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
No no synchronization
Partially local synchronization

7. No Synchronization (0)
“Power-Saving Protocols for IEEE Based Multi-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 2 3 4 2 3 4 6 7 8 5 6 7 8

13. Local Synchronization (0)
“An Energy-Efficient MAC Protocol for Wireless Sensor 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)B C

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

18. Which one? 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
Parameter
Value
Beacon Interval length
100ms
Beacon window length
3ms
ATIM window length
7ms
PFAI T value 4
Quorum-based n value 6
Avg. num. of sch. in border nodes for Local Synch.
2.7
Total nodes in Local Synch.
100

20. Analysis of energy saving (2)
Clock synchronization method
Awake time ratio
No Synchronization
DAI
53%
PFAI
32.5%
Quorum-based
35.4%
Global Synchronization
10%
Local Synchronization
2 schedules
19%
3 schedules
28.4%
4 schedules
37.87%

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

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

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

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

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: pros and cons
More scalable
Inefficient with multiple schedules
Protocols incomplete
Broadcast/multicast is inefficient?

29. All of them To sync or not to sync? Which one? 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 Ad Hoc Networks,” ICPP’03 (Best paper award)

33. No Synchronization Quorum-based 1 2 3 4 2 3 4 6 7 8 5 6 7 8 10 11 12

34. View T as a matrix and pick a row and a column as the quorum
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.

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.

36. Questions Feasible quorum systems: quorum systems with Property PSP.
How to characterize 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

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 like
CSMA/CA

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

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