## 4 Outline of the Approach
### User Profile Structure  
The task of building a flexible ubiquitous user model ontology that represents knowledge of all possible user profiles from providers and consumers actually used by a person and to come is very ambitious. As we described in the related work, developing a commonly agreed standard ontology for sharing and reusing information in heterogeneous sources for different domains is not a feasible solution. We build a ubiquitous user model that starts with a user profile structure representing generic settings according to recommended standards and most commonly used applications and devices. From this setting on, the ontology and therefore the ubiquitous user model evolves and changes the profile structure to adapt to new actors of the user modeling interoperability process. With the purpose of defining the start-up user profile structure for our ubiquitous user model, we reviewed some international initiatives towards standardization of user profile structure; we analysed the nature of the information to determine its relevancy and permanence in the ubiquitous user model; we set up our user representation for some selected commonly used applications and devices. We reviewed the following initiatives towards standardization of user profile structure: European Telecommunications Standards Institute (ETSI) [33], 3 rd Generation Partnership Project (3 GPP) GUP [34] and MAGNET Project [35]. With the above guides in mind, we established standard requirements for our user profile structure taking into account the recommendations relevant to our purposes. * The user profile structure must at least consider information of basic profile with demographic data and general user data, device and service profiles, user preferences and interest. The profile should be flexible to be used in changing environments. * Redundancy must be avoided. And if a specific data exists in one supplier profile, other profile actors considered in our system must be able to consume it or enrich its profile with it. (_Cfr._ ETSI Guideline 4.1.4.a). * The user profile model provided must be directly usable by profile providers and consumers or for it to be translated. (_Cfr._ ETSI Guideline 4.1.5a). According to the first request, the user profile structure contains concepts to define knowledge in the dimensions shown in Figure 1. Each of these dimensions is modeled with a corresponding skos:Collection of our skos:ConceptScheme in the ubiquitous user modeling ontology described in Section 4.4. The profile structure is flexible because new concepts and sub collections can be added to these dimensions. Adding new dimensions dynamically is been considered. If we analyse the nature of the information of user profiles, we find that some data are static like name or home town, some data are semi-static for example allergies or disabilities, and some data are changing constantly for instance current location or daily calorie intake. Every data is important for personalization and adaptation of one particular application in current user situation, but dynamic and semi-static data lose their validity in a period of time. The nature of the information influences the permanency and validity in the ubiquitous user model, but it is not determined by itself. For this reason we included static, semi-static and dynamic concepts in the start-up configuration of the ubiquitous user model. In addition, we have to provide the ontology set up with representation of the profiles of most commonly used applications. We justified our selection of FOAF, Facebook and personal devices with sensors as profile suppliers and Web services as profile consumers in Section 2. We consider they are representative of most commonly used profiles and consumers. Personal Health Records contain valuable health information and will be included in the ubiquitous user model, but they are not widely adopted yet.