A Fair Scheduling for Wireless Mesh Networks

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A Fair SchedulingforWireless Mesh Networks

• Naouel Ben Salem and Jean-Pierre Hubaux
• Laboratory of Computer Communications and
  Applications (LCA)
• EPFL - Lausanne, Switzerland

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Wireless Mesh Networks

• An extension of WiFi networks
• One wired hot spot HS

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Problem Statement

• If the medium access protocol is poorly designed
• severe unfairness (starvation)
• low bandwidth utilization

• We propose a fair scheduling mechanism that
  optimizes the bandwidth utilization.
• Our solution assigns transmission rights to the
  links in the WMN and maximizes the Spatial Reuse

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State of the Art

• 1 V. Gambiroza, B. Sadeghi, and E. Knightly,
• End-to-End Performance and Fairness in Multihop
  Wireless
• Backhaul Networks" in Proceedings of MobiCom
  2004.

• Three main differences with 1
• The definition of fairness Per-Client fairness
  vs Per-TAP fairness

• The network topology The whole network vs One
  branch

• Traffic model No inter-TAP communications vs
  possibility of inter-TAP communications

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System Model

• A directed graph
• VHS, TAPi, 1 ? i ? n

• Assumptions
• One operator and fixed topology
• Omni directional antennas
• All the clients pay the same flat rate
• All the clients send and receive data at
  saturation rate
• Orthogonal channels for upstream and downstream
  traffic
• All communication links have the same capacity C

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An Example of Fair Scheduling

• Link (i,j) is activated during li,j time slots
• Each client sends the same amount of data C.ts
• The number of time slots in the cycle is T? li,j
  24
• Each client sends the same throughput ? C/T
• No spatial reuse
• The solution is not optimal

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Spatial Reuse

• Some links can be activated at the same time
• A shorter cycle (T19 instead of 24)
• Optimal spatial reuse
• We have to minimize T

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Our Solution

• A scheduling mechanism
• Fair The per-client fairness condition is
• ?a C/T ? a ? M
• Optimal bandwidth utilization Minimize T

• Three main components
• Construction of the compatibility matrix/graph
• Construction of the cliques
• Definition of the fair scheduling (FS)

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Our Solution Compatibility Matrix

• Three main components
• Construction of the compatibility matrix/graph

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Our Solution Cliques

• Three main components
• Construction of the cliques

A clique is a set of links which can all be
enabled at the same time.

• We define all the possible cliques.

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Our Solution The Fair Scheduling (FS)

• Three main components
• Definition of the fair scheduling (FS)

A scheduling s is a set of cliques that fulfills
? ?
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Our Solution The Fair Scheduling (FS)

• Rationale of FS

1. s ?
2. G compatibility graph
3. Search for the clique Clmax with the maximal
   gain in G
4. s s ? Clmax
5. G G\ Clmax
6. if Ggt0, go to step 3

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Simulations

• Matlab simulations
• Two network topologies
• One-dimensional 10, 15, 20 and 25 nodes
• Two-dimensional 8, 16, 24 and 32 nodes
• Nodes distribution m2.n
• Uniform distribution
• Peripheral distribution
• Central distribution
• We compare the performance of our solution with
  the scheduling without spatial reuse

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Results One-dimensional Topology
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Results Two-dimensional Topology
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Conclusion

• If the medium access protocol is poorly designed
• severe unfairness
• low bandwidth utilization
• We propose a scheduling mechanism that
• is fair, and
• optimizes the bandwidth utilization.
• We prove the efficiency of our solution by means
  of simulations
• Future work
• Relax some of the assumptions
• Security issues