Realistic and Efficient Multi-Channel Communications in Wireless Sensor Network

1. Realistic and Efficient Multi-Channel Communications in Wireless Sensor Network

2. Reference • Yafeng W., Stankovic J.A., Tian H, Shan L, “Realistic and Efficient Multi-Channel Communications in Wireless Sensor Networks,” In proceedings of INFOCOM 2008, Phoenix, AZ, USA, April 13-18, 2008.

3. Outline • Introduction • Experiments on Multi-Channel Reality • Tree Based Multi-Channel Protocol (TMCP) • Minimum Interference Channel Assignment Problem • Performance Evaluation • Conclusion

4. Introduction • Wireless sensor network (WSN) (Base Station)

5. Introduction • Current WSN hardware such as Micaz and Telos provide multiple channels • Improve network throughput • Provide reliable and timely communication services • Recently MAC layer multi-channel protocols are proposed • To assign different channels to two-hop neighbors (hidden terminal problem) and coordinate channel switching • Also called node-based schemes • E.q., MMSN(INFOCOM2006), TMMAC(ICC2007), MCMAC(CIT2006).

6. Introduction • Practical issues for node-based scheme • A large number of channels are needed in dense networks. • Require precise time synchronization at nodes. • Channel switching delay and scheduling overhead. • Complex.

7. Outline • Introduction • Experiments on Multi-Channel Reality • Tree Based Multi-Channel Protocol (TMCP) • Minimum Interference Channel Assignment Problem • Performance Evaluation • Conclusion

8. Experiments on Multi-Channel Reality • Number of available channels • Adjacent Channel Interferences • Interferences with 802.11 network • Impact of time synchronization errors • Experiment setup • CC2420 radio chip used in Micaz motes • 16 non-overlapping channels, with 5MHz spacing

9. Adjacent Channel Interferences • Place three Micaz motes in a line • One transmitter, one receiver, and one jammer • The jammer’s transmission is synchronized with the transmitter. Transmitter : channel 11 Receiver : channel 11 Jammer : channel 12 (adjacent) channel 13 (2 channel away)

10. Adjacent Channel Interferences • Adjacent channel interference can cause collisions and packet losses.

11. Interferences with 802.11 networks • Put 8 pairs of Micaz motes closely in a department office with 802.11 networks • Each pair uses unique channel and all 8 channels are non-adjacent. • Multi-channel protocols must have capabilities to work well with a small number of available channels.

12. Impact of Time Synchronization Errors 1 2 3 4 5 • Put 5 Micaz motes on a line • Node-based scheme • Each node with unique channel and all are synchronized. • A time period is divided into 2 time slots • 1st time slot • Nodes in odd positions send packets • Nodes in even positions receive packets • 2nd time slot is vice versa

13. Impact of Time Synchronization Errors

14. Outline • Introduction • Experiments on Multi-Channel Reality • Tree Based Multi-Channel Protocol (TMCP) • Minimum Interference Channel Assignment Problem • Performance Evaluation • Conclusion

15. TMCP • To overcome two problems in practical networks • The small number of available non-adjacent channels. • Unavoidable time errors. • Data collection traffic • Multiple information flows generated at sensor nodes converge to the base station.

16. TMCP • Main idea • Partition the whole network into multiple vertex-disjoint subtrees all rooted at base station • Allocate different channels to each subtree. • Forward each flow only along its corresponding subtree. • 3 components • Channel detection (CD) • Channel assignment (CA) • Data communication (DC)

17. TMCP • CD finds available non-adjacent channels • Two motes are used to sample the link quality, and we selected good link qualities with non-adjacent channel. • Assume we have k channels at this point. • CA partitions the whole network into k subtrees and assigns one unique channel to each subtree • Intertree interference is eliminated • Intra-tree interference is minimized

18. TMCP • DC manages the data collection through each subtree • Assume the base station is equipped with multiple radio transceivers. • Without time synchronization

19. Outline • Introduction • Experiments on Multi-Channel Reality • Tree Based Multi-Channel Protocol (TMCP) • Minimum Interference Channel Assignment Problem • Performance Evaluation • Conclusion

20. Model and Problem Definition • The goal is to minimize intra-tree interferences. • Assume that a sensor network is a static. • Interference value: int(u) = the number of other node by which the reception at u can be distrurbed • The intra-tree interference value of a tree T is defined as • int(T) = max{int(u): u is a non-leaf of T} * M. Burkhart, P. V. Rickenbach, R. Wattenhofer, and A. Zollinger, “Does topology control reduce interference,” in ACM MobiCom, 2004.

21. The intra-tree interference value of the tree is 4.

22. PMIT Algorithm • Apply Breadth-First search algorithm from the base station to construct a fat tree. • Nodes keep height and have multiple parents on the fat tree. • The tree is a shortest path tree. • Execute the channel allocation one-by-one level from top to bottom on the fat tree • For each node, choose an optimal tree and add this node to bring the least interference to this tree. • Selects a parent which has the least interference value. • Nodes with fewer parents first, because they are less free to choose channels.

24. Greedy PMIT • K = 3

25. Evaluation of the PMIT Algorithm • Simulations parameters • 200m x 200m field • 250 nodes are uniformly distributed • Communication range is 10~35m • Interference range is 1.5 times as the communication range

26. Evaluation of the PMIT Algorithm Prim’s algorithm constructs “Minimum Spanning Tree” (single channel) Eavesdropping is a node-based protocol (multi-channel) Lower bound : maximum interference value among all nodes divided by k channels

27. Outline • Introduction • Experiments on Multi-Channel Reality • Tree Based Multi-Channel Protocol (TMCP) • Minimum Interference Channel Assignment Problem • Performance Evaluation • Conclusion

28. Performance Evaluation of TMCP • Simulation parameters • 200m x 200m field • 250 nodes are uniformly distributed • Communication range is 10~40m • Interference range is 1.5 times as the communication range • 50 Many-to-one CBR streams • 40 packets / second

29. Performance with different node density

30. Performance with different workload workload workload workload

31. Performance comparison of • TMCP and MMSN • 50 CBR streams • Node density: 38 • The radio range: 40m

32. Evaluation in a Real Testbed • Experiment setup • A real testbed with 20 Micaz motes. • Four motes are laid closely together to act as a base station with four transceivers.

33. Saturated data rate (reception radio above 80%) • TMCP effectively reduces interferences and mitigates congestion at nodes. • TMCP works well in a real sensor network.

34. Outline • Introduction • Experiments on Multi-Channel Reality • Tree Based Multi-Channel Protocol (TMCP) • Minimum Interference Channel Assignment Problem • Performance Evaluation • Conclusion

35. Conclusion • This paper studies how to efficiently use multiple channels to improve network performance in WSNs. • TMCP • Work with a small number of channels. • Work without the need of time synchronization. • Decrease potential radio interferences.