Investigating Mac Power Consumption in Wireless Sensor Network

Javier BonnySupervised by Jun Luo

IC-29 – LCA – 02/09/20052

Contents

Introduction Source of energy waste Existing Solution

S-MAC, B-MAC Our proposition Conclusion

IC-29 – LCA – 02/09/20053

Energy in WSN

Battery is the most critical resource It is important to reduce the waste of

energy to improve network lifetime How to improve energy efficiency

Routing Mobile Base Station Energy efficient MAC protocol …

IC-29 – LCA – 02/09/20054

MAC Attributes for WSN Energy efficiency Collision avoidance

Basic task of a MAC protocol Scalability and adaptability

Network size, node density and topology change Secondary concerns

Latency Channel utilization Throughput Fairness

IC-29 – LCA – 02/09/20055

Source of energy waste (MAC) Idle listening

Node listens to an idle channel

Overhearing Node listens for a message sent to another

node

Collision Two nodes emit at the same time and messages

must be retransmitted

Control packet overhead Required frame header and signaling to implement

the MAC

IC-29 – LCA – 02/09/20056

S-MAC: Coordinated Adaptative Sleeping

Ye, Heidemann (USC), Estrin (UCLA) 2002

IC-29 – LCA – 02/09/20057

S-MAC

Reducing the waste of energy: Idle listening: by Periodic Sleep Overhearing: by switching the radio off

when the transmission is not meant for that node

Collision: by using RTS and CTS

IC-29 – LCA – 02/09/20058

Idle Listening avoidance Periodic listen and sleep

Turn off radio when sleeping Duty cycle is fixed (application

dependant)

Listen

Listen

Listen

Sleep

Sleep

Sleep

Sleep

Listen

IC-29 – LCA – 02/09/20059

Choosing and Maintaining Schedules

Each node maintains a schedule table that stores schedules of all its known neighbors.

Periodic timer synchronization among neighbors are needed to prevent the clock drift.

IC-29 – LCA – 02/09/200510

Overhearing Avoidance Idea: Sleep when neighbors talk Who should sleep?

E C A B D F

All immediate neighbors of sender and receiver How long to sleep?

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

IC-29 – LCA – 02/09/200511

SMAC: Pros and Cons Pros

Significant low power operation Schedules sleep and transmit times to

enable low-power data transfer with reasonable-latency.

Cons Implementation is quite complex for WSN Significant state maintenance (schedules) Neighbors synchronization Sleep and listen period are predefined and

constant (not efficient for variable traffic load)

IC-29 – LCA – 02/09/200512

B-MAC: Versatile Low Power MAC

Polastre, Hill, Culler (Berkeley), 2004

IC-29 – LCA – 02/09/200513

B-MAC

Unscheduled sleep Reduces control overhead But sender incurs greater overhead to

wakeup unsynchronized receiver from sleep (long preamble)

Unscheduled wakeup Keep wakeup intervals very short CSMA/CA or some other app-specific

scheme can be used

IC-29 – LCA – 02/09/200514

B-MAC

Preamble Data Tx

Data RxReceiver

Sender

Sleep Sleep

Preamble sampling

Sender sends a long preamble to overlap with the receivers “carrier sense” duration.

Data transmission can use RTS/CTS or some other strategy.

IC-29 – LCA – 02/09/200515

B-MAC

Preamble Data Tx

Data RxReceiver

Sender

Sleep Sleep

Duty cycle and preamble length are tunable

Preamble length ≥ Check interval Long sleeping time trades transmission latency for low power

consumption (suitable for sparse transmission) A long preamble increases the power consumption of all nodes

in the sender’s transmission coverage due to overhearing Sender and Receiver should be tuned together (Loose Sync)

Check Interval

IC-29 – LCA – 02/09/200516

S-MAC vs B-MAC

B-MAC is the MAC protocol chosen for TinyOS!

S-MAC B-MAC

Solve Idle Listening yes yes

Solve Overhearing yes no

Synchronisation needed

Yes less

Simplicity/scalability no yes

Overall performance good better

IC-29 – LCA – 02/09/200517

Our proposition: RTS Preambling

Take the best of both world Based on B-Mac + add Overhearing

avoidance Idea

Send useful information (RTS) in the preamble instead of a constant

IC-29 – LCA – 02/09/200518

RTS Preambling

IC-29 – LCA – 02/09/200519

RTS Preambling

Comments Basic rule: if you hear something while

listening, listen until the end Listening period (duty cycle) > DIFS What to do during SIFS period?

Turn off radio or continue listening? It depends on the Radio performance!

IC-29 – LCA – 02/09/200520

RTS Preambling

IC-29 – LCA – 02/09/200521

RTS Preambling

Advantage Avoid overhearing Preamble (RTSs, CTS, DIFS and SIFS)

no longer than B-MAC preamble

IC-29 – LCA – 02/09/200522

RTS Preambling

Our proposal is good for unicast We must use an alternative for

broadcast, multicast Repeat message in the preamble (like

WiseMAC)

IC-29 – LCA – 02/09/200523

Conclusion

We think RTS Preambling could improve B-MAC performance…

To do Figure out how to set the NAV Finalize alternative for broadcast Finish implementation Simulation + comparison…

IC-29 – LCA – 02/09/200524

Conclusion That’s all Folks!

Any question?