Wireless Sensor Networks:

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Wireless Sensor Networks
An overview and experiences.Matthew Grove
ltm.grove_at_rdg.ac.ukgtPEDAL Seminar Series, January
9th 2008
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Outline

• Introduction - what are wireless sensor networks?
  What have we been doing with them?
• Terminology.
• Some sample hardware from Crossbow.
• Commonly used software architecture.
• An example application - greenhouse monitoring.
• Hardware and software experiences.
• Summary and recommendations.
• Further reading.

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Introduction

• A Wireless Sensor Network is a wireless network
  consisting of spatially distributed autonomous
  devices using sensors to cooperatively monitor
  physical or environmental conditions, such as
  temperature and sound at different locations.
• We bought a development kit in October 2006 - the
  aim was to work with Soil Science and get a
  reasonable understanding of the technologies and
  capabilities.
• Over the past year we have been writing code and
  generally experimenting with the hardware.
• The insights and experiences are being used for
  proposals and knowledge transfer within the
  University (like this talk).

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Terminology

• WSN - Wireless Sensor Network.
• Mote - a wireless sensor node. Sensors are
  connected to sensor nodes.
• Patch - a group of motes.
• Mesh - mesh networking is a common way of doing
  comms with wireless sensors.

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Wireless Sensor Board Hardware

• There are several manufacturers. The motes we
  have been using are from Crossbow.
• Pictured below are a Mica2 Mote, Mica2Dot Mote,
  UK stamp and American quarter.

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Specification for the Mica2 Mote

• Processor
• Program Flash memory 128 K
• Measurement Flash 512 K
• Wired Communications UART
• Current Draw (Sleep) 8 mA (lt15 µA)
• Radio
• Frequency (in UK) 433 Mhz
• Data Rate 38.4 Kbaud
• Outdoor Range 150-200 M
• Misc
• Battery 2x AA
• UI 3 LEDs

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Other Hardware

• Programming boards Used to upload programs to
  Motes. We have serial and Ethernet programmers.
  They can also be used as a base station linking
  the Motes to a PC.
• Gateways Act as a bridge between the Motes and
  another network. Our gateway is a 400 MHz board
  running embedded Linux.

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Software

• Motes
• Run TinyOS (event based embedded OS).
• Applications are written in embedded C (nesC).
• Lots of code snippets and some good tutorials.
• Tool chain available for nix and Windows.
• Familiar to anyone who uses GCC and Make.
• Simulators available for limited debugging and
  testing.
• Base Station / Gateway
• Java libraries provided.
• Tools available to automatically make Java stubs
  to convert Mote messages into Java objects.
• Can use your normal Java development process and
  IDE.

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Monitoring the temperature in greenhouses on
campus
Simple Example Application
Lyle Building
SSE Car Park
Greenhouse 1
Greenhouse 2
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Setup For Greenhouse Monitoring

• Two Mica2 Motes with temperature sensor boards,
  one for each greenhouse
• Both Motes periodically measure temperature.
• Mote 1 sends its measurements directly back to a
  base station in the Lyle building.
• Mote 2 sends the measurements via Mote 1 because
  it is too far away for direct communications with
  the Lyle building.
• An embedded Linux base station on the top floor
  of Lyle building inserts the measurements into a
  database.
• A web application providing a user interface for
  people to analyse the temperature measurements.

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Setup For Greenhouse Monitoring
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Tasks for the Greenhouse WSN

• Build enclosures for the motes to protect them
  from the environment in the greenhouse (humidity
  etc).
• Calibrate the motes and store that information in
  the database.
• TinyOS nesC application for measuring the
  temperature to run on the motes.
• Some code can come from existing open source
  applications.
• Conserve the batteries.
• Route messages between motes.
• Convert raw ADC sensor values into engineering
  units.
• Java base station
• Receive messages from the motes and store the
  measurements in an SQL database.
• PHP / AJAX web-client
• Dynamically visualise the data (graphs).
• Allow the user to run queries on the data (for
  instance view measurements for a specific date).
  

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Hardware Experiences

• The connectors on the Crossbow Mica2 motes are a
  nightmare to work with.
• There is no accurate hardware clock onboard these
  motes, this means more complex software is needed
  for motes to wake up from low power states
  simultaneously.
• The UI for a mote is 3 LEDs and if you attach one
  you can have a buzzer. This makes it difficult
  for motes to communicate with humans.
• Transmission distance outside is around 200
  meters line of sight (next slide). The way to
  determine where you will need to place motes for
  reliable communications is to get out there and
  do a survey with some motes.

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Transmission Distance
200 metres line of sight
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Software Experiences

• Normal concurrency and communications problems
  seen in distributed systems. TinyOS is event
  driven and messaging over the radio has no
  guaranteed quality of service.
• It is relatively simple to write applications
  that leave the radio on full power all the time,
  complexity of code increases the more you try and
  conserve power.
• An end-to-end application is requires several
  programs to be written likely in different
  languages.
• Debugging outside of the simulator can be very
  tough.

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Web-Based User Interfaces
Live AJAX Charts
Dynamic Plots in a JSR168 Portlet
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Summary and Recommendations

• WSN software is not accessible for non computer
  scientists.
• Tools need to be improved if application
  scientists are going to use WSNs to replace
  existing wired solutions in the field.
• Make the motes self-configuring, it makes
  deploying them easier.
• If you buy motes get these features
• USB interfaces.
• An accurate hardware clock (we have not found an
  off-the-shelf solution to this).

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Further Reading

• My research blog -
• http//acet.rdg.ac.uk/mjeg/blog.php
• Reading Wireless Sensor Networks project -
  http//acet.rdg.ac.uk/projects/resn/
• Crossbow Motes -
• http//www.xbow.com/Home/HomePage.aspx
• TinyOS -
• http//www.tinyos.net/