Fasika Assegei

Segal’s Law

A man with a watch knows what time it is.

A man with two watches is never sure.

http://www.chess.nlFasika Assegei

A Decentralized Frame Synchronization of a TDMA-based

Wireless Sensor Network

Fasika Assegei

Advisors: Frits van der Wateren – Chess B.V. dr.ir. Peter Smulders – TU/e

Fasika Assegei

This research is ….

DevLab project

a project to build a functional WSN for research on protocols, power management, programming models, and security.

What is MyriaNed ?

• Conducted at Chess B.V. in Haarlem , the Netherlands

• Part of MyriaNed project

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Outline of presentation

Synchronization in Wireless Sensor Networks

What is wrong with Median algorithm ?

Proposed algorithms

Energy and its conservation

Simulation Results

Conclusion and future work

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Why synchronization?

TDMA Slots

Data Integration

NTP

Having the same notion of time

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Isn’t this a solved problem by now ??? NTP, MACs with sync built in (802.11), time broadcasts (GPS,

WWVB), high-stability oscillators (Rubidium, Cesium)

New problems arise in Wireless Sensor Networks Important assumptions no longer hold

(fewer resources -- such as energy, good connectivity, infrastructure, size, and cost -- are available)

Sensor apps have stronger requirements (…but we have to do better than the Internet anyway)

What now ?

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Wireless Sensor Networks: Why different ?

Energy limitation Each node has a limited battery life

Dynamic nature of the network, as well as inaccessibility Mobility and interference

Diverse applications Relative and absolute reference

Cost of node Cheap node

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Synchronization Methods- Previously

Centralized synchronization Central reference time Global or Relative

Receiver-Receiver RBS ( Receiver Broadcast Synchronization)

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Synchronization Methods- Previously

Decentralized synchronization Estimation of the other’s clock time

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What is NEW here ?

Decentralized synchronization Relative time references

No time stamping Low message overhead and no sync message

Primarily using estimation (biased and unbiased)

Energy efficient Able to use as low energy as possible

Unnecessary synchronization wastes energy, and insufficient synchronization leads to poor performance.

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Definitions

Phase error Time difference between the clocks

Frequency error The difference in the rates of the

clocks

Clock cycle (clk) The time between adjacent pulses of

the oscillator

Wake-up time The time that the node starts to listen

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Sources of Error

Oscillator characteristics: Accuracy: Difference between ideal frequency and

actual frequency of the oscillator. Stability: Tendency of the oscillator to stay at the same

frequency over time.

Network and System Parameters Receive and Transmit Delay: Time duration between

message generation and network injection. Propagation Delay: Time to travel from sender to

receiver. Access Delay: Time to access the channel.

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Clock drift

1)(

1dt

tdC

The nodes clock time is thus bounded as :

where ρ is the maximum clock drift.

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Synchronization algorithms

Synchronization Parameter Space:(max error, lifetime, scope, convergence,

stability)

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Synchronization frequency

T

Tt syncdiff 2

xguardslot Ttt

1T

Tsync

The period in which the network can stay synchronized without the application of the synchronization algorithm.

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)()()( nj

ni

nij ttt

io

n

ni

ni tTt )()(

)()()1( ni

ni

ni

ni Ttt

)( )()( nij

ni tf

Mathematical Model

What is f ???

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Median as a method for synchronization

Very simple method.

Calculates the Median of the phase errors

Adjusts the offset in the next wakeup time of the node.

With the simplicity, comes the price……

Not stable in a highly dynamic networks.

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What is wrong with Median ?

Got message from Node 3Calculating

offset

Got message from Node 10

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Node 9 drifted from

its neighbors

A WSN scenario for Median contd.

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)()()1( ni

ni

ni

ni Ttt

N

j

nijij

ni

ni

ni twTtt

0

)()()()1(

N

j

njij

ni

ni twTt

0

)()()1(

Weighted measurements

ijw,1 ij

,ij

Weight selection : two steps in determining the weight factor

Offset calculation:

5.0)( ijmean

5.0)( ijmean .1ijwwhere

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Weighted Measurements Contd.

)()1( nij

nij ww

ijtbij ae

The larger the phase error is, the lower the value it is assigned.

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)log(),( 21 ii xxf

),)...(,(),,(),,( 332211 nn yxyxyxyx

n

iirS

1

2

),( iii xfyr

jkj

kj 1

j

iij

rJ

yJJJ Tj

T )(

Non Linear Least Squares Contd.

Set of data points :

Squares of error :

where:

Iteration of parameters:

where:

Non Linear Least Squares - Curve:

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Non Linear Least Squares approach

Upper bound is the synchronization error to be permitted….

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Discrete time kalman filter

Kalman filter estimates a process by using a form of feedback

control the filter estimates the process state at some time

then obtains feedback in the form of (noisy) measurements.

Predict equations

Update equations

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1

0lim

HKk

Rk0lim

0

kP

Kk

Kalman Filter Contd.

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Reducing the duty cycle ……..

xslot TxT 2

T

NTD slot

T

TxND x )2(

T

TxND x

n

))(2(

T

ND

)2( A decrease in the duty cycle:

For a performance improvement:

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Simulation setup

Mixim Octave

Input

Output

Discrete Event Simulation

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Abstraction

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Simulation results

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Simulation results Contd.

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Simulation results Contd.

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Simulation results Contd.

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Simulation results

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Simulation results Contd.

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Simulation results Contd.

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Simulation results Contd.

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Active Idle Sleep

CPU 3.5mA 0.01mA 5μA

Radio 11.3mA(TX) 12.3mA 5μA

Radio 12.3mA(RX) 12.3mA 5μA

Tradeoff? Directly comparing computation/communication energy cost not

possible but: put them into perspective! Energy ratio of “listening for 1 clk ” vs. “computing one instruction

per cycle”: In the range of 5 to 10 times Try to compute instead of communicate whenever possible

Communication is more expensive than computing.

Computation vs. communication energy cost

900 mAh

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Energy Consumption

Battery life of a MyriaNode vs Guard time

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Conclusion

A decentralized synchronization for a TDMA-based WSN algorithm is proposed using KF, WM and NLLS.

WM and NLLS have a better tolerance to the dynamic nature of Wireless Sensor Network.

KF performs the best in all synchronization space, both in static as well as dynamic environments.

Median is still the choice in static environment, when considered in all aspects (energy and performance).

Reducing the communication cost and increasing cost of computing is worthy bait.

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Future work

Software power minimization techniques to reduce the power consumption of the algorithms.

Implementation of the algorithms on the MyriaNode and further investigation for perfection.

Additional tools for frequency error minimization, using the available resources.

More advanced estimation techniques and low power implementation.

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Thank you!

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What causes the clock to drift ?

The frequency of the clock is given as :

a is the aging factor fe is the environmental factor (temperature..) fr is the noise instability

fo is the nominal frequency

)()()()( 0 tftfttaftf reo

where :

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Lower bound of synchronization

where n is the number of nodes in the network. This means that synchronization is not only a local

property, in the sense that the clock skew between two nodes depends not only on the distance between the nodes

but also on the size of the network.

))1log()1(8

log(

)1log(

)1(8

n

ndL

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What is synchronization ?

Having the same time of reference and count the same time [either global (UTC) or local (relative)]

Operate a system in unison

Same notion of time

We are synchronized!!!!

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Wireless Sensor Networks

Wireless network of low cost sensors. Nodes communicate by broadcasting. Multihop communication needed in order to reach

far-away destinations.