Wireless Sensor Networks

Papers referenced:

Overview

• Smaller technology provides potential for new kinds of applications
• Example: scientists can use sensors to monitor environments that are hazardous/hard to reach/cannot be disturbed
  • http://www.coa.edu/html/motes.htm
  • http://www.eecs.harvard.edu/~mdw/proj/volcano/
  • http://prisms.cs.umass.edu/dome/index.php?page=terrapinnet

Requirements

These are the requirements listed for the GDI deployment, but may are broadly applicable.

• Accessible via the Internet
• Hierarchical network spread over a large geographic area
• Long lived
• No AC power access
• Managed via the Internet (deploy new code, etc)
• Non-disruptive to the habitat

System Architecture

http://www.acm.org/crossroads/xrds9-4/gfx/sn1.jpg

• Multi-hop network
  • radio range of sensors is limited - SunSPOT 802.15.4 radio = 80 meters
• Gateway
• Base station

GDI Setup

• Mica Motes (32 deployed) - 40kbps, 4MHz, 512KB storage, AA batteries
• Sensor board - temperature, photoresistor, barometric pressure, humidity, passive infrared
• Energy budget - need to run for 9 months = 1.4 hours of processing (only processing, no sensing, etc) per day
  • Energy budget must be divided between sensor sampling, data collection, routing, health monitoring, and network retasking

Volcano Setup

• TMoteSky-based (16 deployed) - 10KB SRAM, 48KB program ROM, 1MByte flash
• Sensors - single-axis seismometer or three seismometers, microphone
• Energy budget - 2 D-cell batteries, 1-week lifetime, researchers returned to change batteries during 19-day deployment

Challenges

• Metrics: energy usage, fidelity (quality), yield (quantity)
• Network forwarding
• In-network aggregation
• Data collection interval

GDI Discussion

• Data sampling
  • sample/sense period is dependent on application and energy budget
  • data may be compressed before sending if energy to compress < energy to transmit
• Communication
  • must be scheduled (requires time synchronization!)
  • each level of the routing tree forwards to the level above at a given time and returns to sleep/low power state
• Health and Status Monitoring
  • use status info to aid with retasking
  • example: adjust duty cycle to alter lifetime

Volcano Deployment

• Network topology and status monitoring
  • nodes transmit status every 10 seconds (position in routing tree, buffer status, timestamp info, battery voltage)
• Data collection
  • nodes continuously collect, but do not continuously transmit data
  • 1MByte flash is a circular buffer
  • if an interesting event is observed, it is reported
  • if enough nodes (30%) report event, the base station asks all nodes for last 60 seconds of data

Date: 2008-04-18