Stabilizing Tracking :
Demos on Berkeley’s Mote Platform
Anish Arora, Murat Demirbas, Sandip Bapat, Sohail Munir
Ohio State
Tracking Problem
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Consider an undirected graph where any two nodes
connected by an edge are called neighbors
There is an evader that moves from a node to a
neighboring node each Te seconds
There is a pursuer that moves from any node to a
neighboring node each Tp seconds, where Tp < Te
Associate a “sensor process” with each node so that the
pursuer catches up with the evader in short time
Two Approaches to Stabilization
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OSU tracking protocol
 sensor processes communicate periodically with neighbors
 stabilizes and tracks faster
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UT tracking protocol
 sensor processes communicate with neighbors only upon
request
 minimizes number of messages and energy efficient
OSU Tracking Protocol
Sensor process
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Processes communicate periodically (four times per
second) with their neighbors
Message format
 Src: Id of sender
 Parent: Id of parent of Src
 Dist: Distance from root to Src
 Value: The logical timestamp when the bug was detected by
the root mote of Src
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Processes run stabilizing tree algorithm to form the
tracking tree
Stabilizing Tree Algorithm
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Dynamic-rooted minimum-distance spanning tree
algorithm
Self stabilizing to the invariant that the tree is rooted at
the bug
Asynchronous
 Does not require any underlying synchronization
mechanism, or local clocks
Ad hoc
 Works for any topology
A Tracking Tree Forms
…with Bug at Root
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Bug is on mote 2
Visualization tool shows tracking tree formed with root at
mote 2
Video
Screenshot
Tree Stabilizes
…when Motes State gets Corrupted
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Tree rooted at mote 2, where bug is located
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State of mote 16 is transiently corrupted
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Two trees, with roots at motes 2 and 16, are formed
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Source of corruption at mote 16 is removed
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A single tree, potentially different from the original one, is
(re)formed
Video
Screenshot1
Screenshot2
Tree Stabilizes
…even when Motes Failstop
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Tree is rooted at mote 2
Mote 14 points to mote 10 as the next hop on its shortest
path to the root
Mote 10 fails (is turned off, radio unit fails, or runs out of
battery)
Mote 14 now points to mote 13 or mote 15 instead of the
failed mote 5
Tree (re)stabilizes
Video
Screenshot1
Screenshot2
Spider Modes
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Line following
 If spider goes off line, it searches for line and follows it until it
reaches the end of the line
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Direction tracking
 Colored lines encode the four directions
e.g. Long Yellow + Short Black = North
 Spider calculates its direction after a complete line traversal
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Reading from mote
 Motes blink IR LEDs at four different frequencies to
communicate the four directions
 On reaching a mote, spider detects the frequency of emitted IR
signals and decides how to turn
The Spider Converges to the Grid
...and Tracks the Tree
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Spider converges to the grid from any point within grid,
without falling off the edge
Upon converging, spider regains its sense of direction
within one complete line traversal
Upon regaining sense of direction, spider tracks tree by
following direction signals from the motes
Video
The Spider Catches the Bug
…assuming Spider is Faster
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Spider follows the tree all the way to the root, where the
bug is located and catches the bug
Even if the bug moves, the tree reconfigures when the
bug is detected at the next mote
So, if the bug stops or is slower than the spider, the spider
eventually catches the bug
Video
The Spider Catches the Bug
…even if Motes Failstop
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If spider reaches a failed mote, it cannot get a direction
signal
Spider then chooses a random direction to follow. If this
direction leads it off the grid, it backs up and retries till it
finds a grid direction
On reaching the next non-failed node, the spider gets a
proper direction to follow and catches the bug eventually
Video
Engineering tricks
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Main problem – Lack of a fat communication pipe between
spider and motes
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Encoding directions using line colors
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Using IR LEDs on motes to communicate with spider
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Ensuring that motes do not detect spider as the bug using
a light source
Future work: Hybrid approaches
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Proxy Pursuer or Agent
Publish/Subscribe service for interest based information
propagation to pursuer
On-demand stabilization of the spanning tree