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