1. 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

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? Source Destination

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

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

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

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

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

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 Fast/Short links = low range = many hops/transmissions to get to destination Slow/Long links = long range = few hops/transmissions 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 Physical configuration of the nodes Fundamental properties of wireless communication Fundamental properties of wireless communication MAC protocol MAC protocol

16. Wireless Shared Medium Transmission blocks all nearby activity to avoid collisions MAC protocol provides channel arbitration Carrier Sense Range 12

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. 123

19. Effect of Transmission SourceDestination Request to Send (RTS)Clear to Send (CTS)DATAACK 12345678 XXXXXXX

20. Multi-Hop Throughput Loss (TCP)

21. Analysis General Model of ad hoc network throughput Multi-rate transmission graph Multi-rate transmission graph Interference graph Interference graph Flow constraints 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 1 4.55 Mbps 0.85 Mbps 2.5ms 13.9ms = 2.5 Path Throughput Path Medium Time Metric (MTM) = 13.9 Link Rate 1 Mbps SourceDestination 11 Mbps

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

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

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

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

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

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

30. MTM Example Source Destination 11 Mbps 5.5 Mbps 2 Mbps 1 Mbps 5.5 + 2 1 1.04 Mbps 0.85 Mbps 2.5ms 3.7ms 7.6ms 13.9ms 11 Mbps 5.5 Mbps 2 Mbps 1 Mbps 7.6ms3.7ms 13.9ms = 11.3 ms Medium Time Usage 4.55 Mbps 3.17 Mbps 1.54 Mbps 0.85 Mbps Path Throughput Path Medium Time Metric (MTM) = 13.9 ms Link Throughput

31. MTM Example Source Destination 11 Mbps 5.5 Mbps 2 Mbps 1 Mbps 11 + 2 5.5 + 2 1 1.15 Mbps 1.04 Mbps 0.85 Mbps 2.5ms 3.7ms 7.6ms 13.9ms 11 Mbps 5.5 Mbps 2 Mbps 1 Mbps 2.5ms7.6ms 3.7ms 13.9ms = 10.1 ms = 11.3 ms Medium Time Usage 4.55 Mbps 3.17 Mbps 1.54 Mbps 0.85 Mbps Path Throughput Path Medium Time Metric (MTM) = 13.9 ms Link Throughput

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

33. Advantages It’s an additive shortest path metric Paths which minimize network utilization, maximize network capacity Global optimum under complete interference Global optimum under complete interference Single flow optimum up to pipeline distance (7-11 hops) Single flow optimum up to pipeline distance (7-11 hops) Excellent heuristic in even larger networks 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 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 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 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 Therefore performance could vary as a function of node density When density is low: MTM = Min Hop When density is low: MTM = Min Hop

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

37. MTM Throughput Increase Under 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 Increase over 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?? More Information: http://www.cnds.jhu.edu/networks/archipelago/ Herb Rubens herb@cs.jhu.edu