Ubiquitous Computing has become increasingly important, thanks to its attractive aims to provide invisible computing everywhere and anytime. Dealing with Non-Functional Requirements (NFRs), such as usability or privacy, during requirements engineering for Ubiquitous Computing, however, is oftentimes a di cult task, given the unique features of this paradigm (e.g. context-awareness or technological unobtrusiveness). In this paper, we present an approach in which pieces of knowledge about recurring situations in ubiquitous systems are identied and clearly captured as patterns. The approach is illustrated through an example of an e-Learning ubiquitous system.
Dealing with Non-Functional Requirements (NFRs), such as usability or privacy,
during requirements engineering for Ubiquitous Computing [
Consider an e-Learning ubiquitous system, as shown in Figure 1. It depicts
three situations, where some concerns have not been addressed properly: (a)
phones start to ring during a class, when they are not supposed to; (b) devices
expose sensitive data to some people that should not be aware of the data; and
(c) the security mechanism to access the classroom/lab asks a person to enter a
password to open the door. These three situations are contrary to some of the
key principles of Ubiquitous Computing, such as context-aware adaptation or
unobtrusiveness of the technology in the user's daily activities. These situations
have something in common - a bad assessment of di erent Non-Functional
Requirements (NFRs), such as usability, privacy or security [
Undoubtedly, clearly capturing knowledge about such problems and
solutions to them for later explicit reuse would be highly desirable. In this paper,
we present an approach in which pieces of knowledge about recurring situations
in ubiquitous systems are identi ed and clearly captured as patterns, and later
explicitly reused across di erent domains. The use of patterns during
Requirements Engineering is not new. It has been explored in previous works [
The rest of this paper is organized as follows. Our pattern approach to dealing with NFRs for Ubiquitous systems is introduced in Section 2, where di erent types patterns are discussed, together with their bene ts and drawbacks. The approach is illustrated through the application of patterns to an e-Learning Ubiquitous System in Section 3. Related works are described in Section 4. Finally, a summary and contributions of the paper, together with future work, are described in Section 5.
In this section, the core of this proposal is presented. It consists of a set of Non-Functional Requirements Patterns for Ubiquitous Systems that have been identi ed from the study of existing systems and the experience of the authors in the construction of ubiquitous applications. The patterns presented in this section are a small subset which has been chosen to illustrate their need. The patterns presented above are a subset from a larger set that have been identi ed. Moreover, there are di erent dimensions to organize them that can enable the appearance of new patterns by their generalization, specialization, composition or instantiation. 2.1
Ubiquitous systems are essentially interactive. Thus, the user and the system
needs to exchange information with each other using natural interfaces that,
at the same time, are not obtrusive to the user. The way the system outputs
information to the user and the user provides input to the system should not
be xed. In fact, it depends on contextual circumstances. Depending on the
environment that the user is, the system may decide to provide the information
in a di erent way, or even do nothing, if that causes a disruption in the user's
activities [
The Context-sensitive I/O Pattern provides a possible solution to model this situation, which is recurring in di erent ubiquitous systems. Consider that there is a goal, G, that aims to provide or receive information from the user. Many di erent operationalizations, O1 : : : On, may contribute to the satisfaction of this objective, with di erent levels of contribution. Finally, some context situations, C1 : : : Cm, are relevant to the system. The pattern proposes to relate the operationalizations to the goal, conditioned by the occurrence of some context situations. Formally speaking, Oi contributes to G under the context situation Cj (Figure 2, top). 2.2
Preservation of sensitive data is a general challenge for software designers, which
also holds true for ubiquitous systems [
To achieve this task, either the user provides data to feed the adaptation mechanisms, or the system records events from the user's daily life and applies mining techniques to infer the user pro le from user's behavior, and try to anticipate the customization. Dangerous scenarios can happen when all this sensitive data have to be exchanged with others. Adaptations in new environments or data exchange with other users, or between applications on their behalf, can lead to the exposure of sensitive data that the user does not want to share. In order to overcome this situation, the Context-sensitive Data Sharing Pattern proposes to take into account the trustworthiness of the context situation to determine how much information is shared. To achieve this, user sensitive information is organized in a hierarchical structure, where the bottom level nodes provide few user information, and the top level nodes provide a lot of information. Formally, operationalization Oi (that enables the exchange of data Di) contributes to the satisfaction of goal G under context situation Cj (Figure 2, middle).
When this structure has been created, each node represents the amount of
information that may be shared. These nodes are related to the context situations
where they have to be shared, in such a way that the lower the trustworthiness
of the situation is, the less information is shared. Also, it enables to provide
the right amount of information at each time, enhancing the accuracy of the
information transmission.
In many cases, users need to do security-sensitive activities, either because they
deal with private information or because they access to certain devices which
use is restricted. Thus, the ubiquitous system has to determine the identity of
the user to guarantee access to the desired data or operations. However, the
paradigm claims that this authentication should be done in a transparent way,
making the analyst to face a tradeo between security and unobtrusiveness [
The application of the proposed patterns to the running scenario is as follows:
{ Context-sensitive I/O: When students are communicating with each other,
the system may need to deliver messages (i.e., output some information).
Depending on whether they are in class or not, the device should vibrate or
ring, respectively, in order to avoid disturbances in the class (Figure 3, top).
{ Context-sensitive Data Sharing: The e-Learning system supports
communication between users through nearby devices discovery. In these cases,
the user needs to provide some information about his/her identity and
sensitive data. In order to provide accurate data, s/he may decide to share
his/her student records to School administrative sta , and not to share any
information to unknown users. Outside school, s/he may, for instance, share
his/her medical data to hospital services sta (Figure 3, middle).
{ Unobtrusive Authentication: Consider two possible situations with
different security levels: accessing a lab and accessing to someone's private
account. In this case, unobtrusiveness and security are decomposed taking
this into account. For the former, using Near Field Communication (NFC)
[
Figure 4 depicts the resulting scenario after the application of the patterns. The problems that were presented at the beginning of this paper have been addressed through the application of the proposed patterns. Also, some NFRs have been enhanced. Since I/O is adapted to the context, the number of unnecessary interruptions decreases and the users are more likely to accept the system. The amount of data that are shared is more accurate to the target receiver, and it avoids sending private data to unknown users as well. Finally, users can perform normal activities, like accessing a class, in the usual way, but still being secured. 4
The notion of design pattern in software engineering has been present for several
years since the appearance of the book by the Gang of Four with a set of
patterns for Object-oriented design [
The use of patterns is not limited to design; requirements patterns can also
be de ned and used, although systematic treatment of NFRs through the
application of patterns has not been widely explored. For instance, Withall [
The Context-sensitive I/O pattern can be de ned from Mark Weiser's initial
de nition of Ubiquitous Computing [
Dealing with NFRs for Ubiquitous Systems has to address their unique features,
which oftentimes requires the use of a rich body of knowledge. In this paper,
we have presented a pattern-based approach to explicitly and clearly
capturing, organizing and reusing such knowledge, with promising results in some of
our developments [
For future work, we are planning to build a repository of patterns, which can be searched and grow with new knowledge. Currently, we are investigating how to achieve a high quality design based on a pattern-based requirements analysis. Also, we are planning to study the implications of these patterns in the software design stage, as well as analyzing those techniques that enable model transformation to (semi-automatically) derive designs and implementations following a model-driven engineering approach.
This research work is funded by the Innovation O ce from the Andalusian Government through project TIN-6600, the Spanish Ministry of Economy and Competitiveness through the project TIN2012-38600, and The Spanish Ministry of Education, Culture and Sports through the FPU Scholarship.