Wireless Sensor Networks (WSN)

Introduction

M. Schölzel

Difference to existing wireless networks

Infrastructure-based networks e.g., GSM, UMTS, …

Base stations connected to a wired backbone network

Mobile entities communicate wirelessly to these base stations

Traffic between different mobile entities is relayed by base stations and wired backbone

Infrastructure-free networks Try to construct a network without

infrastructure, using networking abilities of the participants

This is an ad hoc network – a network constructed “for a special purpose”

Simplest example: Laptops in a conference room –a single-hop ad hoc network

IP backbone

Server

Router

Gateways

A Wireless Sensor Network

Sensornet

Internet, LAN, GSM-Network, …

Network is embedded in environment

Nodes in the network are equipped with sensing and actuation to measure/influence environment

Nodes process information and communicate it wirelessly

wireless communication

wired communication

Roles of Participants in WSN

sources of data: Measure data, report them “somewhere” Typically equip with different kinds of actual sensors

sinks of data: Interested in receiving data from WSN May be part of the WSN or external entity, PDA, gateway, …

actuators: Control some device based on data, usually also a sink

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WSN Application Examples

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Goal: Collecting data

Environmental Monitoring Each node measures temperature Derive a “temperature map”

Intelligent buildings, bridges, or machines Measuring vibrations Control of leakages in chemical plants Monitor mechanical stress (e.g. during

earthquakes) Predictive maintenance

Example: Parking Space Management

Each parking space is equipped with a sensor node AMR sensor senses disturbances on the magnetic field of the earth

(resistance changes, if a car is above the sensor) Hardware:

8-bit microcontroller RF transceiver: CC1120

with 4kbps data rate 3.6V Li-ion battery

Rather simple network topology: Single hop from sensor node to sink.

Other WSN Application Domains

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Precision agriculture Bring out fertilizer/pesticides/irrigation only where needed

Medicine and health care Wearable sensor nodes monitoring movement of patients

Logistics Equip goods (parcels, containers) with a sensor node Track their whereabouts – total asset management E.g. Bosch already provides commercial solutions

Telematics Provide better traffic control by obtaining finer-grained information about traffic conditions Intelligent roadside Cars as the sensor nodes

Examples Car2Car and Car2X communication

Communication Standard WLAN-p already exists

Cars have on-bord-units, infrastructure has road-side-units

Cohda-Box commercially available 32-bit ARM-like processor

runs a Linux system

up to 27 mbps

Cars and infrastructure can send various kinds of messages CAMs: position, direction, speed, etc.

DENMs: Detected hazards

SPATs: State of the traffic light

…

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Cohda OBU

WSN Application Examples

Goal: Building control loops (sensing -> computing -> acting)

Communication in industrial environments Communication between mobile machines

Requirement real time demands

Wireless heart standard

much higher reliability of communication required probability of packet error rate ~10-9

typical bit error rate in wireless channels: 10-3 to 10-4

wireless ad-hoc networks: problems

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Without a central infrastructure, things become much more difficult

Problems are due to

Lack of central entity for organization available

Limited range of wireless communication

Battery-operated entities

Mobility of participants

Problem: Lack of Central Entity

Without a central entity (like a base station), participants must organize themselves into a network (self-organization)

Pertains to (among others):

Medium access control – no base station can assign transmission resources, must be decided in a distributed fashion

Finding a route from one participant to another

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Problem: Limited Range

For many scenarios, communication with peers outside immediate communication range is required

Direct communication limited because of distance, obstacles, …

Solution: multi-hop network

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?

Problem: Battery-Operated Entities

Often (not always!), participants in an ad hoc network draw energy from batteries

Desirable: long run time for Individual devices

Network as a whole

Required: Energy-efficient networking protocols E.g., use multi-hop routes with low energy consumption (energy/bit)

E.g., take available battery capacity of devices into account

How to resolve conflicts between different optimizations?

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Problem: Mobility

In many (not all!) ad hoc network applications, participants move around (-> mobile ad hoc networks (MANET)) Car2Car-communication

Moving robots in industrial environments

In cellular network: simply hand over to another base station

In MANET Mobility changes neighborhood relationship

Routes needs adaptation

Complicated by scale• Large number of such nodes difficult to support

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Requirements for WSN(user perspective)

Quality of Service (QoS) Detect events and deliver data reliable

events may be detected by multiple nodes reliability of a single node doesn’t matter, system reliability is important!

