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5.3. The Artifact-Based Approach
75
then related to the domain of the application, e.g. it is tagged with knowledge or
related to a plan.
The output of the application-layer is tightly related to each particular application.
In general it’s output is reflected in the functionality and user interface of the ap-
plication. Typically there are changes in the presentation of the application to the
user. It may be screen based or by use of actuators. The output also includes the
application classes, such as context-triggered actions and contextual information as
specified by Schilit
[
Schilit et al. 1994
]
.
The application layer is the least structured one. Its functionality and what is
implemented is highly dependent on the actual application. However, providing a
defined interface to settings is very helpful when implementing new applications.
5.3.2 Implementation
This section describes the implementation details of the described model. We start
with a short introduction into the underlying hardware including its specific require-
ments and then present the details of the implementation.
Ubiquitous Computing Hardware
Custom hardware developed during the Smart-Its project
[
Beigl et al. 2003
]
was
the basis for the design of the artifact-based approach. The hardware was built
up in a modular fashion in order to separate concerns as much as possible. It is
composed of a sensing and a communication part. Both modules are built upon a low
cost micro-controller (Arizona Microchips PIC16F87x). The communication board
runs with a time-slotted protocol and features a radio frequency transceiver with
a digital potentiometer to vary signal strength. The general sensor board contains
a variety of sensors including a temperature sensor, a light sensor, an acceleration
sensor, a pressure sensor, and a microphone. The sensor selection is similar to
what comparable systems use
[
Farringdon et al. 1999
] [
Mainwaring et al. 2002
]
.
Additional sensors or specific sensor boards can easily be attached using either I2C
or serial communication. The standard way of building self-contained units is by
connecting the RF-module with sensor modules via the I2C communication protocol.
When the sensor boards are used with a PC or PDA hardware serial communication
is the default. Combined, they form a basic building block for ubiquitous computing
applications. During the project, 160 of these devices were built and distributed to
the various project partners.
Besides the custom-built hardware, PDA’s, PC’s, and notebook computers were
used, either as additional mobile units or as background infrastructure.
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