Power Saving and Clock Sync

1. Power Saving and Clock Sync Ten H. Lai

2. Problem, Problem, Problem! ???

3. Energy Efficiency • Done at every level from physical to application. • Energy-efficient routing. • Energy-efficient MAC. • Energy-efficient everything.

4. Power Saving at MAC Layer Beacon interval awake sleep Beacon window ATIM window

5. Time Sync Is Necessary/Important • Really? • What if it is difficult or impossible to synchronize clocks?

6. To sync or not to sync? • Yes global synchronization • Nono synchronization • Partially local synchronization

7. No Synchronization (0) • “Power-Saving Protocols for IEEE 802.11-BasedMulti-Hop Ad Hoc Networks” • INFOCOM 2002 • Y.C.Tseng, C.S. Hsu, T.Y. Hsieh • NCTU

8. No Synchronization (1) • Basic idea: nodes be awake more frequently. • Extreme case: awake all the time. Beacon interval awake sleep

9. No Synchronization (1) • Dominating-Awake-Interval • Awake ≥ BI/2 + BW Beacon interval awake sleep

10. No Synchronization (1) • Dominating-Awake-Interval • Awake > BI/2 + BW Beacon interval awake sleep

11. No Synchronization (2) • Periodical-Fully-Awake-Interval

12. No Synchronization (2) • Quorum-based 1 4 16 1 4 16 1 2 3 4 • 2 3 4 • 6 7 8 • 10 11 12 • 13 14 15 16 5 6 7 8 • 10 11 12 • 13 14 15 16

13. Local Synchronization (0) • “An Energy-Efficient MAC Protocol for WirelessSensor Networks” • INFOCOM 2002 • W. Ye, J. Heidemann, D. Estrin • UCLA

14. Local Synchronization (1) • Offset 10:10 10:04 -0:01 0:05 0:01 10:09 - 0:05

15. Local Synchronization (2) • Nodes of same color -- synchronize with each other. • Nodes of different colors – know each other’s timing

16. Local Synchronization (3) A B C

17. Problem, Problem, Problem! Power saving ??? Physical MAC Routing Awake-sleep Global no partial sync Analysis & Comparison Clock Sync

18. To sync or not to sync? • Yes (global sync) • No (no sync) • Partially (local sync) • Which one?

19. Analysis of energy saving (1) • No data traffic • Parameters

20. Analysis of energy saving (2)

21. Global Synchronization: pros and cons • Best performance in energy saving • Needs a good synchronization algorithm

22. No Synchronization – pros and cons • Simple -- no need for clock sync • Less efficient in power saving 1 4 16 1 4 16

23. No Synchronization: Analysis • A has a packet for B in interval 4. • Q: When should A send it? • In every yellowinterval • Or when yellow meets red. • Q: When will yellowmeetred  ? 1 4 16 1 4 16

24. The Wisdom of Diamond Sutra • No synchronization, is not reallyno synchronization,it is just called no synchronization.

25. No Synchronization – pros andcons • Less efficient in power saving • Simple -- no need for clock sync • Simpler – clock sync is simpler and more scalable ? 1 4 16 1 4 16

26. Time Sync in the “No Sync” Scheme • Why is it simpler, more scalable? Beacon window ATIM window

27. A major drawback with no sync • Broadcast/multicast is inefficient

28. Local Synchronization: prosand cons • More scalable • Inefficient with multiple schedules • Protocols incomplete • Broadcast/multicastis inefficient?

29. To sync or not to sync? • Yes (global sync) • No (no sync) • Partially (local sync) • Which one? Normal situation Neighbor discovery Transient situation All of them

30. Possible Protocol • Normally, use the global sync scheme. • Switch to the no sync scheme when necessary (for neighbor discovery). • Use the partial sync scheme while merging. ?

31. Problem, Problem, Problem! Power saving ??? Physical MAC Routing Awake-sleep Global no partial sync Analysis & Comparison Clock sync

32. Follow-ups on no-sync • “Asynchronous Wakeup for Ad Hoc Networks,” Mobihoc’03 • “Quorum-Based Asynchronous Power-Saving Protocols for IEEE 802.11Ad Hoc Networks,” ICPP’03(Best paper award)

33. No Synchronization • Quorum-based 1 4 16 1 4 16 1 2 3 4 • 2 3 4 • 6 7 8 • 10 11 12 • 13 14 15 16 5 6 7 8 • 10 11 12 • 13 14 15 16

34. T = {0, 1, …, n-1} • Quorum: a subset of T • View T as a matrix and pick a row and a column as the quorum • Property A: No matter how asynchronous, every two nodes have at least one overlap in every T intervals. 0 3 15 0 3 15

35. Desired Property for the Power Saving Problem • Property PSP: No matter how asynchronous, every node’s beacon window is covered by every other node’s active period at least once per T intervals. 0 3 15 0 3 15

36. Questions • Feasible quorum systems: quorum systems with Property PSP. • How tocharacterize all feasible quorum systems? • Any optimal feasible quorum system? • What if we want to have m overlaps?

37. Feasible Quorum System • A sufficient condition (rotation closure property): For any two quorums A, B in the system, A ∩ rotate (B, i) ≠ Φ 0 3 15 0 3 15 0 3 15

38. Quorum Size • T = {0, 1, …, n-1} • Quorum: the smaller, the better (energy efficient) • Closure property |quorum| ≥ √n

39. Specific Feasible Quorum Systems • Grid Quorum System (≈2√n) • Torus Quorum System (≈√2n ) • Cyclic Quorum System (≈√n) • Finite Projective Plane Quorum System (≈√n)

40. Quorum Systems with a Single Quorum • T = {0, 1, …, n-1}. H is a subset of T. • {H} is a quorum system iff … H is a difference set of T. • H is a difference set of T iff for every i in T, i = x-y mod n for some x, y in H. • {0, 1, 2, 4} is a difference set of {0,1, …, 7}.

41. Quorum Systems with multiple overlaps • E-Torus Quorum System • e-torus(k1) and e-torus(k2) have (k1+k2)/2 overlaps. • Can be used to dynamically adjust the number of overlaps.

42. K=4

43. Problem, Problem, Problem! Power saving ??? Physical MAC Routing Awake-sleep S-MAC Global no partial sync Analysis & Comparison Clock sync

44. S-MAC: an energy-efficient MAC • In IEEE INFOCOM 2002, • By Ye, Heidemann, Estrin • IEEE 802.11-like • CSMA/CA

45. 802.11 MAC S-MAC RTS(t1) DATA(t3) A B C CTS(t2) ACK Back off Turn

46. What’s next? Power saving ??? Physical MAC Routing Awake-sleep(802.11) S-MAC Global no partial sync Analysis & Comparison Clock sync