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High Throughput Route Selection in Multi-Rate Ad Hoc Wireless Networks

High Throughput Route Selection in Multi-Rate Ad Hoc Wireless Networks. Dr. Baruch Awerbuch, David Holmer, and Herbert Rubens. Johns Hopkins University. Department of Computer Science. www.cnds.jhu.edu/archipelago. Overview. Problem: Route selection in multi-rate ad hoc network

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High Throughput Route Selection in Multi-Rate Ad Hoc Wireless Networks

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  1. High Throughput Route Selection in Multi-RateAd Hoc Wireless Networks Dr. Baruch Awerbuch, David Holmer, and Herbert Rubens Johns Hopkins University Department of Computer Science www.cnds.jhu.edu/archipelago

  2. Overview • Problem: Route selection in multi-rate ad hoc network • Traditional Technique: Minimum Hop Path • New Technique: Medium Time Metric (MTM) • Goal: Maximize network throughput

  3. What is Multi-Rate? • Ability of a wireless card to automatically operate at several different bit-rates (e.g. 1, 2, 5.5, and 11 Mbps) • Part of many existing wireless standards (802.11b, 802.11a, 802.11g, HiperLAN2…) • Virtually every wireless card in use today employs multi-rate

  4. Advantage of Multi-Rate? • Direct relationship between communication rate and the channel quality required for that rate • As distance increases, channel quality decreases • Therefore: tradeoff between communication range and link speed • Multi-rate provides flexibility 1 Mbps 2 Mbps 5.5 Mbps 11 Mbps Lucent Orinoco 802.11b card ranges using NS2 two-ray ground propagation model

  5. Ad hoc Network Single Rate Example • Which route to select? Destination Source

  6. Ad hoc Network Single Rate Example • Which route to select? • Source and Destination are neighbors! Just route directly. Destination Source

  7. Multi-rate Network Example • Varied Link Rates Destination Source 11 Mbps 5.5 Mbps 2 Mbps 1 Mbps

  8. Multi-rate Network Example • Varied Link Rates Destination Throughput = 1.04 Mbps Source 11 Mbps 5.5 Mbps 2 Mbps 1 Mbps

  9. Multi-rate Network Example • Varied Link Rates Destination Throughput = 1.15 Mbps Source 11 Mbps 5.5 Mbps 2 Mbps 1 Mbps

  10. Multi-rate Network Example • Varied Link Rates • Min Hop Selects Direct Link • 0.85 Mbps Destination Source 11 Mbps 5.5 Mbps 2 Mbps 1 Mbps

  11. Multi-rate Network Example • Varied Link Rates • Min Hop Selects Direct Link • 0.85 Mbps effective • Highest Throughput Path • 2.38 Mbps effective Destination Source 11 Mbps 5.5 Mbps 2 Mbps 1 Mbps

  12. Multi-rate Network Example • Under Mobility • Min Hop • Path Breaks • High Throughput Path • Reduced Link Speed • Reliability Maintained • More “elastic” path Destination X Source 11 Mbps 5.5 Mbps 2 Mbps 1 Mbps

  13. Challenge to the Routing Protocol • Must select a path from Source to Destination • Links operate at different speeds • Fundamental Tradeoff • Fast/Short links = low range = many hops/transmissions to get to destination • Slow/Long links = long range = few hops/transmissions

  14. Minimum Hop Path(Traditional Technique) • A small number of long slow hops provide the minimum hop path • These slow transmissions occupy the medium for long times, blocking adjacent senders • Selecting nodes on the fringe of the communication range results in reduced reliability

  15. How can we achieve high throughput? • Throughput depends on several factors • Physical configuration of the nodes • Fundamental properties of wireless communication • MAC protocol

  16. Wireless Shared Medium • Transmission blocks all nearby activity to avoid collisions • MAC protocol provides channel arbitration Carrier Sense Range Carrier Sense Range 1 2

  17. Transmission Duration 4.55 Mbps 3.17 Mbps 1.54 Mbps 0.85 Mbps Medium Time consumed to transmit 1500 byte packet

  18. Hops vs. Throughput • Since the medium is shared, adjacent transmissions compete for medium time. • Throughput decreases as number of hops increase. 1 2 3

  19. Effect of Transmission Source Destination X X X X X X X 1 2 3 4 5 6 7 8 Request to Send (RTS) Clear to Send (CTS) DATA ACK

  20. Multi-Hop Throughput Loss (TCP)

  21. Analysis • General Model of ad hoc network throughput • Multi-rate transmission graph • Interference graph • Flow constraints • General Throughput Maximization Solution is NP Complete • Derived an optimal solution under a full interference assumption

  22. New Approach: Medium Time Metric (MTM) • Assigns a weight to each link proportional to the amount of medium time consumed by transmitting a packet on the link • Existing shortest path protocols will then discover the path that minimizes total transmission time

  23. MTM Example 11 Mbps Source Destination 1 Mbps Path Medium Time Metric (MTM) Path Throughput Link Rate 11 = 2.5 2.5ms 4.55 Mbps 1 0.85 Mbps 13.9ms = 13.9

  24. MTM Example 11 Mbps 11 Mbps Source Destination 1 Mbps Path Medium Time Metric (MTM) Path Throughput Link Rate 11 + 11 = 5.0 2.5ms 2.5ms 2.36 Mbps 1 0.85 Mbps 13.9ms = 13.9

