Ivano Malavolta

Henry Muccini

A Survey on

the Specification of the Physical Environment

of Wireless Sensor Networks

Roadmap

Background

Contribution

Design of the study

Results

Takeaways

Conclusions

Wireless sensor networks (WSNs)

WSNs consist of spatially distributed sensors that cooperate to accomplish some tasks.

Sensors are:

–  small

–  battery-powered

–  with limited processing power

–  with limited memory

They can be easily deployed to monitor different environmental parameters such as temperature, movement, sound and pollution.

WSN applications

Sensors can be distributed on roads, vehicles, hospitals, buildings, people and enable different applications such as:

•  environmental monitoring

•  medical services

•  battlefield operations

•  crisis response

•  disaster relief

WSN physical environment (1)

What really sets WSNs apart from all the other kinds of distributed systems is:

•  limited processing capabilities of the nodes

•  contingent energy restrictions

•  strict dependence to physical phenomena

à The physical environment in which

WSN nodes are deployed strongly

affects the overall quality of the system

like refraction, reflection, and attenuation…

WSN physical environment (2)

Information from the physical environment, like:

•  exact position of the nodes

•  information about the surrounding obstacles and their material

–  e.g., walls, furniture, windows, or small objects in general

surely helps making an accurate estimate of the physical phenomena affecting the WSN

Such data could allow a more precise measurement of the network in terms of: bit error rate, packets loss, energy consumption, etc. à enables the prediction of how the WSN will globally behave when nodes are deployed in different ways

Examples VeriSensor [1]

GLONEMO [2]

[3]

Roadmap

Background

Contribution

Design of the study

Results

Takeaways

Conclusions

Contribution

To investigate on how practitioners specify the physical environment of a WSN Survey by interviewing WSN practitioners with a special focus on their practical needs and activities •  Many practitioners describe the physical environment

via GIS software or informally •  practitioners not specifying the physical environment

do not see a clear return on investment on doing it or perceive existing algorithms and tools as too complex

•  practitioners rate as definitely useful a potential tool for deploying WSN nodes on a virtual environment

GOAL

HOW

MAIN FINDINGS

Roadmap

Background

Contributions

Design of the study

Results

Takeaways

Conclusions

Research objective

Our main research question is

Why they should define it? To better reason on:

•  the network topology

•  how much power is consumed by the application running on the nodes with respect to the used batteries or harvested energy sources

•  how well an area is covered or tracked by sensors

•  …

How WSN engineers currently define the physical environment, and how they would like to do it?

Research sub-questions

If so, how do they accomplish this task (e.g., formally, informally, etc.)?

If so, how do they do it (do they consider obstacles, hardware configuration, etc.)?

Need to consider the exact shape of obstacles, or only an approximation?

How would WSN engineers prefer to interact with such a potential tool?

Do engineers explicitly specify the physical environment where the WSN is going to be deployed? RQ1

Do engineers specify the sensor nodes and their exact position within the physical environment of a WSN? RQ2

What are the most relevant features a potential tool for specifying the physical environment of a WSN shall expose? RQ3

Population selection (1)

Participant profile:

Two sampling methods:

1.  Convenience sampling - we directly selected WSN engineers from: –  our personal contacts

–  reference websites, newsgroups, and other web resources about WSN OSs, node vendors, and WSN technologies in general

Engineer who has been concretely involved in the development of at least one WSN in the last 10 years

Population selection (2)

2.  snowball sampling [4] - we asked selected participants to nominate additional experts in their network

21 WSN engineers from 18 different organizations in 9 countries Main affiliation types:

–  university –  center of excellence –  company –  research institution

image from: http://www.hsrmethods.org/Glossary/Terms/S/Snowball%20Sampling.aspx

Resulting population

Design of the questionnaire*

A transcript of the questionnare is available here: http://www.di.univaq.it/malavolta/wsn/WSNenv.pdf

a.Introduction b.Personal

information

c. Questions about the WSN environment

specification

c. Questions about why and how the WSN

environment is not specified

Yes

No

c. Questions about digital WSN environment

e. Questions about the potential tool for

WSN environment

d. Questions about WSN Design

f. Concluding questions

Yes

Is the WSN environment

specified?

Is the WSN environment

specified digitally?

Yes No

Involved in the WSN design

phase?

