Transcript The Hitchhiker\`s Guide to Successful Wireless Sensor Network

Network and Systems Laboratory
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The Hitchhiker’s Guide to Successful
Wireless Sensor Network Deployments
Guillermo Barrenetxea, Francois Ingelrest,
Gunnar Schaefer and Martin Vetterli
LCAV, EPFL, Switzerland
SenSys 2008
Jeffrey
Network and Systems Laboratory
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Outline
 Introduction
 Related Work
 SensorScope
 The Hitchhiker’s Guide
 Conclusion
 Comments
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Introduction
 Most theoretical aspects of wireless sensor networks
(WSNs) have been well studied over the past few years
 Synchronization
 Localization
 Routing
 Real-world deployments still remain a challenging task
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Why Challenging?
 Good WSN systems fail to provide expected results
once deployed in the real world
 Such failures may be either due to
 a completely non-working system or
 an inability to meaningfully exploit gathered data
 While certain issues may be anticipated, experience is
still the key asset to ensure a successful deployment
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Main Areas to Successful Deployments
 Three main areas exist, in which expertise is needed,
to access to the “Holy Grail” of successful deployments
 Development
 Testing
 Deployment
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Development
 The first step
 Local conditions must be carefully studied and
considered
 such as the expected weather in case of outdoor
deployments
 Hardware must be well-fitted to the targeted site
 Embedded software must be designed in a way that
eases debugging later on
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Testing
 Ensuring that the system is ready to be deployed
before going on site is mandatory
 Setting up a testbed is often the best solution
 Designing a good one, however, is not so easy
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Deployment
 Last but not least, the deployment is most often the
time to face unexpected problems due to
unanticipated or—even worse—underestimated issues
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SensorScope
 Over the past three years, the authors have worked on
SensorScope
 an environmental monitoring system based on a WSN
 Have engineered a complete framework including
 electronic circuit boards
 a solar energy system
 an embedded communication stack, based on TinyOS
 server-side software
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Six Real-world Deployments
 Have already run six real-world deployments
 Ranging in size from half a dozen to a hundred
stations
 From our university campus to high-mountain sites
 Throughout these deployments, valuable experience in
preparing, conducting, and managing deployments
have been gathered
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On A Rock Glacier
 Have deployed our system on a rock glacier located at
2500m, on top of the G´en´epi, a mountain in the Swiss
Alps
 This environment is rough and the deployment took
place under very harsh conditions
 Thanks to the authors’ experience, it was successful
and led environmental scientists to the modeling of a
microclimate causing dangerous mud streams
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Not For Outdoor Deployments Only
 SensorScope is aimed at outdoor deployments
 Many of the issues we describe in this paper are
common to all kinds of deployments
 The main part of this paper is written as a guide for
readers aiming to deploy a live WSN
 Contains much advice, illustrated with many
examples, all taken from our own experience
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Outline
 Introduction
 Related Work
 SensorScope
 The Hitchhiker’s Guide
 Conclusion
 Comments
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Related Work
 Many known deployments of WSNs
 Wireless sensor networks for habitat monitoring, 2002
 By a group at Berkeley in 2002, on the Great Duck
Island, to help habitat monitoring
 Pioneering Work
 Limited To Single-hop Communications
 Many Lessons Were Learned Regarding The Difficulties
Of Deploying Such A Network
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Berkeley’s Macroscope
 A macroscope in the redwoods, 2005
 A new sensor network built by Berkeley
 Built on top of TASK, a set of WSN software and tools,
also designed at Berkeley
 Extensively used for microclimate monitoring of a
redwood tree
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Drawbacks
 Rather small-scale
 Placed in a tree, at an altitude of 15 to 70m from the
ground
 Most sensor motes, in particular the ones used in
SensorScope, are able to communicate directly over
such a small distance
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Harvard
 Deploying a wireless sensor network on an active
volcano, 2006
 A group at Harvard described their experience in
deploying WSNs on top of active volcanoes
 To study their activity by measuring seismic and
infrasonic signals
 Close to SensorScope, in the sense that
 targeted sites are harsh and difficult to access
 once deployed the network must be robust and reliable
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Differences
 Event-based
 no data is needed when there is no volcanic activity
 Sensor Scope is time-based
 Their deployments were also short-term (a few weeks)
 Some of deployments in this paper lasted for more
than six months
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Delft University
 Murphy loves potatoes: Experiences from a pilot
sensor network deployment in precision agriculture,
2006
 Researchers at Delft University deployed a large-scale
sensor network in a potato field
 The goal of the project was to
 improve the protection of potatoes against a fungal
disease
 to precisely monitor the development of that disease
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Drawbacks
 Unfortunately, the deployment went mostly awry
 so that the work could not be finished, because of time
and money constraints
 Nevertheless, the researchers reported the lessons they
