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Typical rooftop view

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Roofnet radio links

1 kilometer

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A Roofnet Self-Installation Kit

Computer ($340)

533 MHz PC, hard disk, CDROM

802.11b card ($155)

Engenius Prism 2.5, 200mW

Software (“free”)

Our networking software based on Click

Antenna ($65)

8dBi, 20 degree vertical

Miscellaneous ($75)

Chimney Mount, Lightning Arrestor, etc.

50 ft. Cable ($40)

Low loss (3dB/100ft)

Takes a user about 45 minutes to install on a flat roof

Total: $685

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Roofnet Node Software

802.11b eth Linux TCP/IP

Click

Linux Kernel User-space

sshd apache dhcpd antenna Living-room ethernet

NAT srcrr ETX

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Basic Roofnet performance

121 152 6 9 98 168 17 8 72 184 38 7 81 248 43 6 61 216 127 5 44 272 256 4 39 368 354 3 22 784 354 2 12 2528 179 1 Latency ms Avg TCP Kilobits/sec # of Pairs Hops

• High TCP throughput even w/ many hops
• Why is 2-hop b/w less than half 1-hop b/w?

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Multi-hop collisions cut b/w by about 2x

• x-axis: expected multi-hop b/w based on single-hop b/w
• y-axis: actual Roofnet b/w is often much lower

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Roofnet link quality distribution

Node Pair Packet Delivery Probability 1 megabit/second 11 mbits/sec

• Why do most links have intermediate loss rates?

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S/N vs loss w/ cable + attenuator

• In principle, the intermediate-loss S/N region is narrow

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S/N vs loss for Roofnet links

• Roofnet loss rates cannot be explained by S/N

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Effect of transmit power level on Roofnet

• Higher tx power increases radio “range”
• Increase in # of links between 1/r2 and 1/r3

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What is a typical radio range?

Distance (Meters) Delivery probability

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Would a less-dense mesh work?

3.3 320 100% 34 3.2 256 100% 29 3.5 256 100% 24 3.5 224 100% 19 3.0 144 95% 14 2.2 80 50% 9 1.3 16 17% 4 Hop Count Avg TCP Kilobits/sec Connectivity Nodes

• Roofnet is about twice as dense as it needs to be
• Higher densities provide higher throughput

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Mesh versus access points

1856 1608 1152 1144 864 688 160 AP throughput 2296 2184 2040 2096 1880 1616 952 Mesh throughput 41 7 41 6 41 5 40 4 38 3 34 2 25 1 AP connections APs or gateways

• 5 APs are required for full connectivity
• N mesh gateways give higher throughput than N APs

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Conclusions

• Roofnet provides Internet access to 40+ users
• Even 9-hop routes average 150 kilobits/second
• Radio range up to 2km
• Hard to beat mesh performance w/ access points
• Multi-hop packet loss costs about a factor of two

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Transmit bit-rate choice

Node Pair Packets/second received 11 megabits/second 5.5 2 1

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How reliant on the “best” nodes?

Average Throughput (KB/s) Number of Best Nodes Eliminated