KAIST

Medium Access Control with Coordinated Adaptive Sleeping

for Wireless Sensor Network

Wei Ye, John Heidemann, Deborah Estrin

2003 IEEE/ACM TRANSACTIONS ON NETWORKING

Hong Nan-Kyoung

Network & Security LAB at KAIST

2006.09.06

22/22/22S-MAC

Contents

Introduction

MAC attributes

Sensor-MAC (S-MAC) overview

S-MAC major components

Periodic listen and sleep

Collision avoidance

Coordinated Sleeping

Overhearing avoidance

Massage passing

Implementation

Experiments

Conclusion

33/22/22S-MAC

Introduction

Where is the medium access control (MAC)

A sublayer of the Link layer

Directly controls the radio

Only cares about its neighborhood

The role of MAC

Controls when and how each node can transmit

Why do we need MAC?

Shared medium

Same frequency band interfere with each other – collisions

Application layer

Transport layer

Network layer

Link/MAC layer

Physical layer

< Network model from Internet>

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MAC attributes

Characteristics of wireless sensor network (WSN)

Battery powered

Large number of nodes

Sensor-triggered bursty traffic

Topology and density change

In-network data processing

Nodes for a common task

WSN can often tolerate some delay

Primary attributes

Energe-efficiency, scalability, adaptability

Secondary attributes

Latency, fairness, throughput

55/22/22S-MAC

S-MAC overview

Goal

Reducing energe consumption in WSN

Major sources of energy waste

Collision

Overhearing

Control packet overhead

Long idle listening

Tradeoffs

Latency & Fairness Energy

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S-MAC major components

Collision avoidance

Coordinated Sleeping

Periodic listen and sleep

Schedule Synchronization

Adaptive Listening

Overhearing avoidance

Massage passing

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Collision avoidance

S-MAC is based on contention

Collision-Avoidance Strategy ~= 802.11 ad hoc mode

RTS/CTS for hidden terminal problem

Physical carrier sense

Virtual carrier sense: network allocation vector (NAV)

Randomized backoff time

RTSSender

Receiver CTS

Other Sensors

DATA

ACK

NAV (based on RTS)

NAV (based on CTS)

Contend for medium access

defer access

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Coordinated Sleeping

Problem: Idle listening consumes significant energy

Solution: Periodic listen and sleep

Turn off radio when sleeping

Reduce duty cycle to ~ 10% Latency Energy

duty cycle = listen interval / frame length

sleeplisten listen sleep

Frame

Sleepfor SYNC for RTS for CTSReceiver

SenderTx SYNC

CSCS

Tx RTS Got CTS

Send data

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Coordinated Sleeping

Schedules can differ

Prefer neighboring nodes have same schedule

Easy broadcast & low control overhead

Node 1

Node 2

sleeplisten listen sleep

sleeplisten listen sleep

Border nodes

: two schedules or broadcast twiceSchedule 2

Schedule 1

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Coordinated Sleeping

Schedule Synchronization

New node tries to follow an existing schedule

Remember neighbors’ schedules

To know when to send to them

Broadcasts its schedule using SYNC pkt

Re-sync when receiving a schedule update

Periodic neighbor discovery

Keep awake in a full sync interval Syncronization period (SP)

SP SP SP

Listen Sleep

Frame

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Coordinated Sleeping

Adaptive listening

Reduce multi-hop latency due to periodic sleep

Wake up for a short period of time at end of each transmission

Reduce latency by at least half

41 2 3

CTS

RTS

CTS

listen listenlisten

t1 t2

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Latency Analysis

N hop total delay Dn = D1 + D2 + . . . + D10

Delay = Carrier sense (tcs) + Transmission (ttx) + Sleep (ts)

MAC without sleeping

E[D(n)] = N(tcs + ttx)

S-MAC without adaptive listening

E[D(n)] = NTf – Tf /2 + tcs + ttx

S-MAC with adaptive listening

E[D(n)] = NTf /2 –Tf/2 + 2tcs + 2ttx

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Overhearing avoidance

Problem: Receive packets destined to others

Solution: Sleep when neighbors talk

Use in-channel signaling

Who should sleep?

