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?

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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

8
No Synchronization (1)

• Basic idea nodes be awake more frequently.
• Extreme case awake all the time.

Beacon interval
awake sleep
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No Synchronization (1)

• Dominating-Awake-Interval
• Awake gt BI/2 BW

Beacon interval
awake sleep
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No Synchronization (1)

• Dominating-Awake-Interval
• Awake gt BI/2 BW

Beacon interval
awake sleep
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No Synchronization (2)

• Periodical-Fully-Awake-Interval

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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

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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

1010 1004
-001 005
001
1009
- 005
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Local Synchronization (2)

• Nodes of same color -- synchronize with each
  other.
• Nodes of different colors know each others
  timing

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Local Synchronization (3)
A
B
C
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Problem, Problem, Problem!
Power saving
???
Physical
MAC
Routing
Awake-sleep
Global no partial sync
Analysis Comparison
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 Clock synchronization method Awake time ratio
No Synchronization DAI 53
No Synchronization PFAI 32.5
No Synchronization Quorum-based 35.4
Global Synchronization Global Synchronization 10
Local Synchronization 2 schedules 19
Local Synchronization 3 schedules 28.4
Local Synchronization 4 schedules 37.87
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Global Synchronization pro and con

• Best performance in energy saving
• Needs a good synchronization algorithm

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No Synchronization pro and con

• 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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No Synchronization pro and con

• 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 pro and con

• More scalable
• Inefficient with multiple schedules
• Protocols incomplete

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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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Proposed 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
???
Physical
MAC
Routing
Awake-sleep
Global no partial sync
Analysis Comparison
Clock sync
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Follow-ups on no-sync

• Asynchronous Wakeup for Ad Hoc Networks,
  Mobihoc03
• Quorum-Based Asynchronous Power-Saving Protocols
  for IEEE 802.11Ad Hoc Networks, ICPP03 (Best
  paper award)

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Problem, Problem, Problem!

• ???

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??????

• ?? ?? ??
• The so called no synchronization
• is not no synchronization
• its named no synchronization.

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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

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• T 0, 1, , n-1
• Quorum any 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 slots.

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Questions

• T 0, 1, , n-1
• Quorum any subset of T
• Quorum system a collection of quorums
• Feasible quorum systems those that work for the
  Power Saving Problem.
• All feasible quorums?
• 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 n rotate (B, i) ? F

0 3
15
0 3
15
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Quorum Size

• Given T 0, 1, , n-1
• Quorum the smaller, the better (energy
  efficient)
• Closure property quorum sqrt(n)

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Specific Feasible Quorum Systems

• Grid Quorum System
• Torus Quorum System
• Cyclic Quorum System
• Finite Projective Plane Quorum System

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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 (k1k2)/2
  overlaps.
• Can be used to dynamically adjust the number of
  overlaps.

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Problem, Problem, Problem!
Power saving
???
Physical
MAC
Routing
Awake-sleep
S-MAC
Global no partial sync
Analysis Comparison
Clock sync
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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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S-MAC
???
RTS(t1)
DATA(t3)
A B C
CTS(t2)
ACK
Back off
Turn