No

a) purpose of the study + terminology

b) demographical info of participants

c) how environment is specified

d) focus on nodes and positioning

e) potential tool for WSN environment

f) additional comments + snowballing

21

7

close-ended questions

open-ended questions

Roadmap

Background

Contribution

Design of the study

Results

Takeaways

Conclusions

Population

21 practitioners:

14 with experience ≥ 5 years

7 with experience < 5 years

1

14

2

3

1

1

15

3

1

1

0 2 4 6 8 10 12 14 16

1-9

10-49

50-99

100-999

1000 and above

Average number of WSN nodes

Number of nodes in the largest WSN project

53%

19%

23%

5%

#projects < 3

3 ≥ #projects ≤ 6

#projects > 6

No info

43%

24%

28%

5%

Equally indoor and outdoor Mostly indoor Mostly outdoor Indoor only

WSN environment specification (1)

Encouraging for our study since we can investigate on both types of development processes

Major trend in specifying the environment in a precise way, rather than relying on draft specifications.

48% 52%

The WSN environment is explicitly specified

The WSN environment is not specified

20%

30% 30%

10%

10% Always by a draft

Mostly by a precise specification Equally

Not specififed

Always by a precise specification

WSN environment specification (2)

Clear trend in favor of digital representation

Most used file formats:

text-based and images

Basically, those results uncover the great variance about the software used to represent the WSN environment

90%

10%

Digital representation

Paper-based representation

40%

30%

20%

10%

Maps and GIS software

Office software

Dedicated software

Don't know

2D vs 3D

80%

10%

10%

2D

3D

2d and 3D

Due to the complexity of producing 3D models?

Due to the fact that 2D models are perceived to be sufficient for representing the environment of a WSN?

In this case, 2D+3D representation is the main trend

In their last project

20%

30%

50%

2D

3D

2d and 3D

Best options in general?

Obstacles definition

33, 33%

33, 33%

13, 13%

6, 6%

15, 15%

Free space (no obstacles)

Walls, floor, and roof

Walls, floor, roof, windows, and large-sized objects Walls, floor, roof, windows, large and small-sized objects No choice

Clear winners:

•  free-space environment

•  only very large obstacles (e.g., walls, roofs, etc.)

Hardware and nodes positioning

94%

10%

Definetely useful

Not useful

Indeed, WSN engineers must have at least some knowledge about the hardware features of the nodes used in the WSN.

Examples:

–  transmission power of the antenna

–  available sensing devices

–  batteries voltage

Do analytical models and simulation tools fit well with practitioners’ needs?

Usefulness of having a hardware specification

84%

0% 0% 16%

By deploying them on site (real-world testbed)

Analytically

By simulating the network

Other

Instrument for evaluating the optimal nodes positioning

“Simulation is performed only if simple, feasible and meaningful, otherwise deployment”

Why not specifying the WSN environment?

54%

46%

No perceived usefulness

Lack of satisfactory tools, algorithms or models

Why not?

37%

27%

27%

9%

Not needed (adaptable WSN)

Measure the WSN on the field, after deployment

Preliminary measures of the area and network simulation

Based on their experience

How do they proceed to the deployment of the WSN?

“Because up to now it has been sufficient just to know the main features of the environment”

“We mainly worked on networking protocols, able to adapt to the changes of the environment”

“Unclear whether the modeling effort is going to pay off”

“It is simpler not to model the environment and compensate for time dynamic failure with robust algorithms”

Potential tool (1)

Proposal: potential tool that allows engineers to virtually deploy a WSN in the environment.

Such a potential tool could simulate an environment where to virtually deploy a set of defined sensor nodes into a digital version of its physical environment.