Deliver data in within a specified delay

Lifetime The network should fulfill its task as long as possible – definition depends

on application Lifetime of individual nodes relatively unimportant

Fault tolerance Be robust against faults

node failures (often permanent): running out of energy, physical destruction link failures (often temporarily): weather, moving obstacles, …

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Requirements for WSN (administrator perspective)

Deployment Simple deployment even for thousands of nodes (self-organization)

Scalability Support large number of nodes (several thousands)

in existing practical applications < 100

Maintainability WSN has to adapt to changes, self-monitoring, adapt operation

Incorporate possible additional resources, e.g., newly deployed nodes

Programmability Re-programming of nodes in the field might be necessary, improve flexibility, fixing

bugs

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Mechanisms to Overcome Problems and to Meet Requirements

Multi-hop wireless communication to overcome limited communication range requires self-organization mechanisms

Energy-efficient operation For communication, computation, sensing, actuating Very typical: computation far less energy hungry than communication

Collaboration & in-network processing• Nodes in the network collaborate towards a joint goal• Pre-processing data in network (as opposed to at the edge) can greatly improve efficiency

Locality • Do things locally (on node or among nearby neighbors) as far as possible

Data centric networking• Focusing network design on data, not on node identifies (id-centric networking)• To improve efficiency

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Mechanisms to meet requirements

Auto-configuration/Self-organization

Manual configuration just not an option

Right after deployment

But also periodically during operation to overcome link and node failures

mobility problems

Exploit tradeoffs during the design phase

E.g., between invested energy and accuracy

Design-space-exploration

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Design Space

Due to the many options for tackling problems, WSNs offer a very large design space

Trade-offs between many design space parameters must be found in order to meet the requirements

Design space parameters:

Form Factor

Heterogeneity

Communication Modality

Sensor Coverage

Radio Coverage

Network Topology

Connectivity

Design Space

Form factor of the moteclasses: brick, matchbox, grain, dust Depends on application (microscopic to shoebox) Costs may range from cents to hundreds of euro's Impacts

life time (e.g. size of battery), processing resources, costs

Heterogeneity of the WSNclasses: homogeneous/heterogeneous First approach: identical nodes only In practice: a variety of nodes can be very useful

e.g. cluster heads with more resources special capabilities only required for some nodes (e.g. GPS) gateways to external networks (GSM, satellite, Internet)

Impacts Complexity of software

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Design Space

Communication Modalityclasses: radio (169MHZ, 434MHz, 868MHz, …), visual light communication (VLC), ultrasound, … How do nodes communicate ? most common: radio waves, usually sub-gigahertz bands light beams or laser: smaller, more energy efficient (cf. Smart Dust) Impacts

data rate, life time, communication range

Connectivityclasses: connected / intermittent / sporadic Nodes always connected or only sometimes (regularly or sporadic)? Impacts

Protocols, data gathering, life time

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Design Space

Sensor-Coverageclasses: sparse / dense / redundant sensors could cover only part of area of interest, or all, or the same area is

covered multiple times Impacts:

observational accuracy, number of nodes, costs

Radio-Coverageclasses: from sparse to dense / static /dynamically How many other nodes will be in the communication range? May be adapted at runtime Impacts:

reliability, energy consumption due to collisions, transmission power, etc., number of nodes

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Design Space

Network Topologyclasses: infrastructure or ad-hoc (single-hop / star / networked stars / tree / graph)

infrastructure-based: motes communicate via base stations only often costly

ad-hoc: direct communication between nodes no expensive infrastructure

diameter is the max number of hops between any two nodes single hop (d=1), infrastructure based (d=2), multi-hop (d big for ad-hoc networks)

Impacts: Communication delay, protocol complexity

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Development Flow and Design Space Exploration

Models used for an early evaluation of possible design

parameters

Simulations: Evaluate the prediction of the model in the full

dynamic of the network Test and debug to find software bugs Not all aspects of the hardware are accurately

reflected

Testbed uses real hardware Test the software on real hardware Test and Debug in a distributed system difficult Systematic test of fault-tolerance techniques not easy

(often the code is instrumented) real sensors and actors maybe not available

virtualization

Deployment test in a real setup is required Not always possible Test-bed infrastructure not available anymore

• Over-the-air update• power measurement

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Implementation(+Software)

Simulation

Test/DebugTestbed

(+Hardware)

Start with requirements

Deployment(+Topology)

Models Evaluation

Test/Debug

Required Knowledge for Developing WSN Applications

Knowledge of Models and Simulators

Microprocessor architecture and programming Communication with peripherals

Usage of operating systems

Power saving techniques (node-level)

Architecture and Design of Protocols Power saving techniques (communication-level)

Fault Tolerance

• Forward- and backward-error correction