period. Cessation of service Parameter 88: Contractual The ratio (%) of the number of contractual cessations achieved cessations requested to the total number of such requests made within a specified period. 7.3 Interconnection and QoS means that are available to them). They should be simple to use and measure, to provide accurate representation of the customer perception. In some cases QoS parameters are defined with standards. One may distinguish between QoS parameters standardized for different service types. For example, regarding the voice services there are several ITU-T standards that define voice-related QoS parameters (ITU-T G.107, G.108.2, G.109, P.862/P.863, and others). Also, the E-model (also discussed in Chapter 6) provides a scalar rating of transmission quality, but it requires knowledge of the end-to-end configuration (e.g. networks, terminals). Another classification of QoS parameters is regarding whether they refer to ser- vice utilization or non-utilization. So, besides service utilization (or one may name them as operational QoS parameters) such as packet delay, loss, jitter, etc., there are non-utilization QoS parameters such as those standardized in ITU-T E.803 [5], which lists 88 different generic QoS parameters. Examples of non-utilization generic QoS parameters are given in Table 7.2. Generally, QoS parameters are also called QoS metrics, QoS indicators, QoS mea- sures, or QoS determinants. Regardless of that, all QoS parameters (including those that refer to service utilization and those that are non-utilization based) have a goal to charac- terize the quality level of the service being offered and the level of customer satisfaction (the latter refers to human end-users). In cases when services are used by humans as end-users, the QoS parameters represent subjective and abstract user-perceived "qual- ity" in terms of numeric (i.e. quantified) values. The QoS parameters can be obtained with two main methods: (i) objective measurement, and/or (ii) subjective measurement (based on subjecti