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Optimizing Throughput with Medium Time Metric in Multi-Rate Ad Hoc Networks

Explore the challenges of route selection in multi-rate ad hoc wireless networks and learn how the Medium Time Metric (MTM) can maximize network throughput and adapt to various link rates. The interactive examples illustrate how MTM enhances path selection in dynamic network scenarios.

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Optimizing Throughput with Medium Time Metric in Multi-Rate Ad Hoc 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. MTM Definition

  34. 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

  35. 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

  36. 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

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

  38. 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

  39. 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

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

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