# Consensus and Collision Detectors in Wireless Ad Hoc Networks - PowerPoint PPT Presentation

Consensus and Collision Detectors in Wireless Ad Hoc Networks

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Consensus and Collision Detectors in Wireless Ad Hoc Networks

## Consensus and Collision Detectors in Wireless Ad Hoc Networks

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1. Consensus and Collision Detectorsin Wireless Ad Hoc Networks

2. Consensus and Collision Detectorsin Wireless Ad Hoc Networks

3. Consensusand Collision Detectorsin Wireless Ad Hoc Networks

5. Wireless Ad Hoc Networks • Challenges: • Unknown number • No unique ids • Fault-prone • Collision-prone communication

6. Wireless Ad Hoc Networks • Challenges: • Unknown number • No unique ids • Fault-prone • Collision-prone communication

7. How to solve problems • in wireless ad hoc networks with • unreliable communication?

8. How to solve consensus • in wireless ad hoc networks with • unreliable communication? • Consensus: • Fundamental agreement problem • in fault-tolerant computation

9. Outline • Introduction • Collision-prone Networks • Collision Detectors • Algorithms and Lower Bounds • Conclusions

10. Single-hop network • Synchronous rounds • Unknown number of nodes • No unique identifiers

13. “Ethernet” Collisions

14. “Ethernet” Collisions ? ? ? ? ? ? ? ? ? ?

15. More Realistic Collisions

16. Unfortunately… • Consensus is impossible with • non-uniform collisions.

17. Collision Detection

18. Collision Detection ! ! ! ! !

19. Collision Detection ! ! ! ! ! • Receiver-centric • How many messages lost? • Who sent lost message?

20. Collision Detector Classes If ½ messages are lost, then report collision. If all messages are lost, then report a collision. Consensus is impossible with C

21. Consensus with CD V is the value domain

22. Consensus with CD (Always) Accurate V is the value domain

23. Consensus with CD Eventually Accurate V is the value domain

24. Eventual Collision Freedom • Eventually, if only 1 node broadcasts…

25. Eventual Collision Freedom • Eventually, if only 1 node broadcasts, then no collision occurs. • Use a contention manager. • Outputs “active/passive” at each node. • Implementation: randomized backoff, e.g.

26. Eventual Collision Freedom • Eventually, if only 1 node broadcasts*, then no collision occurs. • Use a contention manager. • Outputs “active/passive” at each node. • Implementation: exponential backoff, e.g. • If ≤ b nodes broadcast, then no collisions. • b is an unknown MAC layer constant • b could be as low as 1

27. Consensus with CD V is the value domain

28. Consensus with CD V is the value domain

29. Consensus with CD V is the value domain

30. Consensus with AC • Algorithm executes in super-rounds: • Round 1: • Active nodes vote on a value. • Round 2: • Veto round. • Anybody can veto.

31. Consensus with AC v2 v1 v2 Round 1

32. Consensus with AC v2 v1 v2 Round 1

33. Consensus with AC v2 {v1} v1 v2 Round 1

34. Consensus with AC v1 v1 v2 Round 1

35. Consensus with AC v1 v1 {v2} v2 Round 1

36. Consensus with AC v1 v1 v2 Round 1

37. Consensus with AC v1 v1 v2 {v2, } Round 1

38. Consensus with AC v1 v1 v1 v1 v2 v2 Round 1 Round 2

39. Consensus with AC v1 v1 veto v1 v1 veto v2 v2 • Continue… Round 1 Round 2

40. Consensus with AC v1 v1 v2 Round 1

41. Consensus with AC v1 v1 v2 Round 1

42. Consensus with AC v1 v1 {v1} v2 Round 1

43. Consensus with AC v1 v1 v2 Round 1

44. Consensus with AC v1 v1 v2 {v1} Round 1

45. Consensus with AC v1 v1 v1 Round 1

46. Consensus with AC v1 v1 v1 v1 v1 v1 Round 1 Round 2

47. Consensus with AC v1 v1 Decides at most 3 rounds after stabilization* *Stabilization: accuracy and collision-freedom v1 v1 v1 v1 Decide v1 Round 1 Round 2