All immediate neighbors of sender and receiver

How long to sleep?

The duration field in each packet informs other nodes the sleep interval

Use NAV to schedule the sleep timer

E C A B D FE C A B D FE C A B D FE C A B D F

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Massage passing

Problem: Sensor net in-network processing requires entire message

Solution: Don’t interleave different messages

Sending a long message?

Fragmented & sent in burst

RTS/CTS/ACK –duration fields

Fragment-level error recovery — ACK

Extend Tx time and re-transmit immediately

Other nodes sleep for whole message time

RTSSender

Receiver CTS

Other Sensors

FRAG1

ACK

NAV (based on RTS)

NAV (based on CTS)

defer access

FRAG-N

ACK

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Massage passing

Difference between 802.11 & S-MAC

Medium is reserved upfront for the whole transmission in S-MAC

Fairness Energy & Msg-level latency

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Implementation on Testbed Nodes

Platform

Mica Motes (UC Berkeley)

TinyOS: event-driven

Configurable S-MAC options

Low duty cycle with adaptive listen

Low duty cycle without adaptive listen

Fully active mode (no periodic sleeping)

Layered model on Motes

MAC layer: S-MAC

Physical layer

Radio state control, Carrier sense

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Experimentation

Two-hop network at different traffic load

Avoiding overhearing

Effeciently transmitting

long messages

Periodic sleeping

Source 1

Source 2

Sink 1

Sink 2

<Mean energy consumption on radios in each source node>

0 2 4 6 8 10

200

400

600

800

1000

1200

1400

1600

1800Average energy consumption in the source nodes

Message inter-arrival period (second)

Ener

gy c

onsu

mpt

ion

(mJ)

802.11-like protocolwithout sleep

S-MAC without periodic sleep

S-MAC with periodic sleep

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Experimentation

Ten-hop linear network at different traffic load

<Aggregate energy consumption on radios in the entire ten-hop network using three S-MAC modes>

0 2 4 6 8 100

5

10

15

20

25

30

Message inter-arrival period (S)

En

erg

y co

nsu

mp

tion

(J)

10% duty cycle without adaptive listen

No sleep cycles

10% duty cycle with adaptive listen

Energy consumption at different traffic load

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Experimentation

Adaptive listen significantly reduces latency causes by periodic sleeping

0 2 4 6 8 100

2

4

6

8

10

12

<Mean latency under highest traffic load>

Number of hops

10% duty cycle withoutadaptive listen

10% duty cycle with adaptive listen

No sleep cycles

0 2 4 6 8 100

2

4

6

8

10

12

<Mean latency under lowest traffic load>

Number of hops

Ave

rag

e m

essa

ge la

tenc

y (S

)

10% duty cycle withoutadaptive listen

10% duty cycle withadaptive listen

No sleep cycles Ave

rag

e m

essa

ge la

tenc

y (S

)

2020/22/22S-MAC

Experimentation

Adaptive listen significantly increases throughput

Using less time to pass the same amount of data

0 2 4 6 8 100

20

40

60

80

100

120

140

160

180

200

220

<Effective data throughput under highest traffic load>Number of hops

Eff

ectiv

e da

ta t

hrou

ghp

ut (

Byt

e/S

)

No sleep cycles

10% duty cycle with adaptive listen

10% duty cycle without adaptive listen

2121/22/22S-MAC

Experimentation

Periodic sleeping provides excellent performance at light traffic load

With adaptive listening, S-MAC achieves about the same performance as no-sleep mode at heavy load

0 2 4 6 8 100

0.5

1

1.5

2

2.5

3

Message inter-arrival period (S)

Ene

rgy-

time

prod

uct

per

byte

(J*

S/b

yte)

<Energy-time cost on passing 1-byte data from source to sink>

No sleep cycles

10% duty cycle withoutadaptive listen

10% duty cycle with adaptive listen

2222/22/22S-MAC

Conclusion

S-MAC

Medium access control protocol for wireless sensor networks

Main goal :Energy efficiency

Components

Periodic sleeping Low-duty-cycle operation

+ Overhearing avoidance

+ Message passing

Periodic sleeping increase latency and reduces throughput

+ Adaptive listening

2323/22/22S-MAC