PW-MAC: A Predictive-Wakeup MAC

Protocol for Wireless Sensor Networks

Lei Tang, Yanjun Sun, Omer Gurewitz, and

David B. Johnson

Presentation at IEEE INFOCOM 2011, April 2011

PW-MAC objectives

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Minimize energy consumption both at senders

and at receivers while maintaining:

• High packet delivery ratio

• Low delivery latency

Related work: duty cycling

Duty-Cycling MAC protocols:

– Synchronous: e.g., S-MAC, DW-MAC.

– Asynchronous: e.g., B-MAC, PW-MAC, EM-MAC.3

Duty cycle: The percent of time a node is active.

Related work: synchronous protocols

Problems of synchronous protocols:

– Global time synchronization.

– Contention is packed to DATA period.

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SYNC DATA SLEEP

RTS RTS DATA

Cycle

Related work: asynchronous protocols

Asynchronous

– Nodes wake up asynchronously.

– No global time synchronization.

How does sender rendezvous with receiver?

– Sender-initiated or receiver-initiated.

5Picture from http://www.fleetcouriers.com/blog/

Related work: B-MAC (sender-initiated)

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S

Send Receive Node awake

Preamble Data

DataR

Time

Problem:

1. Sender has large duty cycle.

2. High channel contention.

wakeup interval

Related work: X-MAC (sender-initiated)

Preamble is replaced by repeating data packets.

Receiver sends an ACK so the sender can stop.

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S

RDATA

DATADATA DATA

A

A

wakeup interval

Problem:

1. Sender has large duty cycle.

2. High channel contention.

Time

Related work: WiseMAC (sender-initiated)

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S

shortened preamble P.

P. Data

DataR

Time

Problem:

1. Fixed wakeup interval can cause collisions.

2. Use a fixed clock drift ratio.

fixed wakeup interval fixed wakeup interval

Other problem of these protocols

No efficient retransmission mechanism large duty

cycle and high wireless contention.

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S1

R

packetpacket packet

S2 packetpacket packet

packet

packet

collidecollide collide collide

Time

S

RSend wake-up beacon

DATA

Wake up to wait for receiver

B

B DATA

Send DATA

Send ACK

A

A

Problem: sender still has large duty cycle

due to idle listening and overhearing.

Related work: RI-MAC (receiver-initiated)

Time

10

Overview of PW-MAC

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DATA DATASender S1

DATA DATAReceiver R1

High energy efficiency at both senders and receivers.

High packet delivery performance through reducing

collisions and efficient packet retransmission.

DATA DATASender S2

DATA DATAReceiver R2

Time

Predictive Wake-up mechanism of PW-MAC

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pseudorandom time

R1 wake-up

B B B

pseudorandom

time

R1 wake-up R1 wake-up

time

Next wakeup time = now+ pseudorandom(0.5 interval,

1.5 interval)

pseudorandom time

R2 wake-up

B B

R2 wake-up

timeR2:

R1:

Predictive Wake-up mechanism of PW-MAC

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Why pseudorandom wakeup?

• Enable sender to predict receiver reduce energy consumption

• Spread traffic to different times mitigate wireless contention

pseudorandom time

R wake-up

B B B

pseudorandom

time

R wake-up R wake-up

timeR:

Predictive Wake-up mechanism of PW-MAC

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Prediction state obtained by a node S to predict a

node R’s wakeups includes:

• Pseudorandom number generator parameters and

current seed of R.

• The time difference between S and R.

pseudorandom time

R wake-up

B B B

pseudorandom

time

R wake-up R wake-up

timeR:

PW-MAC packet transmission

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S

R

wake up at predicted time

pseudorandom time

DATAB A

B DATA A

B

Through prediction, sender and receiver wake

up at the same time high energy efficiency.

DATA is

generated

time

The problem of packet retransmission

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S2

R2

DATA

Existing work: stay awake and repeat the packets.

DATA

DATA DATA

S1

R1

DATA

DATA

DATA

Large duty cycle and high channel contention.

time

DATA

DATA

DATA

PW-MAC: prediction-based retransmission

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go to sleep

DATAB A

B DATA AS

R DATAB

B DATA

retransmit at the next

predicted receiver wake-up time

failure

Retransmit packets with high energy

efficiency and low wireless contention.

time

Outline

1. PW-MAC

1.1 Predictive Wake-up MAC (PW-MAC)

1.2 Prediction-based retransmission

1.3 On-demand prediction error control

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Prediction error on a pair of MICAz motes

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Small clock drift ratio

Prediction error on another pair of MICAz motes

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Much larger clock drift ratio

PW-MAC: on-demand prediction error control

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DATAB A

B DATA AS

R

Wakeup advance time

Prediction error: the difference between

predicted and actual wakeup time.

time

Use a wakeup advance time to compensate

prediction error.

PW-MAC: on-demand prediction error control

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DATAB A

B DATA AS

R

Detect prediction error

Request current

prediction state

Update

prediction state

Send prediction state

time

Ensure prediction error to be

within sender wakeup window

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PW-MAC effectively controls the prediction error

to be ≤ wake-up-advance time

Low cost: average 1 update per 1400 seconds

Advantages:

Effective

Low cost: average 1 update per 1800 seconds

PW-MAC: on-demand prediction error control

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DATAB A

B DATA AS

R

Detect prediction error

Request current

prediction state

Update

prediction state

Send prediction state

time

Hidden terminal experiment

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Two MICAz senders are hidden to each other

With WiseMAC, two senders’ repeated

retransmissions cause persistent collisions.

Experiment of wake-up schedule conflicts

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Two receivers have the same first wake-up time.

S1

S2

R1

R2

Pseudorandom wakeup scheduling of PW-MAC

avoids persistent wakeup collisions.

Multihop network performance

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A testbed of 15 MICAz motes.

Up to 3 multihop traffic flows.

Sender duty cycle

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PW-MAC has the smallest duty cycle

Packet delivery latency

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PW-MAC achieved lowest latency

Packet delivery ratio (PDR)

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Packet Delivery Ratio (PDR)

PW-MAC achieved 100% PDR

Conclusions

Predictive-wakeup mechanism:

– High energy efficiency at both senders and

receivers.

– Low channel contention.

Prediction-based retransmission mechanism.

On-demand prediction error control.

PW-MAC outperformed other tested single-channel

protocols on a testbed of MICAz motes.

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