Involving Multiple Smartphones Chu Luo, Henri Koski, Mikko Korhonen, - - PowerPoint PPT Presentation

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Involving Multiple Smartphones Chu Luo, Henri Koski, Mikko Korhonen, - - PowerPoint PPT Presentation

Sep 09, 2023 192 likes •463 views

Rapid Clock Synchronisation for Ubiquitous Sensing Services Involving Multiple Smartphones Chu Luo, Henri Koski, Mikko Korhonen, Jorge Goncalves, Theodoros Anagnostopoulos, Shinichi Konomi, Simon Klakegg and Vassilis Kostakos

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Rapid Clock Synchronisation for Ubiquitous Sensing Services Involving Multiple Smartphones

Chu Luo, Henri Koski, Mikko Korhonen, Jorge Goncalves, Theodoros Anagnostopoulos, Shin’ichi Konomi, Simon Klakegg and Vassilis Kostakos chu.luo@unimelb.edu.au

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Smartphone Sensors and Apps

http://funf.org/about.html

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Sensing with Multiple Phones

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Phone Clock Has Drift

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Challenge: To synchronise clock on phones

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Related Work Clock Synchronisation:

  • 1. Network Time Protocol (NTP)
  • 2. Precision Time Protocol (PTP)
  • 3. GPS Clock Synchronisation
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Network Time Protocol (NTP)

A stratum model

http://www.meinberg.in/support/information/ntp-the- network-time-protocol.htm

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Network Time Protocol (NTP)

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Client

NTP can be inaccurate:

Server

NTP Actual Trips NTP Error

Time Time

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To make use of NTP

Collect many samples, do:

  • 1. Linear Programming; or
  • 2. Linear Regression(fitting)

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Error can be minimised:10-100 ms

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Precision Time Protocol (PTP)

A specialised network

http://www.meinberg.in/support/information/ntp-the- network-time-protocol.htm

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GPS

http://geospatialrevolution.psu.edu/educators/trilateratio n.html

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NTP as the base for ubi-sensing

Reasons:

  • 1. Available anywhere on Internet
  • 2. PTP needs LAN infrastructure
  • 3. GPS is restricted by signals, power-

hungry and hurts location privacy.

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How large can this drift be?

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Phone Clock Drift

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Study 1

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Running for 9 days:

  • 1. 3 identical phones
  • 2. Connected to WiFi
  • 3. Collecting NTP sample per minute
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Using the median of several NTP Samples

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Rapid Clock Synchronisation

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Study 2: measure pair-wise synchronisation difference

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http://leacellodp.weebly.com/roller-coaster-project.html

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Difference of sync using 10 samples

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Take-away Points

  • 1. Clock drift may be 150ms per day
  • 2. NTP is most available and flexible
  • 3. Using NTP median is a rapid way:

a) Mean accuracy 12ms, 10 samples b) Worse case, <100ms, 97 samples <40ms, 157 samples

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Rapid Clock Synchronisation for Ubiquitous Sensing Services Involving Multiple Smartphones

Chu Luo, Henri Koski, Mikko Korhonen, Jorge Goncalves, Theodoros Anagnostopoulos, Shin’Ichi Konomi, Simon Klakegg and Vassilis Kostakos. chu.luo@unimelb.edu.au

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