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Multihop Networks: Fact or Fiction?

Multihop Networks: Fact or Fiction?. Ramki Gummadi (MIT). Multihop: Brief history. Multihop wireless successful since 60s More recently, cellular repeaters Large-scale mesh more recent. Applications. Large-scale urban n/w (Meraki, Roofnet) Overcomes last-mile challenges

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Multihop Networks: Fact or Fiction?

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  1. Multihop Networks: Fact or Fiction? Ramki Gummadi (MIT)

  2. Multihop: Brief history • Multihop wireless successful since 60s • More recently, cellular repeaters • Large-scale mesh more recent

  3. Applications • Large-scale urban n/w (Meraki, Roofnet) • Overcomes last-mile challenges • Rural long-distance (WiLDNet) • Low-cost, high throughput • Sensor n/w (several deployments) • Low-power radios • Ad-hoc networks • Disaster recovery, military, neighborhood mesh

  4. Three mesh n/w challenges • Theoretical limits • Practical considerations • Business models

  5. ... Rmax Rmin s ... ... d 1 p n Theoretical limits • Mesh throughput degrades as O( ) • Intuition: direct communication throughput between s and d goes to 0. • So, have to multihop • But multihopping increases load at each node

  6. Workarounds • Local communication, gateway nodes • Reduces aggregate load • Variable-width channels • Reduces interference, exploits spatial diversity • Fixed throughput between any two nodes • Better radios, algorithms • Interference alignment (Grokop,Tse,Yates’08) • Interference cancellation

  7. Practical considerations • Multihop interference more complex than single-hop interference (Rangwala et al.,’06) • Current MACs and TCP don’t work well • CSMA back-off, carrier sense failure • TDMA, interference cancellation • TCP starvation, unfairness (Rangwala et al.,’08) • Rate-based TCP, network coding 4 X 2 3 X 1

  8. Theory and practice intersection • Do multiple radios help? • Only by a constant factor • But what about imposing structure? • Mesh with a tree backbone scales better • Do multiple channels help? • Again, only by a constant factor • But what if we exploit bursty traffic? • Better scalability if all channels usable by all nodes

  9. Space: The third dimension • Poor spatial reuse observed in Roofnet • Contributes to low throughput (O(100) Kbps) • Omni-directional antennas • Directional antennas • New development: phased arrays • But, reduces the rich scattering MIMO needs • PHY and MAC issues less well-understood

  10. Business issues • Different business models • Traditional ISP model: Rice TFA • Commodity hardware, modified firmware (Firetide, Tropos, Belair, etc.) • Ad-based free-ISP model: Meraki • Grassroots deployment

  11. Conclusions • Rich set of cross-cutting issues • Both theoretical and practical • Important question: external interference from unmanaged networks • Scope for cute algorithms (two out of last four best SIGCOMM papers) • Potential to realize the wireless dream • Always on, faster, cheaper, better • Are we there yet? No

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