list of all aspects that could influence the QoS. There are three models for this purpose [13]: Universal model. This is a generic and a conceptual model. Performance model. This model is more suited for determining the performance cri- teria of a telecommunication service. Four-market model. This model is particularly suited for multimedia services. 1.4.1 Universal Model In this model all QoS criteria are grouped under four categories: Performance criteria. These cover the technical parts of the service, and can be qual- itative or quantitative (or both). They are further defined within the performance model (next section). Esthetic criteria. These refer to ease of interaction between the user and the telecom- munication service (ergonomic aspects, ease of use of the service, style and look, design of functionalities of the service, etc.). Presentational criteria. These reflect the presentational aspects of the service to the customer, including the packaging of the service, tariff models for that service, billing options for the service, etc.). Ethical criteria. These are related to the ethical aspects, such as conditions specified for cutting off the given service, services for disabled users, etc. The service is split into functional elements, where each of the is cross-checked against the given four quality components and criteria. However, the definitions and measure- ment methods of the quality parameters are not a part of the universal model, but they are defined within the performance model. 1.4 ITU's QoS Framework 1.4.2 Performance Model The main aim in the performance model is to determine performance criteria which are further used for defining QoS parameters important to both users and providers [13]. In total, there are seven specified QoS criteria [13], as shown in Table 1.2. They are identified to provide easy translation into QoS parameters. The given quality criteria (Table 1.2) are mapped on a set of service functions, which include service management (sales and pre-contract activities, service