48%

38%

14%

0% 0% Definetely useful

Useful

Neutral

Not very useful

Definitely not useful

Potential tool (2)

1

15

3

1

1

0 2 4 6 8 10 12 14 16

1-9

10-49

50-99

100-999

1000 and above

Average number of WSN nodes Number of nodes in the largest WSN project

48%

52%

The WSN environment is explicitly specified

The WSN environment is not specified

20%

30% 30%

10%

10%

Always by a draft Mostly by a precise specification

Equally Not specififed

Always by a precise specification

24%

33%

43%

By importing a file produced by means of an external tool (for example Autocad)

By directly drawing the environment within the tool

By firstly importing an image file to be used as a guide to the drawing phase within the tool

40%

30%

20%

10%

Maps and GIS software

Office software

Dedicated software

Don't know

20%

30% 30%

10%

10% Always by a draft Mostly by a precise specification Equally Not specififed Always by a precise specification

24%

33%

43%

By importing a file produced by means of an external tool (for example Autocad) By directly drawing the environment within the tool

By firstly importing an image file to be used as a guide to the drawing phase within the tool

Tool interaction

When asked about their interest in defining the exact shape of the obstacles, no clear trend has been identified

Potential tool (3)

About the importance of physical effects for the WSN:

weighted sum

Physical effect -2 -1 0 +1 +2 ws

Attenuation 0 0 0 7 14 7

Reflection 0 0 2 11 8 5.4

Scattering 0 1 6 7 7 4

Diffraction 0 2 6 11 2 2.6

Refraction 0 4 5 9 3 2.2

Polarization 0 4 8 7 2 1.4

Roadmap

Background

Contribution

Design of the study

Results

Takeaways

Conclusions

Good number of practitioners explicitly define the WSN environment Almost equal number of practitioners do not

–  mainly they do not see a clear ROI –  no satisfactory tool or method

à Researchers should

•  provide a more concrete evidence about the advantages of explicitly representing the WSN environment

•  work further on methods, algorithms, and tools Majority of participants would prefer to

–  define the physical environment via mapping or GIS software –  use a combination of text and images –  use a combination of 2D and 3D representations

Do engineers explicitly specify the physical environment where the WSN is going to be deployed? RQ1

WSN practitioners typically: –  consider free-space environment

–  consider only very large obstacles (e.g., walls, roofs, etc.)

–  rely on physically measured testbeds

à do current simulation and analysis techniques demand too much effort to WSN practitioners?

Do engineers specify the sensor nodes and their exact position within the physical environment of a WSN? RQ2

“Usually the available simulation tools do not provide a functionality to define and describe the environment. However, I feel that it is equally important to describe the environment and its behaviour in addition to the models that define how the networking part will function. I believe this is due to the difficulties in defining accurate models for the environment.”

WSN practitioners strongly need a tool for:

1.  defining the physical environment of a WSN

2.  virtually deploying WSN nodes into it

The tool may allow engineers to specify the environment in different ways.

For example, by importing an image that will serve as the basis for a subsequent drawing phase.

We believe that this option provides a good trade-off in terms of level of usability and preciseness

Mininal set of physical effects to be considered: attenuation and reflection

What are the most relevant features a potential tool for specifying the physical environment of a WSN shall expose? RQ3

Roadmap

Background

Contribution

Design of the study

Results

Takeaways

Conclusions

Conclusions

“I think that a study on modelling and analysis of the WSN environment is interesting and can give you some new ideas because nowadays in most cases a WSN is intended as a set of hardware nodes, without taking into account the place where the nodes will be deployed”

References

[1] Y. Ben Maissa, F. Kordon, S. Mouline, and Y. Thierry-Mieg, “Modeling and analyzing wireless sensor networks with verisensor: An integrated workflow,” in Transactions on Petri Nets and Other Models of Concurrency VIII, ser. Lecture Notes in Computer Science, M. Koutny, W. Aalst, and A. Yakovlev, Eds. Springer Berlin Heidelberg, 2013, vol. 8100, pp. 24–47. [Online]. Available: http://dx.doi.org/10.1007/978-3-642-40465-8 2

[2] L. Samper, F. Maraninchi, L. Mounier, and L. Mandel, “Glonemo: Global and accurate formal models for the analysis of ad-hoc sensor networks,” in Proceedings of the First International Conference on Integrated Internet Ad Hoc and Sensor Networks, ser. InterSense ’06. New York, NY, USA: ACM, 2006. [Online]. Available: http://doi.acm.org/10.1145/1142680.1142684

[3] http://www.remcom.com/wireless-insite

[4] B. Kitchenham and S. L. Pfleeger, “Principles of survey research: part 5: populations and samples,” SIGSOFT Softw. Eng. Notes, vol. 27, pp. 17–20, September 2002.

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Ivano Malavolta | Gran Sasso Science Institute

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