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: pro and con

Best performance in energy saving

Needs a good synchronization algorithm

No Synchronization – pro and con

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

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

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: pro and con

More scalable

Inefficient with multiple schedules Protocols incomplete

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

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.

?

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)

Problem, Problem, Problem!

???

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

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: 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.

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?

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

Quorum Size

Given T = {0, 1, …, n-1} Quorum: the smaller, the better (energy

efficient)

Closure property |quorum| ≥ sqrt(n)

Specific Feasible Quorum Systems

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

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.

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

S-MAC

RTS(t1)

CTS(t2)

DATA(t3)

ACK

A

B

C Back offTurn

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