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Lessons from Haggle iMote Experiments

Lessons from Haggle iMote Experiments. Erik Nordström Haggle meeting 9-10 January 2007. Background. Investigate contact patterns (human mobility) of users Intel motes handed out to participants at conferences Bluetooth inquiry scan at regular interval Record devices in proximity

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Lessons from Haggle iMote Experiments

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  1. Lessons from Haggle iMote Experiments Erik Nordström Haggle meeting 9-10 January 2007

  2. Background • Investigate contact patterns (human mobility) of users • Intel motes handed out to participants at conferences • Bluetooth inquiry scan at regular interval • Record devices in proximity • Motes can only respond when not inquiring • Two types of iMotes • Mobile mote (dental floss) carried by users • Stationary long range mote, more battery, external antenna • Reset problems motivation for new experiment

  3. The Intel Mote

  4. Overview of iMote Experiments

  5. Challenges • iMote hardware is very restricted • ARM7TDMI at 12 MHz • 64 kB RAM • 512 kB permanent flash storage! • Integrated Bluetooth 1.1 (~30 m range) • Software environment • Windows + Cygwin development • TinyOS implementation for iMote is experimental • Proprietary ARM compiler

  6. Basic iMote Software Features • Loop initiates a Bluetooth inquiry every 120 s • Inquiry for 5 secs • Add ±5 s to avoid synchronized inquiries on motes • Sleep mode in-between inquiries • Store “active contacts” in RAM • Allow one skipped inquiry period • Write to flash when a contact ends • Saves space • May loose contacts in RAM upon reset

  7. iMote Software Problems • Buffer overflow causing resets on nodes with many “active contacts” • Risk of long term contacts never being recorded • Bug in FlashFS implementation causing headaches • Unseeded random function generator • Affects scanning de-synchronization • Limitation in Bluetooth lower layers returning max 8 devices each scan • Hence the motivation for a non-standard 5 sec inquiry period? • Inefficient timer operation • Fired every second even if nothing needed to be done • Drains battery?

  8. Software Fixes • Ended up rewriting most of the code • Fixed buffer overflow • no software resets (that we know of) • Fire timer every 120±5 secs and perform inquiry • Seed random function with MAC address • Write contact to flash when first seen - update with end time when it goes away • Protects against potentially lost contacts • Increase limit on returned contacts to 100

  9. Observations • Few contacts were returned each inquiry scan • Increased inquiry period returned more motes • Trade-off between scan time and overlapping scans • Drains battery? • Differences between type of device • Motes are rarely recorded for more than around 1-4 periods • External devices are recorded for much longer periods • Unclear effect of sleep mode • MAC address corruptions (also in old experiments) • Noisy environment (70+ nodes in same node)

  10. Implications on Collected Data • Short mote contacts • May seriously impact mobility pattern as it favors short contacts… • …or is this reality? • Long external contacts indicate hardware differences • MAC address corruption • May also explain short contacts • False contacts?

  11. Conclusions • iMote hardware limitations? • Bluetooth radio weak, favoring external devices • External contacts do not inquiry regularly • No listen while scanning • Impact of overlapping inquiries on sampling? • Bi-mote, tri-mote experiments • Bundle two imotes, one inquiring one listening • Third imote only listens

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