  25. MTM Example 11 Mbps 11 Mbps 11 Mbps Source Destination 1 Mbps Path Medium Time Metric (MTM) Path Throughput Link Rate 11 + 11 + 11 = 7.5 2.5ms 2.5ms 2.5ms 1.57 Mbps 1 0.85 Mbps 13.9ms = 13.9

  26. MTM Example 11 Mbps Source Destination 1 Mbps Path Medium Time Metric (MTM) Path Throughput Link Rate 11 + 11 + 11 + 11 = 10.0 2.5ms 2.5ms 2.5ms 2.5ms 1.18 Mbps 1 0.85 Mbps 13.9ms = 13.9

  27. MTM Example 11 Mbps Source Destination 1 Mbps Path Medium Time Metric (MTM) Path Throughput Link Rate 11 + 11 + 11 + 11 + 11 = 12.5 2.5ms 2.5ms 2.5ms 2.5ms 2.5ms 0.94 Mbps 1 0.85 Mbps 13.9ms = 13.9

  28. MTM Example 11 Mbps Source Destination 1 Mbps Path Medium Time Metric (MTM) Path Throughput Link Rate 11 + 11 + 11 + 11 + 11 + 11 = 15 2.5ms 2.5ms 2.5ms 2.5ms 2.5ms 2.5ms 0.78 Mbps 1 0.85 Mbps 13.9ms = 13.9

  29. MTM Example Medium Time Usage Link Throughput Destination 4.55 Mbps 11 Mbps 2.5ms 3.17 Mbps 5.5 Mbps 3.7ms 1.54 Mbps 2 Mbps 7.6ms 0.85 Mbps 1 Mbps 13.9ms Source Path Medium Time Metric (MTM) Path Throughput 11 Mbps 5.5 Mbps 1 0.85 Mbps 13.9ms = 13.9 ms 2 Mbps 1 Mbps

  30. MTM Example Medium Time Usage Link Throughput Destination 4.55 Mbps 11 Mbps 2.5ms 3.17 Mbps 5.5 Mbps 3.7ms 1.54 Mbps 2 Mbps 7.6ms 0.85 Mbps 1 Mbps 13.9ms Source Path Medium Time Metric (MTM) Path Throughput 5.5 + 2 11 Mbps = 11.3 ms 1.04 Mbps 3.7ms 7.6ms 5.5 Mbps 1 0.85 Mbps 13.9ms = 13.9 ms 2 Mbps 1 Mbps

  31. MTM Example Medium Time Usage Link Throughput Destination 4.55 Mbps 11 Mbps 2.5ms 3.17 Mbps 5.5 Mbps 3.7ms 1.54 Mbps 2 Mbps 7.6ms 0.85 Mbps 1 Mbps 13.9ms Source Path Medium Time Metric (MTM) Path Throughput 11 + 2 1.15 Mbps 2.5ms 7.6ms = 10.1 ms 5.5 + 2 11 Mbps = 11.3 ms 1.04 Mbps 3.7ms 7.6ms 5.5 Mbps 1 0.85 Mbps 13.9ms = 13.9 ms 2 Mbps 1 Mbps

  32. MTM Example Medium Time Usage Link Throughput Destination 4.55 Mbps 11 Mbps 2.5ms 3.17 Mbps 5.5 Mbps 3.7ms 1.54 Mbps 2 Mbps 7.6ms 0.85 Mbps 1 Mbps 13.9ms Source Path Medium Time Metric (MTM) Path Throughput 11 + 11 = 5.0 ms 2.5ms 2.5ms 2.38 Mbps 11 + 2 1.15 Mbps 2.5ms 7.6ms = 10.1 ms 5.5 + 2 11 Mbps = 11.3 ms 1.04 Mbps 3.7ms 7.6ms 5.5 Mbps 1 0.85 Mbps 13.9ms = 13.9 ms 2 Mbps 1 Mbps

  33. Advantages • It’s an additive shortest path metric • Paths which minimize network utilization, maximize network capacity • Global optimum under complete interference • Single flow optimum up to pipeline distance (7-11 hops) • Excellent heuristic in even larger networks • Avoiding low speed links inherently provides increased route stability

  34. Disadvantages • MTM paths require more hops • More transmitting nodes • Increased contention for medium • Results in more load on MAC protocol • Only a few percent reduction under the simulated conditions • Increase in buffering along path • However, higher throughput paths have lower propagation delay

  35. Sounds great but… • Do faster paths actually exist? • There needs to be enough nodes between the source and the destination to provide a faster path • Therefore performance could vary as a function of node density • When density is low: MTM = Min Hop

  36. Performance Increase vs. Node Density in Static Random Line

  37. MTM Throughput IncreaseUnder 802.11MAC -NS2 Network Simulations -20 TCP Senders and receivers -Random Waypoint mobility (0-20m/s) -DSDV Protocol modified to find MTM path

  38. MTM + OAR Throughput Increaseover Min Hop + 802.11 -NS2 Network Simulations -20 TCP Senders and receivers -Random Waypoint mobility (0-20m/s) -DSDV Protocol modified to find MTM path

  39. Thank You! Questions?? Herb Rubens herb@cs.jhu.edu More Information: http://www.cnds.jhu.edu/networks/archipelago/

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