The Anatomy of a ContextAware Application
Computer Science and Engineering
University of Texas at Arlington
Cheng-Lung Chu
2003/4/21
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Overview
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Context and context-awareness
A platform described in the paper
Bat Teleporting
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Context
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Context means situational information
“Context is any information that can be used
to characterize the situation of an entity. An
entity is a person, place, or object that is
considered relevant to the interaction
between a user and an application, including
the user and application themselves.” Dey,
A.K. and Abowd. G.D., Georgia Institute of
Technology, 1999.
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Context cont’
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Almost any info available at the time of an interaction
can be seen as context info.
Examples
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Identity
Spatial info – location, orientation, speed, and acceleration
Temporal info – time of the day
Environmental info
Social situation
Resources that are nearby
Availability of resources
Physiological measurements
Activity
Schedules and agendas
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Context-Awareness
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Context-awareness
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One is able to use context info.
A system is context-aware if it can extract,
interpret and use context info and adapt its
functionality to the current context of use
Challenge – Complexity of capturing,
representing and processing contextual
data
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A Context-aware application
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A context-aware application adapts its
behavior to a changing environment
Proposed Platform
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A fine-grained location system
A detailed data model
A persistent distributed object system
Resource monitors
A spatial monitoring service
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Indoor Location Sensing
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Ideal location sensor for use indoor
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Fine-grain spatial info
High update rate
Unobtrusive
Cheap
Scalable
Robust
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The Bat Location System –
Bat
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Bats
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Consist of a radio
transceiver, controlling
logic, and ultrasonic
transducer
7.5x3.5x1.5 (cm), 35g
48-bit Globally unique ID
Powered by a single 3.6V
Lithium cell, a lifetime of
around fifteen months
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The Bat Location System –
Ultrasound receiver unit
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Ultrasound receiver units
Placed at known points on the ceiling of the rooms
to be instrumented
Connected by a high-speed serial network in
daisy-chain fashion
Receivers are placed in
a square grid, 1.2m
apart
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The Bat Location System
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Base station Periodically transmits a radio
message containing a single identifier,
causing the corresponding Bat to emit a short
un-encoded pulse of ultrasound
In the mean time, receiver are reset via the
wired network
Receivers monitor the incoming ultrasound
and record the time of arrival of any signal
from the Bat
Times-of-flight of the ultrasound pulse from
the Bat to receivers
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The Bat Location System
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Entity location is
determined based on
the principle of
trilateration
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3D position can be
also deduced
Orientation of an
object can be
deduced
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The Bat Location System
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Reflection error - Statistical outlier rejection
algorithm
Reverberation 20 ms - location updates
50/second
Scalability issues
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Location Quality of service
Scheduling info can also be used to assist power
saving
The set of Bats to be tracked may change over
time
Handover of control between base stations
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The Bat Location System
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The Bat Location System
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Current deployment
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720 receivers and 6 radio cells to cover an
area of around 1000 m2 on three floors.
The system can determine the positions of
up to 75 objects each second, accurate to
around 3cm in three dimensions
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Environment model
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Describing entities in the real world and their
possible interactions
Sets out the types, names, capabilities and
properties of all entities and acts as a bridge,
allowing computer systems to share the user’s
perceptions of the real world
Ouija – a package provides an object-oriented
data modeling language which is used to
generate an object layer on top of the relational
model used by the Oracle DB
3-tier architecture
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Resource Monitor
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Resource monitor
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Installed on all networked machines
Machine activity
Machine resources
Network point-to-point bandwidth and latency
Techniques to ensure the DB not to be a
bottleneck
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Update frequency
Relevancy
caching
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Programming with Space
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Location-aware applications are interested in
relative spatial facts
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The person is standing in front of the workstation
Express relative spatial facts in terms of
geometric containment relationship
Applications receive a stream of events
expressing spatial facts relevant to them
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Spatial monitoring
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Containment tree indexing
system
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The index system
uses a quadtree
called the
containment index
Maximal cover – the
smallest set of
quadtree cells
required to cover
the space at a
particular resolution
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The containment indexing
theorem
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Space s is contained by space t if and
only if, for each cell x in the maximal
cover of s, there exists exactly one cell
in the maximal cover of t that contains
x or is equal to x
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Bat Teleporting
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Improved application based on Active Badge
System
Redirect X Window System environment to
different displays
Virtual Network Computing system
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Provides a windowing-system-independent means
for a user to access his desktop environment from
any networked machine
Event driven application
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Bat Teleporting cont’
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Two buttons on the Bat
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One is used to allow
selection of an alternative
desktop
The other is used to
override the current desktop
owned by other users
Three relative geometry
conditions are registered
with the spatial monitor
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Positive containment
Negative containment
Negative overlap
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Bat Teleporting cont’
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An application provides human users with browsable
model of the world which they can explore
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Conclusions
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A fine-grained sensor system
A rich data model reflecting the resource information
required to support context-aware application
A distributed system of persistent objects which can
be queried by context-aware application
A resource monitoring system for collecting
information about the computing environment
A spatial monitoring system which allows eventbased applications to be written
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References
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Ward, A., Jones, A., Hopper, A. A New Location Technique for
the Active Office. IEEE Personal Communications Magazine, Vol.
4, No. 5, October 1997. pp. 42-47
Hightower, J. and Borriello, G., A Survey and Taxonomy of
Location Sensing Systems for Ubiquitous Computing, UW CSE
01-08-03, University of Washington, Department of Computer
Science and Engineering, Seattle, WA, Aug. 2001.
Korkea-aho, M., Context-Aware Applications Survey, Helsinki
University of Technology, Department of Computer Science, Apr,
2000
http://www.uk.research.att.com/bat/
http://www.uk.research.att.com/
Harter, A., Hopper, A., Steggles, P., Ward, A., and Webster, P.,
The Anatomy of a Context-Aware Application, Wireless Networks,
Vol. 8, pp. 187-197
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