le to evaluate the actual speech quality, and hence the integrity aspects, from its own category in this type of approach. Retainability The concept of retainability refers to the ability to retain use of the service until the session is released by the user or it is disrupted. Different types of ratios have been defined for the different services from an end-to-end perspective, such as the streaming reproduction cut-off ratio, which is described as the probability that a successfully started stream reproduction can be terminated by any reason not involving the user's own will. Other types of similar KPIs defining a service measuring retainability are, for instance, the MMS end-to-end failure ratio or the MMS notification failure ratio. 4.14.4 KPI Monitoring Monitoring KPIs is at the core of the optimisation processes. It consists of the processes and methodologies accounting, in a meaningful way, the values of the KPIs in different dimensions or aspects, usually having time as the horizontal axis reference. Time can be defined for different levels of granularity, which may make a significant difference, depending on the context. A trade-off between a fine time resolution and a significant amount of data is normally one of the dilemmas when setting up the monitoring process. Some of the next sections provide a follow-up in monitoring. Tools and Methodologies The basic tools utilised in packet core optimisation can be classified into two main categories, OSS tools and tracing tools. They are closely related to the fields of operational monitoring and troubleshooting. From the point of view of performance management as the basic competence in optimisation, monitoring is a daily routine task, which will eventually trigger the need for troubleshooting. Monitoring Monitoring is usually one of the main requirements for an OSS, and most of them will provide enough monitoring capabilities to be utilised in the packet core network environment. Normally, the optimisation engineer will be a user of those monitoring tools. T