oS parameters [2] that are important to end-users and with that to telecom operators is given in Table 9.1. One should note that such a list is not exhaus- tive, but it can be used as a reference point regarding definition of QoS parameters that should be assessed with objective or subjective methods. The targets for QoS parameters are defined as a potential value or a range of values that must be reached for QoS to be regarded as satisfactory. In Table 9.1 the QoS parameters are classified into three classes: Customer interface parameters. They are non-technical parameters for a given service which are subjectively assessed by customers (e.g. by questionnaires). Network infrastructure parameters. They are parameters which refer to availability and proper functioning of the network infrastructure, including fixed and mobile broadband access coverage. These parameters are typically used by telecom operators for network design as well as operation and maintenance. Service functionality parameters. They are typically grouped according to the given service type - for example, call setup ratio refers to voice (not to Web sessions), mes- sage transmission ratio refers to SMS or MMS (not to voice or video), etc. Use of the same service functionality parameters provides the possibility of comparability of operators within a given country or between countries. Also, these parameters can be measured by external agencies. Parameters are named according to the same conventions regardless of how they are fully named in different countries. For example, the parameters of type "rate" define the frequency of certain events (e.g. error rate is frequency of error events), "ratio" is used to denote the proportion of successful events (number of successful events VS. total number of events). Further, parameters of type "time" refer to the average time taken by certain successful actions (e.g. packet delay refers to the average delay of all successfully transmitted packets). 9.5.2 Setting Target Levels and Making Measurements The def