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Embedded Systems for Wireless Sensor Network

Rabi Mahapatra

Mahapatra-Texas A&M-Spring'07 2

Background

• Advancement of integration between “tiny embedded processors, wireless interfaces, and “micro-sensors” based on MEMS led to emergence of wireless sensor network.

• Characterized by their ability to monitor the physical environment through ad-hoc deployment of numerous tiny, intelligent, wirelessly networked sensor nodes.


• Large number of heterogeneous sensor devices– Ad Hoc Network

• complex sensor nodes with– communication, processing, storage capabilities

What is Wireless Sensor Networks


Emerging applications

• Indoor Settings: condition based maintenance of equipment in factory

• Outdoor environment:– Monitor natural habitats– Remote ecosystems– Forest fires– Disaster sites– Defense armaments– Spy microsats


Challenges of WSN• Requirements: small size, large number, tetherless and low

cost. Hence constrained by– Energy, computation and communication

• Small form factors => prohibits large long lasting batteries• Cost & energy => low power processors, small radios with

minimum bandwidth & small transmission ranges.• Ad-hoc deployment => no maintenance and battery

replacement• Increase NW lifetime => No raw data to gateway for

computation


Topics to be discussed

• Simulation tools on WSN• A simulation architecture overview• Sensor node model & framework of SN• Battery model• Case studies• Bonus points


Existing SimulatorsJavaSim:

• Pros – Very modular– Easy to use

• Cons– Geared for wired inter-networks– No wireless support, not efficient due to overheadSSFNet: a parallel simulator for wirelessGlomoSim:

• Specific for mobile wireless networks. • Built as a set of libraries. The libraries are built in Parsec( a C-based

discrete event simulation language).• Layered architecture with easy plug-in capability.

SSFNet and Glomosim are not better than NS-2 in terms of design and extensibility.


Existing Simulators • NS-2: De facto standard for network simulations

– Does support wireless simulations– A primitive energy model is present.– Object oriented design and Lots of documentation.– Uses Tcl to specify the Components, and Otcl to glue them

together.Cons:– Difficult to use and learn – Interdependency among modules pose difficult to implement new

protocols.– Originally built for wired networks, later extended for wireless.– Supposedly, does not work well for large topologies.


More Sensor Network Simulator• WSNS

– Based on Low Energy Adaptive Clustering Hierarchy (LEACH) protocol developed by Dr. Wendi Heinzelman

– Has included Network Preserving Protocol (NPP) for better performance along with LEACH

– Not completed for robustness


Sensor Network SimulatorSENSE (www.cs.rpi.edu/~cheng3/sense/)

• Component Features: (2004)– Battery Model: Linear Battery, Discharge Rate

Dependent and/or Relaxation Battery – Application Layer : Random Neighbor; Constant Bit

Rate – Network Layer: Simple Flooding; a simplified verion of

ADOV without route repairing, a simplified version of DSR without route repairing

– MAC Layer: NullMAC; IEEE 802.11 – Physical Layer: Duplex Transceiver; Wireless Channel – Simulation Engine: CostSimEng (sequential)


Status of simulators

• Other simulator: OpNET• All these tools are not equipped to capture all the aspects

of interests in sensor networks.


Simulator: SensorSim from UCLA

• Extension to NS - 2.• Provides battery models, radio propagation models and

sensor channel models.• Provides a lightweight protocol stack.• Has support for hybrid simulation.• Must be integrated with NS - 2.


SensorSim Architecture

monitor and controlhybrid network

(local or remote)

Simulation Machine

Gateway Machine

ns

modified event scheduler

VR

V

VV

GUI appapp

R

real sensor apps onvirtual sensor nodes

gateway

socketcomm

serialcomm

HS InterfaceEthernet RS232

Proxies for realsensor nodes

GUI Interface

app


SensorSim Architecture Overview

• Sensor NW has three types of nodes:– Sensor nodes: monitor immediate environment, with many

transducers– Target nodes: generates various stimuli for sensor nodes– User nodes: client and administration of sensor network

• Separate channels: – Sensor channels: communication among sensor nodes and target– Network channels: to user node or gateways and onward

transmission to other network.– Concurrent transmission possible– Easier to model complex behavior of sensor nodes, reaction to

multiple sensor signals.


Sensor Network Model architecture

Sensor channel

Wireless channel

sensorsensor sensor

Target

user


SensorSim Model

• Sensor node => one wireless NW protocol stack, one or more sensor stack corresponds to as many transducers– Sensor stack detects stimuli, process it and forward them to

application layer, which in turn process and send them to user node through wireless channel

– A power model corresponding to energy producing-consuming hardware components is also provided. These component can stay at different power saving and performance states.

