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Multihop Networks: Fact or Fiction?

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Multihop Networks: Fact or Fiction? Ramki Gummadi (MIT) Multihop: Brief history Multihop wireless successful since 60s More recently, cellular repeaters Large-scale ... – PowerPoint PPT presentation

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Title: Multihop Networks: Fact or Fiction?


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
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,Yates08)
  • Interference cancellation

7
Practical considerations
  • Multihop interference more complex than
    single-hop interference (Rangwala et al.,06)
  • Current MACs and TCP dont 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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