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Power Saving and Clock Sync. Ten H. Lai. Problem, Problem, Problem!. ???. Energy Efficiency. Done at every level from physical to application. Energy-efficient routing. Energy-efficient MAC. Energy-efficient everything. Power Saving at MAC Layer. Beacon interval. awake sleep. - PowerPoint PPT Presentation
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Power Saving and Clock Sync
Ten H. Lai
Problem, Problem, Problem!
???
Energy Efficiency
Done at every level from physical to application.
Energy-efficient routing. Energy-efficient MAC. Energy-efficient everything.
Power Saving at MAC Layer
awake sleep
Beacon window ATIM window
Beacon interval
Time Sync Is Necessary/Important
Really ?What if it is difficult or impossible
to synchronize clocks?
To sync or not to sync?
Yes global synchronization
No no synchronization
Partially local synchronization
No Synchronization (0)
“Power-Saving Protocols for IEEE 802.11-Based Multi-Hop Ad Hoc Networks”
INFOCOM 2002 Y.C.Tseng, C.S. Hsu, T.Y. Hsieh NCTU
No Synchronization (1)
Basic idea: nodes be awake more frequently.
Extreme case: awake all the time.
awake sleep
Beacon interval
No Synchronization (1)
Dominating-Awake-IntervalAwake ≥ BI/2 + BW
awake sleep
Beacon interval
No Synchronization (1)
Dominating-Awake-IntervalAwake > BI/2 + BW
awake sleep
Beacon interval
No Synchronization (2)
• Periodical-Fully-Awake-Interval
T (=3) Beacon Interval
Beacon Window MTIM Window
Host A
Host B
No Synchronization (2)
Quorum-based
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16
1 4 16
1 4 16
Local Synchronization (0)
“An Energy-Efficient MAC Protocol for Wireless Sensor Networks”
INFOCOM 2002 W. Ye, J. Heidemann, D. Estrin UCLA
Local Synchronization (1)
Offset
10:10
10:04
10:09
0:01
- 0:05
-0:01
0:05
Local Synchronization (2)
Nodes of same color -- synchronize with each other. Nodes of different colors – know each other’s timing
Local Synchronization (3)
A
C
B
Problem, Problem, Problem!
???Power saving
MAC
Awake-sleep
Global no partial sync
Analysis &Comparison
Physical Routing
Clock Sync
To sync or not to sync?
Yes (global sync)
No (no sync)
Partially (local sync)
Which one?
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
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%
Global Synchronization: pros and cons
Best performance in energy saving
Needs a good synchronization algorithm
No Synchronization – pros and cons
Simple -- no need for clock sync
Less efficient in power saving
1 4 16
1 4 16
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?
1 4 16
1 4 16
The Wisdom of Diamond Sutra
No synchronization,
is not really no synchronization, it is just called no synchronization.
No Synchronization – pros and cons
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
Time Sync in the “No Sync” Scheme
Why is it simpler, more scalable?
Beacon window ATIM window
A major drawback with no sync
Broadcast/multicast is inefficient
Local Synchronization: pros and cons
More scalable
Inefficient with multiple schedules Protocols incomplete Broadcast/multicast is inefficient?
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
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.
?
Problem, Problem, Problem!
???Power saving
MAC
Awake-sleep
Global no partial sync
Analysis &Comparison
Physical Routing
Clock sync
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)
No Synchronization
Quorum-based
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16
1 4 16
1 4 16
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
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
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?
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
Quorum Size
T = {0, 1, …, n-1} Quorum: the smaller, the better (energy
efficient)
Closure property |quorum| ≥ √n
Specific Feasible Quorum Systems
Grid Quorum System (≈2√n) Torus Quorum System (≈√2n ) Cyclic Quorum System (≈√n) Finite Projective Plane Quorum System (≈√n)
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}.
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.
K=4
Problem, Problem, Problem!
???Power saving
MAC
Awake-sleep
Global no partial sync
Analysis &Comparison
Physical Routing
Clock sync
S-MAC
S-MAC: an energy-efficient MAC
In IEEE INFOCOM 2002, By Ye, Heidemann, Estrin
IEEE 802.11-like CSMA/CA
802.11 MAC
RTS(t1)
CTS(t2)
DATA(t3)
ACK
A
B
C Back offTurn
S-MAC
What’s next?
???Power saving
MAC
Awake-sleep(802.11)
Global no partial sync
Analysis &Comparison
Physical Routing
Clock sync
S-MAC
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