– The algorithm in both the stacks control the mode of power states of hardware components. Also, performance of the algorithm depends on the mode.


Sensor Node Model in SensorSim

Node Function Model

Network Layer

Sensor Node

Applications

Power Model(Energy Consumers and Providers)

Battery Model

Radio Model

CPU Model

Sensor #1 Model

Sensor #2 Model

MAC Layer

Physical Layer

Sensor Layer

Wireless Channel Sensor Channel 1

NetworkProtocol Stack

SensorProtocol Stack

Middleware

Physical Layer

State Change

StatusCheck

Sensor Stack 1

Sensor Layer

Physical Layer

Sensor Channel 2


NetworkProtocol StackNetwork Layer

MAC Layer

Physical Layer

Wireless Channel

User Application

User Node

Sensor Stack

Sensor Layer

Physical Layer

Target Application

Sensor channel

Target Node


Framework of Sensor Network Simulation

• Node Placement & traffic generation– Performance of WSN is affected when topology of node

distribution changes– Application requires a typical distribution (uniform for forest fire,

Gaussian for perimeter defense)– Three types of traffics: user-to-sensor (command & queries),

sensor-to-user (sensor reporting to user) and sensor-to-sensor (collaborative signal processing before reporting)


Sensor Stack & Channel

• Sensor stack is a signal sink that is responsible for triggering the application layer every time a sensing event occurs

• Simple sensing scheme to elaborate signal processing can be implemented on sensor stack

• Sensor stack acts as a signal source in Target Node and contains signatures unique to the model

• Sensor channel model the medium of signal transmission (e.g. ground to carry seismic events).

• A good simulation tool should model varieties of mediums and type of sensors ( acoustic, infra red, ultrasonic)


Battery Model

• Goal: increase the battery life time• Need to study how different aspects of real battery

behavior can affect the energy efficiency of applications• T = C/I, C is capacity in Ah. I is discharge current• Linear Model:

– Linear storage of current. Assumes the maximum capacity is unaffected by discharge rate.

– Allows user to see efficiency of user application by providing how much capacity is consumed. The remaining capacity after td can be expressed as C = C’ - � I(t)dt integral taken over period t = 0 to td

– It assumes that the I(t) will stay same during the period, if operation mode does not change (radio switching from Tx toRx)

– Remaining capacity is computed when discharge rate is changed


Battery Model

• Discharge rate dependent model:– Considers the effect of battery discharge rate on maximum

capacity– Battery capacity efficiency factor K is introduced. K = Ceff /Cmax

– Capacity C = K.C’ – I . Td

– K varies with current I and is close to 1 when discharge rate is low and approaches 0 when discharge rate is high.

A Popular Battery Model: Dual Foil from UC Berkeley.


Battery Model

• Relaxation Model:– Real-life battery exhibit a phenomenon called “relaxation”.(Fuller

94, Linden 95, Chiasserini 99)– When battery discharge rate is high, diffusion rate of active

ingredients through the electrode & electrolyte falls behind. If high discharge is sustained, the battery reaches its end even if the active materials are still available.

– However, if discharge current is either cut-off or reduced during the discharge, active materials catches up with depletion of thematerials. It gives battery to recover the capacity lost at highdischarge rate.

– An analytical model has been used for SensorSim


Case Studies

• Low rate/low power vs. high rate/ high power (Fig. 7 in the reference)

• Monitoring a moving vehicle in a sensor field– Study the effect of traffic on the sensing and communication traffic

and evaluate the power management.


Power Management Model

Transmit Receive

Off

Idle

BZR event

BZR event

BZR event

receive done

transmit

transmitdone

Without Power Management

Transmit Receive

Sleep

Off

Idletransmit

timeout(3 sec)

BZR eventBZR event

transmit

transmitdone

BZR event

receivedone

receivetimeout

With Power Management


Some Important Studies

• Utility-based decision-making in WSN.• Upper bound on network life-time• Impact of mobility on capacity and life-time• Coverage and Density

– Criticality threshold, scalability, integration etc.

• Security• Ease of Deployment• Synchronization


Conclusion

We have looked at some of the issues with SN and discussed the Sensor Network model developed at UCLA.

• Assignments:– Read the reference papers and look for more titles– Prepare a bibliography on each topic mentioned in the previous

page. – Consider one topic as assigned to you in the class and read apex

papers on that topic. – Summarize and comment on the contributions and shortcomings. – Due Tuesday morning by e-mail with file name as (your name-

sensor.doc)


Reference

• Sung Park, A savvides & M B Srivastava, “Simulating Networks of Wireless Sensors”, Proceedings of the 2001 Winter Simulation Conference.

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