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Figure 4: Problems related to size and power consumption.
that enables the users to take control on those circumstances requiring it, but
also optimizes power consumption, as in [17].
In all cases, some form of feedback is not only assumed but it must be fully
understood by the user. This assumption, however, cannot be held true as the
presence of numerous and diverse devices in the environment may make it
difficult to recognize which system is operating at any given time and where
feedback is to be expected from. In general, it is preferable to embed feed-
back and control capabilities in everyday objects [18]. Diagnostics must be
embedded into the systems and not based on dedicated displays or consoles;
moreover, it should clearly indicate the exact location it is referring to, the
systems involved and suggested actions (if possible) to correct the situation. At
the same time, the diagnostics systems must also continuously interact with the
user, ensuring that everything is working properly [4]. Control loss and lack of
feedback problems and issues are summarized in Figure 4.
5.3 Breaking the traditional mental model
The design of a PC-based system cannot abstract from the design of a usable
interface that complies with Schneiderman's golden rules [19]. Besides these
physical criteria, the design must be in harmony with the user's mental model
of overall system behaviour [18,20]: the user must know and somehow
expect device capabilities and limitations. These arguments, however, are
completely extraneous to the ubiquitous computing paradigm, where a
central PC does not exist and many interacting devices leave often the user
without choice but that available for the specific interaction context. In a
ubiquitous system, there is the need to build a new mental map of the devices
at work without any explicit information on their possible uses. Rather, a user
must rely on the natural way to evolve with own knowledge, based on two
complementary modes: analogies between new and known already exper-
ienced situations and stimuli derived from the surrounding environment.
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