 provision, alteration, service support, repair, and cessation), connection quality (connection establishment, information transfer, connection release), billing, and network or service management by the customer. The mapping between the service functions and service quality criteria is referred to as a performance model (in ITU-T E.802). The QoE is influenced by all seven QoS criteria given in the performance model. For example, speed impacts the available bitrates (in downlink and uplink) and latencies (i.e. delays), which is crucial for the end-user's experience of the service. Upgrading to higher access bitrates (including fixed and mobile networks) improves the overall QoE. Further, availability and reliability are also very important and are directly related to planning and dimensioning of the network (for a given number of users and for a given service) as well as to its operation and maintenance functions and procedures. For example, one typical quality metric for network availability which is used in the traditional telecom world (at the end of the twentieth century) is so-called "five nines," that is, 99.999% of the time service to be available to the end-users, and that poses certain requirements regarding the survivability mechanisms which need to be implemented in the network (e.g. auto- matic traffic redirections over alternative paths in the network in a case of link or path failures). Also, one may note that security aspects - accuracy (e.g. billing accuracy), sim- plicity of use of the service (the user should not be required to read a manual in order to use an offered telecommunication service), and flexibility regarding the service use (e.g. ease of change of tariff model or billing method, or even changing the operator in a case of QoS degradations) - influence the QoE. Table 1.2 QoS criteria. QoS criteria Applying QoS criteria to different service functions Speed Service supply time, call setup time, one-way delay, release time, billing frequency, etc. Accuracy Unsuccessful call ratio, speech