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
CW 2008

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
CW 2008

4. Three mesh n/w challenges
Theoretical limits
Practical considerations
Business models
CW 2008
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5. Theoretical limits 1 p n 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 1 p n
Rmax
Rmin
... s d
CW 2008

6. Workarounds Local communication, gateway nodes Variable-width channels
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
CW 2008

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
CW 2008

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
CW 2008

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
CW 2008

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
CW 2008

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
CW 2008