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A High-Throughput Path Metric for Multi-Hop Wireless Routing

A High-Throughput Path Metric for Multi-Hop Wireless Routing. Douglas S.J. Couto Daniel Aguayo John Bicket Robert Morris Presented by: Eric Rozner. Overview. Hop count metric Describe ETX Implementation Experiments and Comparisons Related Work Conclusions Discussion. Hop Count Metric.

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A High-Throughput Path Metric for Multi-Hop Wireless Routing

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  1. A High-Throughput Path Metric for Multi-Hop Wireless Routing Douglas S.J. Couto Daniel Aguayo John Bicket Robert Morris Presented by: Eric Rozner Eric Rozner - ETX.ppt

  2. Overview • Hop count metric • Describe ETX • Implementation • Experiments and Comparisons • Related Work • Conclusions • Discussion Eric Rozner - ETX.ppt

  3. Hop Count Metric • Paths decided upon by shortest route • Maximizes the distance traveled by each hop • Minimizes signal strength -> Maximizes the loss ratio • Uses a higher TxPower -> Interference • Possibly many shortest routes • Avoid lossy links? Eric Rozner - ETX.ppt

  4. Hop Count vs. “Optimal” x axis: throughput y axis: fraction of pairs with less throughput Eric Rozner - ETX.ppt

  5. Hop Count Route Selection Eric Rozner - ETX.ppt

  6. Motivation for a Better Routing Metric Bidirectional loss rates Fine-grained choices Intermediate loss rates Eric Rozner - ETX.ppt

  7. Potential Ideas (and their cons) • Product of per-link delivery ratios • A perfect 2-hop route is viewed as better than a 1-hop route with 10% loss ratio • Throughput of a path’s bottleneck link • Same as above • End-to-End delay • Changes with network load as interference queue lengths vary… can cause oscillations • Everyone convinced? Eric Rozner - ETX.ppt

  8. ETX • The predicted number of data transmissions required to send a packet over a link • The ETX of a path is the sum of the ETX values of the links over that path • Examples: • ETX of a 3-hop route with perfect links is 3 • ETX of a 1-hop route with 50% loss is 2 Eric Rozner - ETX.ppt

  9. ETX continued… • Expected probability that a transmission is successfully received and acknowledged is df x dr • df is forward delivery ratio • dr is reverse delivery ratio • Each attempt to transmit a packet is a Bernoulli trial, so… Eric Rozner - ETX.ppt

  10. Hooray for ETX! • Based on delivery ratios, which affect throughput • Detects and handles asymmetry by incorporating loss ratios in each direction • Uses precise link loss ratios measurements to make fine-grained decisions between routes • Assumes you can measure these ratios precisely • Penalizes routes with more hops, which have lower throughput due to inter-hop interference • Assumes loss rates are generally equal over links • Tends to minimize spectrum use, which should maximize overall system capacity (reduce power too) • Each node spends less time retransmitting data Eric Rozner - ETX.ppt

  11. ETX always the best? Eric Rozner - ETX.ppt

  12. Acquiring ETX values • Measured by broadcasting dedicated link probe packets with an average period τ (jittered by ±0.1τ) • Delivery ratio: • count(t-w,t) is the # of probes received during window w • w/τ is the # of probes that should have been received • Each probe contains this information Eric Rozner - ETX.ppt

  13. Implementation and such… • Authors modified DSDV and DSR • See paper for details • τ = 1 packet per second, w = 10 sec • Multiple queues (different priorities) • Loss-ratio probes, protocol packets, data packets • Are these experiments unfair or unrealistic? • In DSDV w/ ETX, route table is a snapshot taken at end of 90 second warm-up period • In DSR w/ ETX, source waits additional 15 sec before initiating the route request Eric Rozner - ETX.ppt

  14. DSDV Performance One hop Asymmetric ETX inaccurate Eric Rozner - ETX.ppt

  15. DSDV and High Transmit Power Eric Rozner - ETX.ppt

  16. Packet Size Problems • Less throughputadvantage than whendata packetsare smaller(134 bytes) Eric Rozner - ETX.ppt

  17. Packet sizes continued ACK’s smaller than probe packets ETX underestimates ACK delivery ratios -> overestimates total number of transmissions per packet Eric Rozner - ETX.ppt

  18. DSR Performance Link-layer transmission feedback disabled Link-layer transmission feedback enabled Eric Rozner - ETX.ppt

  19. Related Work • Yarvis et al. [33] propose a path metric which approximates the product of the per-link delivery ratios. • Link hand-shaking to avoid “gray areas” [8], [22] • SNR ratio is also a possible path metric [14] • SNR threshold value to filter links discovered by DSR Route Discovery. • See Related Works section of “Comparison of Routing Metrics for Static Multi-Hop Wireless Networks “ Eric Rozner - ETX.ppt

  20. Related Work Continued… • WCETT proposed in MR-LQSR • Weighted Cumulative Expected Transmission Time Eric Rozner - ETX.ppt

  21. Conclusions • Pros • ETX performs better or comparable to Hop Count Metric • Accounts for bi-directional loss rates • Can easily be incorporated into routing protocols • A lot of other work built off this paper • Cons • May not be best metric for all networks • Mobility, Power-limited, Adaptive Rate (multi-rate), Multi-radio • Predications of loss ratios not always accurate • Experiments (30 sec transfer of small packets) may not complement real-world scenarios Eric Rozner - ETX.ppt

  22. Questions? Eric Rozner - ETX.ppt

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