To Sync or Not to Sync in
Power Saving?
Ten H. Lai

Zen ( 禪 ) Networking
 Essential
 Fundamental
 Simple
 Effective
 Fresh
Problems
Protocols

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
Beacon window ATIM window

Time Sync Is Necessary/Important

?
 What if it is difficult or impossible to synchronize clocks?

To sync or not to sync?


Yes
No global synchronization no synchronization
 Partially local synchronization


“Power-Saving Protocols for IEEE 802.11-
Based MultiHop Ad Hoc Networks”
 INFOCOM 2002
 Yu-Chee Tseng, Chih-Shun Hsu, Ten-Yueng
Hsieh
 National Chiao-Tung University

 Basic idea: nodes have to awake for long enough time.
 Extreme case: awake all the time.

No Synchronization (1)
 Dominating-Awake-Interval
 Awake > BI/2 + BW

No Synchronization (2)
• Periodical-Fully-Awake-Interval
T (=3) Beacon Interval
Host A
Host B
Beacon Window MTIM Window

(3)
 Quorum-based

Local Synchronization

“An Energy-Efficient MAC Protocol for
Wireless Sensor Networks”
 INFOCOM 2002
 Wei Ye, John Heidemann, Deborah Estrin
 UCLA

Local Synchronization (0)
 Offset
10:09
-0:01
10:10
0:05
10:04

B
C
Local Synchronization (1)
A
 Awake/sleep schedules
1:00 2:00 3:00

A
B
C
Time Synchronization in 802.11
1:00 2:00 3:00

A
B
C
Local Synchronization (2)
1:00 2:00 3:00

A
B
C
Local Synchronization (3)

Local Synchronization (4)
 Nodes are free to choose their schedules
 Or synchronize with others
 Or adopt multiple schedules

A
B
C
D
Local Synchronization (3)

A
B
C
Communications

 Yes
 No
 Partially
( global sync )
( no sync )
( local sync )

?

Analysis of energy saving (1)
 No data traffic
 Parameters
Parameter
Beacon Interval length
Beacon window length
ATIM window length
PFAI T value
Quorum-based n value
Avg. num. of sch. in border nodes for Local Synch.
Total nodes in Local Synch.
Value
100ms
3ms
7ms
4
6
2.7
100

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

Global Synchronization: pro and con
 Best performance in energy saving
 Have to assume:
– Formation : MANET initiated by a single node.
– Connectivity : MANET remains connected.
 Overhead

No Synchronization – pro and con
 Simple -- no need for clock sync
 Less efficient in power saving
 Any other disadvantages?

No Synchronization: Analysis
 B has a packet for A in interval 14.
 Q: When should B send it?

– In every blue .
– Or when blue meet gray .

Q: When will blue meet gray
?

金剛經的智慧
 所謂 … 即非 … 是名 …
 The so called no synchronization is not no synchronization ; it’s named no synchronization .

No Synchronization: Analysis
Clock synchronization method
No Synchronization
DAI
PFAI

Quorum-based

Global Synchronization
Local Synchronization
2 schedules
3 schedules
4 schedules
Awake time ratio
53%
32.5%
35.4%
10%
19%
28.4%
37.87%

No Synchronization – pro and con
 Less efficient in power saving


Simple -- no need for clock sync
Simpler – clock sync is simpler and more scalable, but less energy efficient

Time Sync in the “No Sync” Scheme
 Simpler, more scalable, less energy efficient
Beacon window ATIM window

Local Synchronization: pro and con
 Infocom 2002, Ye & Heidemann & Estrin
 Multiple schedules
 Inefficient with multiple schedules
 Many issues unaddressed

 Yes ( global sync )
 No ( no sync )
 Partially ( local sync )

?
Normal situation
Neighbor discovery
Transient situation

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.
?

 Yes ( global sync )
 No ( no sync )
 Partially ( local sync )

Problem, Problem, Problem!
Power saving
MAC
Awake-sleep
???
global no partial sync