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Reassessing Min Hopcount: Flaws in Wireless Networks

This study challenges the effectiveness of minimum hopcount in multihop wireless networks, highlighting issues with link quality, path equality, and throughput optimization. Explore why alternative metrics are needed for better performance.

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Reassessing Min Hopcount: Flaws in Wireless Networks

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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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