On-demand Time Synchronization for Wireless Sensor Networks

[Plan B project]

Advised by: Prof. Tian He

Presented by:

Abhishek Rawat

Introduction

Time synchronization middleware service provides time-reference for nodes

Time reference Global Peer-node

Introduction: applications

Example Applications Sniper detection Seismic activity detection Structural monitoring Object tracking Habitat Monitoring

Introduction: services

Provide time-reference for some sensor node functions TDMA scheduling LPL communication Distributed processing Aggregation techniques

Time Synchronization: approaches Proactive techniques

periodically synchronize the nodes periodicity based on precision requirement

Reactive techniques

Actuated by the event

Periodicity based on event frequency

Existing Techniques

Several techniques application in post and pre-event scenarios Varying accuracy and communication cost

Notable techniques RB TPSN FTSP ETA

Existing Techniques [continued] RBS

Receiver- receiver Broadcast based

TPSN Sender-sender MAC time-stamping

Existing Techniques [continued] FTSP

Broadcast-based MAC time-stamping and skew calculation High accuracy Reactive technique Hardware calibration

Existing Techniques

ETA State-of-art Elapsed Time-of-Arrival primitive Reduced communication cost Elapsed time in data-item-no separate time-

synchronization messaging Skew calculation is a problem

Problem Statement

How to minimize the cost of time-synchronization? Optimizing communication requirement with

desired accuracy

Motivation

Reducing communication cost post-facto technique caching: time-reference present in network

Assumptions

Spatiotemporal events Adjacent nodes will detect the event Time reference would be available in mearby

nodes Unicast messaging will reduce communication

cost

Sources of Error

Time-reference communication Sources of delay in timestamp delivery

Send/Receive Time Access Time Transmission and Reception Time Propagation Time Interrupt Handling Encoding/Decoding Time Byte Alignment Hardware Calibration/Clock-Skew

Proposed Approach

On-demand Time Synchronization Protocol

Post-facto approach

Primitive: seeker provider determination post-event

Time-reference exchange using 3-way handshake

Clock offset and Clock skew rate calculation

Approach Details

Seeker Determination

Data > threshold value

Previous time reference : Expired !!!

Approach Details

Provider determination

Approach Details: time reference exchange Time-Synchronization Messaging

3-way handshake

Tx2Tx1initMsg

t1

provider-node

t0

t2 t5

t4t3

seeker-node

Fig2. Messages exchanged during time-synchronization between two nodes

Approach Details [continued]

Skew Calculation

Approach Details [continued]

Offset Calculation

Implementation: platform

Platform

Simulator

Implementation: system design Modules:

TimeLibService TimeSyncModule TimeSyncCtrl TimeSyncCommModule Routing Module SenseDB

Implementation: system design [continued] Diagram

Analysis: applications

Accuracy requirement

Habitat monitoring : order of seconds Seismic activity detection : order of 10 milliseconds Sniper detection : order of 1 millisecond Structural monitoring : order of 10 milliseconds

Results

Data Analysis for Real Time Experiment Seismic activity detection

Data Analysis from TOSSIM based simulations Error- analysis Communication cost with event frequency Communication cost with event density Delivery success rate

Results: Data Analysis for Real Time Experiment

Analysis of Data from Seismic Activity Detection 21 days – 230 events 60 seconds per activity Sampling frequency of 100 Hz.

Results: real-time data analysis Communication complexity for various skew errors-rates over the

experiment-span

0

100

200

300

400

500

600

1 2 3 4 5 6 7 8 9 10

Skew Error in order of 10 ppm

Mes

sag

es E

xch

ang

ed in

10

00's

FTSP OTSP OTSP w ith caching

Results: tossim simulation Average error per hop

Error per hop (for first hop): 4.52 milliseconds Error per subsequent hop : 1.24 milliseconds

Error in Time Synchronization

0

2

4

6

8

10

1 2 3 4

Hop distance from base station

Err

or

in m

illiseco

nd

s

Error in Time Synchronization

Results: performance with frequency

Results: performance with event density

Results: miscellaneous observations Congestion Delivery probability

Drawbacks

Discussion

Further Work