An Architecture to Support Context-Aware Applications

Anind K.DeyDaniel Salber

Gregory D. AbowdMay 1999.

Introduction What is Context?

Information Application’s operating environment Sensed by the application Actions based on current state

Context Aware Applications Use context to provide task-relevant information and/or

services. eg.tour guide, conference assistant, context-aware mailing

lists, Dynamic Ubiquitous Mobile Meeting Board (DUMMBO).

Problems with using context

Lack of standard architecture Ad-hoc design, very little reusability Acquired from non-standard sources Abstraction to provide meaning to

application Dynamic

Context Handling Direct Hardwiring of Context

Code to handle sensor details embedded in application

Bulky applications Loss of generality, difficult to reuse or use by

multiple applications

Using Servers Polls sensor network Maintains current location information Abstracts details of sensors from application Multiple applications Applications must be proactive Each server offers a different interface

Widget Software component

Provides access to context information Hide complexity of actual sensor network used Abstract context information according to need Provide reusable and customizable building blocks

State Set of attributes that can be queried by an application

eg. Location, identity

Behavior Responds to changes in environment Provides callbacks to applications registered with it

Differences from GUI widgets

Distributed Architecture 3 components viz.

Generators – acquire context information Interpreters – abstract information Servers – Aggregate information

Active all the time Activation not driven by other applications

Distribution of context sensing network

Remote context sensing devices Eg. Indoor infrared positioning system

Multiple applications use same context information

Heterogeneous platforms for collecting context

Interoperability of context widgets and applications.

Context Abstraction

Need for interpretation to make context usable Transparency of low-level layers needed

Extension of existing mechanisms

Aggregation – Applications need to connect to only one component for each entity

Aggregation Improved maintainability and efficiency

Persistence and History

Independent execution of context widgets

Persistence required at all times

Maintenance of history necessary Storage of history on applications not possible always

Architecture Requirements Requirements summary

Allow applications to access context information from distributed machines in the same was that input information is accessed

Support execution on different platforms and the use of different programming languages

Support for the interpretation of context information

Support for the aggregation of context information

Support for independence and persistence of context widgets

Support for storage of context history

Architecture

Application

ServerInterpreter Interpreter

Widget Widget

Sensor Sensor ContextArchitecture

Context Widgets Attributes

Interface provided to other components Accessed via polling and subscribing

Callbacks Events used to notify subscribing components

Methods sendToSubscribers() – notifies concerned subscribers about

change in context state store() – stores historical data for future reference

(mechanism can be modified)

Context Server Single point of access/subscription to all widgets of

interest – proxy behavior Further separation, simpler design

In/Out Board

Building Aggregatio

ns

LocationWidget

LocationWidget

ID to Name

InterpreterFace

Recognition

Smart Card

Reader

Context Interpreter Convert or interpret context to higher level of

information and representation Context not available for appropriate level

In/Out Board

Location Widget

Location WidgetID to name

interpreter

Face Recognitio

n

Smart Card Reader

Callback Model

Transparent communications, always available

SensorLocationWidget

BaseObject

BaseObject

Application Handle

a.Subscribe Gregoryin Room 343

b.Sensor Dataarrives

c.Callback if dataMatches

Gregory in 343:

d.Callback deliveredto a handler

Conference Assistant

Application

Recommender/InterpreterUser

ServerPresentation

Server

QuestionWidget

ContentWidget

LocationWidget

MemoWidget

RegistrationWidget

Applications

In/Out Board

Serendipitous Meetings/Whiteboard

Aware home

Conference Assistant

Conclusions Context …

Enables context-aware applications to build smart environments

Treats context like user input Makes it easer for applications to deal with context Provides reusable solutions(widgets) for context

handling Achieves separation of concerns between context-

sensing and application semantics