learned
 especially how much more difficult it is to set up a WSN
in the real world

rather than in a simulator or in a laboratory
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University of Virginia’s LUSTER
 LUSTER: Wireless sensor network for environmental
research, 2007
 Designed mainly to gather light measurements
 Built on top of a low-power MAC layer
 Makes use of distributed storage on embedded flash
cards, providing fault-tolerance
 Deployed outdoors, in two different environments
 in a forested area, close to the laboratory
 on Hog Island, a research site in the Virginia Coast
Reserve
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Deployment Methodology Remains The Same
 Although these projects are different from each other
 Hardware
 Protocols
 Applications
 The deployment methodology remains the same
 While the targeted site may be either a volcano or a
giant tree, most difficulties regarding the deployment
itself are common to all scenarios
 Preparation
 Management
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Outline
 Introduction
 Related Work
 SensorScope
 The Hitchhiker’s Guide
 Conclusion
 Comments
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SensorScope
 SensorScope is an environmental monitoring system
 Based on a time-driven WSN
 The network’s sensing stations regularly transmit
environmental data to a sink
 wind speed and direction
 The sink in turn, uses a gateway to relay the data to a
server
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Different Gateways
 Depending on the deployment scenario and the
available communication resources, different gateways
are used
 GPRS, Wi-Fi, or Ethernet
 All data is published on our real-time Google Maps-
based web interface and on Microsoft’s SensorMap
website
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Architecture
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Results of Collaboration
 SensorScope is developed in collaboration between
two research laboratories at EPFL:
 LCAV (signal processing and networking)
 EFLUM (hydrology and environmental fluid mechanics)
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Goal
 The goal is to improve current environmental data
collection techniques
 Commonly based on a single, very expensive sensing
station (€ 60,000)
 Such stations use data loggers with limited capacity,
requiring manual on-site downloads
 Using a WSN is highly relevant to this area of research
 Realtime feedback (e.g., storms, pollution)
 Long-term monitoring (e.g., snow level) in areas of
varying size
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Hardware
 Shockfish TinyNode sensor motes are used
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Why Shockfish TinyNode?
 Long communication range
 Low power consumption
 A transmission power of 15 dBm allows for a
communication range of up to 500m with the onboard antenna
 Up to 1 km using an external quarter-wavelength
omni-directional antenna
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Powered by Solar Energy System
 To allow for long-term deployments, we designed a
complete solar energy system in the spirit of
Heliomote
 Composed of a solar panel and two rechargeable
batteries
 one of them being used as a backup buffer
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Sensors
 Stations are equipped with seven sensors, measuring
nine environmental quantities
 air temperature and humidity
 surface temperature
 solar radiation wind speed and direction
 soil water content and suction
 precipitation
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Sensing Station
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Sensor Box
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Price
 The average price of a station is around € 900
 The price is kept down by using lower-end sensors
 A key goal of the project is to obtain dense spatial
measurements
 This is achieved by deploying multiple low-cost—
possibly less accurate—sensing stations, rather than a
single expensive, but very accurate one
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Network
 Multi-hop
wireless
networking
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Packet Format
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Neighborhood Management
 Motes maintain a neighborhood table in which they
store the neighbors they can hear from
 Chose an overhearing method in the spirit of
MintRoute
 There are no dedicated neighborhood discovery
packets
 Neighbors are discovered by listening to data traffic
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Discovery Process
 the sink starts the discovery process by emitting
beacons
 A cost—currently the hop distance to the sink—and a
timestamp are associated to each neighbor
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Synchronization
 To allow for a meaningful exploitation of gathered
data, it must be time-stamped by the nodes, as part of
the sensing process
 Because our power management mechanism relies on
duty-cycling, we opted for global synchronization of
all motes
 Use SYNC REQUEST/SYNC REPLY messages to
propagate the local time of the sink (the network
time), so that all nodes share its clock
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Clock Update
 When a node wants to update its clock, it sends a
request to a neighbor closer to the sink than itself
 This neighbor, if it knows the network time,
broadcasts it back, and all receivers, which are further
from the sink, update their clock
 The network time always propagates away from the
sink, which acts as the global reference
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Power Management
 Even with solar energy, power management at the
MAC layer is essential for long-term deployments
 As the radio chip is a greedy energy consumer
 Turning on the radio of a TinyNode increases its
energy consumption approximately eightfold
 Opted for a synchronous duty-cycling scheme
 made this decision based on interactions with EFLUM
 allowed us to determine that overall data traffic would
be low
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Routing
