1. Low Latency Broadcast in Multirate Wireless Mesh NetworksChun Tung Chou, Archan Misra, Junaid Qadir Keon Jang 2007. 10. 18

2. Outline • Introduction • Impact of Multirate Links on Efficient Broadcasting • Problem Formulation • Heuristic Algorithm • Simulation and Performance Study • Conclusion

3. Introduction • Wireless mesh network (WMN) is considered to be promising technology for suburban and urban community-based networks • Two aspects of WMN research is popular • Use of multichannel, multi-radio mesh nodes • Multi-rate Mac protocols 1 2 3 100m 11Mbps 400m 1Mbps

4. Introduction(cont’d) • How low-latency (and possibly high-throughput) network-layer broadcast can be realized? • WMN broadcast applications • Broadcast community specific content • Neighborhood soccer game or etc. • Wide-are content to a group of receivers

5. Introduction(cont’d) • Effect of multi-rate links on efficient broadcasting • Is multi-rate multicast necessary? • How effective it is? • Choice of transmission rates in multi-rate networks • How many different transmission rate is needed? • Are some rates more efficient than others?

6. Impact of Multi-rate Link on Efficient Broadcasting • Broadcast from node 1 to all the other nodes 5 1 2 3 4 400m 1Mbps 100m 11Mbps 400m 1Mbps 400m 1Mbps

7. Case 1: 1 Transmit / 1 Node 5 1 2 3 4 400m 1Mbps 100m 11Mbps 400m 1Mbps 400m 1Mbps 1 -> (2,5) 2 -> 3 3 -> 4 Time 0 11 22 33

8. Case 2: Allow multiple transmit 5 1 2 3 4 400m 1Mbps 100m 11Mbps 400m 1Mbps 400m 1Mbps 1 -> 2 3 -> 4 1 -> 5 2 -> 3 Time 0 1 12 23

9. Problem Formulation • Build an algorithm to minimize broadcasting latency in wireless mesh networks • Assumptions • MAC-layer multi-rate multicast capability • Each node has single radio • Radio on all nodes are tuned to common channel • etc This problem is NP Hard!

10. Heuristic Algorithm • 3 Stages • Broadcasting Tree Construction • Multicast Grouping • Scheduling Transmissions 2 1 s 2 2

11. Weighted Connected Dominating-set Construction (WCDS) V = nodes N(x,y)= reachable nodes from x with rate y C = covered nodes T = tree R = transmission rates WCDS is modification of MCDS

12. Multicast Grouping Algorithm • a->b, 1 • a->c, 8 20 . . . c 8 31 a Time 1 +Max(8+20,30) ----------------- 31 30 1 b . . .

13. Multicast Grouping Algorithm(cont’d) • a->(b,c) 8 20 . . . c 8 38 a Time 8 +Max(20,30) ----------------- 38 30 8 b . . .

14. Multicast Grouping Algorithm(cont’d) • a->b, 1 • a->c, 8 20 . . . c 8 31 a Time 1 +Max(8+20,30) ----------------- 31 30 1 b . . .

15. Scheduling of Transmissions • From WCDS and Grouping, all the multicasts transmissions(sender, recipient group) are known • Scheduling determines a time that each multicast happens based on dependency and interference • Dependency: a node can relay only after it receives

16. Simulated Performance Studies • 4 Different Tree Construction Algorithm • WCDS • BIB(author’s previous work) • SPT (shortest path using Dijkstra) • CDS (lowest transmission rate only) • 100 randomly generated topologies

17. Normalized LatencySingle Transmission Case

18. Normalized ThroughputSingle Transmission Case

19. Multiple Transmission Case • Only 2 out of 100 topologies required multiple transmission • Multiple may not be required in single radio and single channel case • However, multi-radio and multi-channel is more likely to benefit from multiple transmission

20. Conclusion • Proposed the novel concept of multi-rate link-layer multicast for network-layer broadcast • Showed that exploiting multi-rate can reduce latency and increase throughput