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Transcript
Routing in Multi-Radio, Multi-Hop Wireless Mesh Networks Richard Draves, Jitu Padhye, Brian Zill Microsoft Research Self-Organizing Neighborhood Networks • Key Properties – No network engineer – Very little mobility – Energy not a concern Internet 101 Bus Stop 206 Gas Station (Internet TAP) Mesh Router 7 EXIT Mesh Router 5 Mesh Router 2 Mesh Router 3 Mesh Zone Mesh Router 1 Mesh End Device End Device (Guest to Router 1) 90 • One challenge: network capacity • Our approach: multiple radios Results • Ad-hoc routing at layer 2.5 works well • Link quality is important, but not all metrics are created equal • Multiple radios provide significant capacity improvement if the routing utilizes channel-diversity, data rate, loss rate (Please see our SIGCOMM & Mobicom papers for more details.) Layer 2 vs Layer 3 • Layer 2 (link layer): like ethernet switches − Limited to single link technology + Supports multiple protocols (IPv4, IPv6, IPX) + Preserves link abstraction • Layer 3 (network layer) + Supports multiple link technologies − Limited to single network protocol − Link-local mechanisms don’t work • DHCP, RA/RS Our Approach: Routing at Layer 2.5 • A virtual link-layer + Supports multiple link technologies + Supports IPv4, IPv6 etc unmodified + Preserves the link abstraction + Agnostic to choice of ad-hoc routing algorithm IPv4 IPv6 IPX Mesh Connectivity Layer (with LQSR) Ethernet 802.11 802.16 Mesh Connectivity Layer (MCL) • Virtual ethernet adapter – Virtual ethernet addresses – Multiplexes heterogeneous physical links – Physical links need not be ethernet Packet Format Ethernet MCL Payload: TCP/IP, ARP, IPv6… Link-Quality Source Routing (LQSR) • Source-routed link-state routing protocol – Derived from DSR – Part of Mesh Connectivity Layer (layer 2.5) – Supports link-quality modules • Both on-demand/proactive mechanisms – Route Discovery – Route Maintenance – Metric Maintenance LQSR Metric Support • HOP: shortest-path routing – closest to DSR • • • • RTT: round-trip time latency PktPair: packet-pair latency ETX: expected transmission count WCETT: designed for multiple radios Multi-Radio Routing • Previous metrics (HOP, ETX) not suitable for multiple radios per node – Do not leverage channel, range, data rate diversity • Weighted Cumulative Expected Transmission Time – Weight links according Expected Transmission Time (ETT) • Takes link bandwidth and loss rate into account – Combine link ETTs into Weighted Cumulative ETTs (WCETT) • Takes channel diversity into account – Incorporated into source routing WCETT: Combining link ETTs All hops on a path on the same channel interfere – Add ETTs of hops that are on the same channel – Path throughput is dominated by the maximum of these sums Given a n hop path, where each hop can be on any one of k channels, and tuning parameter β: n WCETT (1 ) ETTi max 1 j k X j i 1 where Need to avoid unnecessarily long paths - bad for TCP performance - bad for global resources Xj ETT i hop i is on channel j Select the path with min WCETT Testbed • 23 nodes in building 113 • Cheap desktop machines 201 220 205 203 227 221 207 Approx. 61 m – NetGear WAG or WAB – Proxim OriNOCO – Cards can operate in a, b or g mode. 226 204 – HP d530 SF • Two radios in each node 210 206 208 225 211 224 223 209 214 215 217 219 218 216 Approx. 32 m TCP Throughput Test • Select 100 sender-receiver pairs at random (out of 23x22 = 506) – 2-minute TCP transfer • Two scenarios: – Baseline (Single radio): • NetGear cards in 802.11a mode • Proxim OFF – Two radios • NetGear cards in 802.11a mode • Proxim cards in 802.11g mode • Repeat for shortest-path, ETX, WCETT Results Median Throughput of 100 transfers 3500 2990 Single Radio Throughput (Kbps) 3000 Two Radios 2500 2000 1601 1379 1500 1508 1155 844 1000 500 0 WCETT ETX Shortest Path WCETT uses 2nd radio better than ETX or shortest path. Two-Radio Throughput CDF Fraction of Connections with Lower Throughput 1 HOP ETX WCETT 0.8 0.6 0.4 Better 0.2 0 0 2 4 6 8 10 Throughput (Mbps) 12 14 16 Two-Radio Path Length vs Throughput ETX 18 18 16 16 14 14 Throughput (Mbps) Throughput (Mbps) WCETT 12 10 8 6 4 2 12 10 8 6 4 2 0 0 0 1 2 3 4 5 Average Path Length (Hops) 6 7 0 1 2 3 4 Average Path Length (Hops) 5 6 7 WCETT Improvement by Path Length Percentage Improvement in Median Throughput 140 120 100 80 60 40 20 0 1 2 3 4 Path Length (with 2 radios) >=5 Conclusions • Ad-hoc routing at layer 2.5 works well • Link quality is important for performance • Previous routing metrics do not work well in heterogeneous multi-radio scenarios • WCETT improves performance by making judicious use of additional capacity and channel diversity provided by the 2nd radio