 To route data to the sink, a randomizing solution is
chosen
 Each time a packet has to be routed, the forwarding
node randomly selects a next hop between the
neighbors closer to the sink
 To give priority to the better neighbors, two
thresholds, based on link quality, are used
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Deployments
 Have conducted six deployments
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Outline
 Introduction
 Related Work
 SensorScope
 The Hitchhiker’s Guide
 Conclusion
 Comments
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The Hitchhiker’s Guide
 Hardware and Software Development
 Testing and Deployment Preparation
 Deployments
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Hardware and Software Development
 Development is the first step towards the construction
of a new system
 During this phase, it is of prime importance to ensure
that both hardware and software fit the intended
application, considering
 the expected results
 the conditions in which deployments will take place
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Consider Local Conditions
 You must investigate how local conditions will affect
your deployments
 Because we knew that our deployments were going to
be outdoors, we carefully considered, with the help of
the EFLUM, how weather conditions would impact our
system
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Not Always Obvious
 However, it is not always obvious how possibly drastic
variations in temperature and humidity will affect
hardware devices in general
 A lack of testing under real conditions may lead to
serious issues
 Already knew that Li-Ion battery should not be
charged when the temperature is below freezing
 as it could explode
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Many Hardware Failures
 hydrologists brought a disdrometer, an expensive
instrument that can distinguish between different
kinds of rain by analyzing the water drops
 It was supposed to be used as a high-quality
benchmarking tool.
 Unfortunately, it turned out that it worked only during
a few days, simply because it was too cold on top of the
mountain
 Crucial to simulate the anticipated deployment
conditions as accurately as possible
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Use A Climate Chamber
 To study the impact of weather conditions on
hardware devices, the best solution is to use a climate
chamber
 arbitrary temperature/humidity conditions can be
created
 In most cases, basic tests inside a household freezer
will expose potential points of failure
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Time’s a Drifter
 The crystals, used in embedded devices to measure
time, are not perfect
 temperature greatly impacts their precision
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Hard Shell – Soft Core
 Packaging sensors for outdoor deployments is a
difficult task
 As it must protect electronic parts from humidity and
dust while being unobtrusive at the same time
 IP codes are used to specify the degree of
environmental protection for electrical enclosures
 The required protection for outdoor deployments is
IP67, which provides full protection against dust as
well as water, up to an immersion depth of one meter
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Sensirion SHT75 sensor
 Used to measure both air temperature and humidity
comes unpackaged
 Took quite some time to figure out a suitable
packaging, protecting from direct sunlight, while still
letting the wind reach the sensor
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Corrosion Problem-1
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Corrosion Problem-2
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There Is no Light at the End of the Tunnel
 There shall be no light because it strains thy battery
 On our motes, a single LED consumes about 3 mA
 That makes a total of 9mA for the typical three LEDs,
while the radio chip, when on, consumes “only” 15 mA
 There is thus no reason to efficiently manage the radio
while carelessly using the LEDs
 LEDs are the most useful debugging tools for WSN
developers (and often the only ones)
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Keep It Small and Simple
 Both code and algorithms must be well-fitted to the
intended application
 Sometimes, you will not be able to avoid complexity,
but whenever the benefits are questionable, you
should prefer simple solutions
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Remote Control
 If sensor motes are to be deployed in difficult-to-access
places (and sometimes even in easy-to-access places),
the ability to remotely control the deployment is
highly desirable
 When going back to the deployment site is difficult or
costly, being able to adjust certain parameters
remotely, such as the sampling frequency, may be
necessary
 More drastically, you will also want to be able to
reprogram the motes of an ongoing deployment,
without leaving your office
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SensorScope
 When we developed SensorScope, we added routines
to the software running on the GPRS module
 enable us to control it remotely, using simple GSM text
messages, sent from a standard mobile phone
 Allows to query its status or to reboot either the GPRS
or the sink’s mote
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SensorScope
 Can also ask the GPRS to download a new version of its
binary image from an FTP server, and to reboot using
this new version
 Still cannot, however, change parameters of the entire
network, as this requires a mechanism to disseminate
information from the sink, which is currently not
implemented
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Don’t Be a Black Box
 Programming embedded devices requires a different
philosophy than traditional programming
 In the latter case, it is easy to debug the code by using
any kind of debugging statements or tools
 It is far more difficult with embedded devices, such as
sensor motes, as the simplest way for them to
communicate with the outside world is by blinking
their LEDs or using their serial port
