ng the relevant procedure. It must be noted, however, that those subtypes of the mentioned procedure are also strongly linked to the actual network topology and size, and even to the actual architecture of the network nodes implemented by the concrete vendor. 4.14.3 Key Performance Indicators Since every vendor has a unique type of network node and unique capacities, it is important to have a set of well-defined KPIs that help the performance to be understood in a consistent manner, even in the case of a multivendor network or even for a mobile network operator having several operations in different countries. The next sections will present briefly some of the most relevant KPIs to be considered during packet core network optimisation. It should be noticed that most of them are actually generic, and they are also used in BSS or RAN optimisation. In fact, these are the same KPIs calculated in different parts of the network, and therefore their values can be different. Nevertheless, a proper study and correlation of the different values of a given KPI, as measured in different parts of the network, will provide invaluable information on where to concentrate optimisation efforts. It should be noted also that the area of performance management is in constant evolution, following the developments of the new capabilities enabled by the mobile networks. Therefore, a limited set of KPIs will be presented, which should be enough to illustrate the concepts. Network Accessibility With regards to accessibility the KPIs actually measure the capability of the network to provide the user with an access to the service as fast as possible. Firstly, a service-independent network access, or network accessibility, needs to be considered, i.e. the capability of the mobile station to attach to the expected GPRS network. More precisely, it can be defined as the probability that mobile data services offered to a given subscriber, as displayed in the network indicator available on the mobile equipment. In more concrete terms, service-ind