1. A Fair Scheduling for Wireless Mesh Networks Naouel Ben Salem and Jean-Pierre Hubaux Laboratory of Computer Communications and Applications (LCA) EPFL - Lausanne, Switzerland

2. 2 Wireless Mesh Networks  An extension of WiFi networks:  One wired hot spot HS Internet HS M8M8 M1M1 M2M2 M3M3 M5M5 M6M6 M 10 M9M9 M7M7 M4M4  The TAPs are not directly connected to the Internet:  They rely on HS relays to get Internet connectivity for their clients. TAP 3 TAP 1 TAP 2 TAP 5 TAP 4 TAP 6 TAP 7  Several Transient Access Points (TAPs)  Wireless communications  Possible interference

3. 3 HS TAP 5 TAP 4 TAP 7 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 f4f4 f5f5 f7f7

4. 4 [1] V. Gambiroza, B. Sadeghi, and E. Knightly, “End-to-End Performance and Fairness in Multihop Wireless Backhaul Networks" in Proceedings of MobiCom 2004. State of the Art  The network topology: The whole network vs One branch  Traffic model: No inter-TAP communications vs possibility of inter- TAP communications TAP 3 TAP 1 TAP 2 TAP 6  Three main differences with [1]:  The definition of fairness: Per-Client fairness vs Per-TAP fairness HS TAP 5 TAP 4 TAP 7 f4f4 f7f7 f4f4 f7f7 TAP 5 TAP 4 TAP 7 HS f 7 = f 4 f 7 = 2*f 4 f 5-4

5. 5 System Model Internet HS TAP 3 TAP 1 TAP 2 TAP 5 TAP 4 TAP 6 TAP 7  A directed graph:  V ={HS, TAP i, 1  i  n}  Communication links Upstream Downstream  Interference links  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

6. 6 An Example of Fair Scheduling HS TAP 3 TAP 1 TAP 2 TAP 5 TAP 4 TAP 6 TAP 7 (1,0) (4,0) (2,1) (3,2) (5,4) (7,5) (6,5) M8M8 M1M1 M2M2 M3M3 M5M5 M6M6 M 10 M9M9 M7M7 M4M4  Link (i,j) is activated during l i,j time slots  Each client sends the same amount of data = C.ts  The number of time slots in the cycle is T=∑ l i,j =24  Each client sends the same throughput  = C/T  No spatial reuse  The solution is not optimal

7. 7 Spatial Reuse HS TAP 3 TAP 1 TAP 2 TAP 5 TAP 4 TAP 6 TAP 7 (1,0) (4,0) (2,1) (3,2) (5,4) (7,5) (6,5) M8M8 M1M1 M2M2 M3M3 M5M5 M6M6 M 10 M9M9 M7M7 M4M4  Some links can be activated at the same time  A shorter cycle (T=19 instead of 24)  Optimal spatial reuse:  We have to minimize T

8. 8 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: 1.Construction of the compatibility matrix/graph 2.Construction of the cliques 3.Definition of the fair scheduling (FS)

9. 9 Our Solution: Compatibility Matrix  Three main components: 1.Construction of the compatibility matrix/graph HS TAP 3 TAP 1 TAP 2 TAP 5 TAP 4 TAP 6 TAP 7 (1,0) (4,0) (2,1) (3,2) (5,4) (7,5) (6,5) M8M8 M1M1 M2M2 M3M3 M5M5 M6M6 M 10 M9M9 M7M7 M4M4 0 0 0 0 0 1 1 0 0 0 0 1 1 1 0 0 0 1 1 1 1 CM = 0 0 1 0 0 0 0 0 1 1 0 0 0 0 1 1 1 0 0 0 0 (1,0) (4,0) (2,1) (3,2)(5,4) (7,5) (6,5) (1,0) (4,0) (2,1) (3,2) (5,4) (7,5) (6,5) 

10. 10 Our Solution: Cliques  Three main components: 2.Construction of the cliques A clique is a set of links which can all be enabled at the same time.  7 cliques of size 1  8 cliques of size 2  No clique of size  3  We define all the possible cliques. Cl d links

11. 11 Our Solution: The Fair Scheduling (FS)  Three main components: 3.Definition of the fair scheduling (FS) t 1cycle links Cl 1 d1d1 Cl 2 d2d2 dpdp Cl p A scheduling s is a set of cliques that fulfills: 

12. 12 Our Solution: The Fair Scheduling (FS)  Rationale of FS: 1. s =  2. G = compatibility graph 3. Search for the clique Cl max with the maximal gain in G 4. s = s  Cl max 5. G = G\ Cl max 6. if |G|>0, go to step 3

13. 13 Simulations  Matlab simulations  Two network topologies:  One-dimensional: 10, 15, 20 and 25 nodes  Two-dimensional: 8, 16, 24 and 32 nodes  Nodes distribution: m=2.n  Uniform distribution  Peripheral distribution  Central distribution  We compare the performance of our solution with the scheduling without spatial reuse

14. 14 Results: One-dimensional Topology

15. 15 Results: Two-dimensional Topology

16. 16 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