Power Scheduling at the Network Layer for wireless sensor networks Barbara Hohlt Eric Brewer UC Berkeley NEST Retreat June 2004.

Power Scheduling at the Network Layer for wireless sensor networks

Barbara HohltEric BrewerUC Berkeley

NEST RetreatJune 2004

Wireless sensor networks

Lifetime constrained by limited energy stores Communication is the dominant energy cost Turning the radio off during idle times reduces

power consumption Flexible Power Scheduling

Adaptively schedules nodes to save radio power Decentralized Multihop sense-to-gateway applications Typically 5X power savings

Time in seconds

Cur

rent

in

mA

1.4

Overall: 5X Savings

Six Design Principles

Avoid idle listening Use a schedule Two-layer architecture Schedule traffic flows (not packets) Schedules must be adaptive Nodes that want change do the most

listening

Power Schedule

MAC

FPS Two-Layer Architecture

Coarse-grain slotted scheduling At the network layer Plan radio on-off times

Fine-grain CSMA MAC underneath Reduces contention and increases end-to-end fairness

Distribute traffic Decouple correlated events from traffic Reserve bandwidth from source to sink

Does not require perfect schedules or precise time synchronization

FPS Experiments

10 MICA motes plus base station

6 motes send 100 messages across 3 hops

One message per cycle (3200ms)

Begin with injected start message

Repeat 11 times

1 2 3 4 5 6

Two Topologies Single Area

one 8’ x 3’4” area Multiple Area

five areas, motes are 9’-22’ apart

Scheduled (FPS) vs Unscheduled (Naïve)

Contention is Reduced

CDF of Single Area Tests

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

90.00%

100.00%

0 5 10 15 20 25 30 35

Backoff Bins

CD

F

Naive

FPS

End-to-end FairnessThroughput and Fainess in Multiple Area Tests

0

10

20

30

40

50

60

70

80

90

100

1 2 3 4 5 6

Pe

rce

nt

FPS

Naive

AVG STDDEV Max/Min

FPS 96.4 1.13 1.03

Naïve 24.7 6.19 2.48

Evaluation with TinyDB

Three implementations TinyDB duty cycling TinyDB low power listening TInyDB FPS

Berkeley Botanical Gardens

3 Step Methodology

Estimate radio-on time for each scheme

For FPS, validate the estimate at one mote

Use current measurements to estimate power consumption

TinyDB Redwood Deployment

17

18

BTS

1 2

3

0

• 2 trees• 35 nodes

• 1/3 two hops• 2/3 one hop

No radio power management = 3600 sec/hour

TinyDB Duty Cycling

4 seconds

2.5 minutes

All nodes wake up together for 4 seconds every 2.5 minutes. During the waking period nodes exchange messages and take sensor readings.

Outside the waking period the processor, radio, and sensors are powered down.

24 samples/hour * 4 sec/sample = 96 sec/hour

Low-Power Listening

Radio-on time = listening + transmitting + receiving

.003 sec/poll * 10 polls/sec * 3600 sec/hour= 108 sec/hour to listen

( 24 samples/hour ) * ( 2/3 * 1 hop + 2/3 * 1 hop ) = 32 hops/hour

32 hops/hour * 0.1 sec/hop = 3.2 sec/hour to transmit

108 (L) + 3.2 (T) + 1.6 (R) = 112.8 sec/hour

Flexible Power Scheduling

18 slots * 128 ms = 2.3 sec/cycle per 3 nodes

= 0.767 sec/cycle (per node)

24 samples/hour * 0.767 sec/cycle = 18.4 sec/hour

0

2

3

1

TrafficComm

Node 1: 2 T, 3 ANode 2: 3 T, 2 R, 3 ANode 3: 2 T, 3 A

5 (node 1) + 8 (node 2) + 5 (node 3) = 18 slots

FPS Validation

Metric Slots Idle PercentPredicted Idle Slots 56/64 89.1%Measured Idle Slots 56/64 89.1%Measured Radio Idle Time 56/64 91%

10

100

1000

10000

100000

None DutyCycling

LPL FPS

Cu

rren

t (m

A)

10

100

1000

10000

Sec

on

ds

Off Current

On Current

Total Current

Radio On Time

Power ratios: 160x 4.4x 5.1x 1

Summary

Flexible Power Scheduling Two-level architecture Schedules flows (not packets) Adaptive and decentralized schedules

Reduced contention and increased end-to-end fairness and throughput

Improved power savings of 4.4X over duty cycling and 160X over no power management

Thank You

Barbara [email protected]

Radio-on Times

Scheme Radio On Time RatioNone 3600 196Duty Cycling 96 5.2LPL 113 6.1FPS 18.4 1.0

Radio on: 8 mARadio off and node on: 0.4 mARadio off and node asleep: 0.01 mA

Power Savings

Scheme Radio On Radio Off On (mA) Off (mA) Total (mA) RatioNone 3600 0 28800 0 28800 157Duty Cycling 96 3504 768 35.04 803 4.39LPL 113 3487 904 34.87 939 5.13FPS 18.4 3582 147 35.816 183.0 1

Scheme Radio On Radio Off On (mA) Off (mA) Total (mA) RatioNone 3600 0 28800 0 28800 18.2Duty Cycling 96 3504 768 1401.6 2170 1.37LPL 113 3487 904 1394.8 2299 1.46FPS 18.4 3582 147 1432.64 1579.8 1

Radio Off and Node Asleep

Radio Off but Node On (Worst Case)

END