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Multihop Wireless Networks: What’s Wrong With Min Hopcount?

Multihop Wireless Networks: What’s Wrong With Min Hopcount?. Douglas S. J. De Couto Daniel Aguayo, Benjamin A. Chambers, and Robert Morris http://pdos.lcs.mit.edu/grid. Wired Nets: Why Min Hopcount?. Prefer route with fewest links (“hops”) Many more sophisticated alternatives

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Multihop Wireless Networks: What’s Wrong With Min Hopcount?

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  1. Multihop Wireless Networks: What’s Wrong With Min Hopcount? Douglas S. J. De Couto Daniel Aguayo, Benjamin A. Chambers, and Robert Morris http://pdos.lcs.mit.edu/grid

  2. Wired Nets: Why Min Hopcount? • Prefer route with fewest links (“hops”) • Many more sophisticated alternatives • End-to-end latency, lowest congestion, etc. • Min hopcount works in practice • Can engineer a decent network • Alternatives complex, less tested

  3. Min Hopcount Assumption • All important route state in hopcount • Link quality is bimodal • Links are either “good”, or “bad” • All “good” links are equivalent • Sufficient condition for success • What about wireless?

  4. Indoor Wireless Network 802.11b radios (fixed tx power), PCs, DSDV 5th floor 6th floor

  5. Min Hopcount Underperforms Packets/sec Worse Better Node Pair DSDV 124-byte packets for 30 seconds “Best” static route “Best” is maximum pkts/sec over multiple static source routes

  6. Not All Min Hopcount Paths Are Equal 3-hop paths between a single node pair

  7. 171 non-zero links Links are not bimodal Min hopcount assumptions are false Broadcast delivery rate Not All Links Are Equal Link

  8. Intermediate Quality Wireless Links Always Exist • There are nodes at every distance • Link quality depends on signal strength (distance) • Min hopcount maximizes link distance • Marginal links are more likely! N d1 d2

  9. Capacity = 1 Capacity = 1/2 Should We Give Up Hopcount? • Intuition from wired networks is wrong • Links share spectrum • Capacity penalty for more hops • What should we optimize? • Per-route throughput, network capacity, power?

  10. Low Quality 802.11 Links • 802.11 has link-level retransmissions • Lossy links become narrow links • 0% effective loss rate • Good pings • But, low packet-per-second throughput • Retransmissions waste capacity • Other nodes could have transmitted

  11. B 100% 50% C A 51% 51% D A Reasonable (Wrong) Idea • Maximize bottleneck throughput: A-D-C A-B-C: ABBABBABB = 1/3 A-D-C: AADDAADD = 1/4 Actual throughput:

  12. A Better Idea • Insight: spectrum use is important! • Idea: minimize transmissions per packet Num tx 1 2 2

  13. Transmissions Per Packet B Transmissions per packet (A-B-C) = 3 Transmissions per packet (A-D-C) = 4 100%1 tx 50% 2 tx C A 51% 2 tx 51% 2 tx D

  14. Research Agenda • Explore performance of other protocols, e.g. DSR, AODV • Explain route performance by underlying link performance • Confirm usefulness of transmissions per packet as metric • Handle link variation over time

  15. Summary • Minimum hopcount protocols are unlikely to achieve best performance • Quality varies between links • Transmissions per packet nicely quantifies link and path quality for routing

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