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Recommendation
 researchers from Delft University recommend that
each software component should be able to produce a
set of statistics about its recent activity
 In SensorScope, besides traditional sensing packets,
sensor motes generate three kinds of status packets
 Energy
 Network
 Neighborhood
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Energy Status Packets
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Network Status Packets
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Publish or Perish
 At some point, your system will—hopefully—be
functional and deployed, and your next step will
certainly be to get publications out of it
 Similar to the system itself, you should carefully plan
these publications during the development phase
 to make sure that all required data will be gathered
during deployments
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Choose Your Partner
 There are two major components of a successful
deployments
 gathering the data
 exploiting the data
 Generally, networking laboratories only care about the
first component, while the second one actually plays
an equal role
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Testing and Deployment Preparation
 To prepare for our deployments, we use two different
testbeds
 One is indoors, conveniently located in our building,
used mostly to test our communication software
 The second testbed is a pseudo real-world
deployment, located on our campus, composed of
actual sensing stations
 used to ensure that all code which is not in use on our
indoor testbed (e.g., sampling sensors, managing solar
power) does not interfere with the rest
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Efficiency Matters
 When setting up a testbed, you must keep in mind
that it will be used to develop and to test many
software components
 Because that is a long process, composed of many testit-and-fix-it cycles, it is important for the testbed to be
easily and quickly accessible
 While programming motes one by one with a serial
cable may be acceptable for deployments, because it is
done only once, this is not the case for a testbed
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Indoor Testbed
 Regularly replacing batteries is not a good idea either,
and this will be necessary, even if some slick power
saving algorithm is used
 As our indoor testbed is solely used to evaluate
network code, its motes are not wired to any external
sensors.
 All of them are, however, plugged into AC power,
allowing us to disregard any problems linked to energy
management
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Indoor Testbed
 Furthermore, all 50 motes are equipped with a Digi
Connect ME module which makes it possible to access
and program them over a simple Ethernet connection
 Each Digi module is indeed assigned an IP address
which, in combination with the appropriate PC-side
drivers
 Allows for transparent PC–mote serial communication
 Such equipment is very important to allow for quick
testing
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Time to Flash Motes
 On our indoor testbed, it takes only between 10 and 45
seconds to flash all 50 motes, depending on the size of
the image,
 while it takes us about 45 minutes to flash the 10 motes
of our outdoor testbed and to put them back
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Labels that Stick
 Our indoor testbed is distributed among a number of
offices in our building, some of them belonging to
other laboratories
 We frequently discover that some of the motes are
disconnected, or have even disappeared
 Because people do not know exactly what these strange
devices are and what they are used for
 When we first installed our indoor testbed, we put
stickers on the motes
 stating that “this device belongs to LCAV, please contact
. . . for further information”
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Know Your Enemies
 When setting up a deployment or a testbed (especially
indoors, or close to urban areas), the first order of
business should be to inspect the radio spectrum used
by your platform to detect possible external
interferences
 The optimal way to do this is to use a spectrum
analyzer
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A Simpler Way
 A simpler way to check for interferences is to run a test
program to determine losses over time for the various
frequencies that your selected platform can use
 A run of 100 transmissions was started for each
payload length with an interval of two seconds
between transmissions
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The Value of Simulation
 In place of a testbed, or in addition to it, simulations
can be used to test protocols
 Many simulation tools are available, the most
(in)famous one certainly being ns-2
 A new kind of simulation tool, called Worldsens [4],
has been developed
 Most of it is actually not a simulator, but a sensor mote
emulator
 WSim
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Image is Needed
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Outline
 Introduction
 Related Work
 SensorScope
 The Hitchhiker’s Guide
 Conclusion
 Comments
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Network and Systems Laboratory
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Conclusion
Copyright © 2008 Jeffrey Hsiao
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Network and Systems Laboratory
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Outline
 Introduction
 Related Work
 SensorScope
 The Hitchhiker’s Guide
 Conclusion
 Comments
Copyright © 2008 Jeffrey Hsiao
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Network and Systems Laboratory
nslab.ee.ntu.edu.tw
Comments
 Provide rather practical advices for WSN deployments
 Useful for outdoor WSN deployments
 Might not be directly applicable to indoor WSN
deployments
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Network and Systems Laboratory
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Thank you very much for your
attention!
Any Questions?
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84