harging day classes, change group, charging zones, charging case definition, charging attribute subanalysis and results, as well as origin analysis and digit analysis. 4.7 CS Core Network Optimisation The CS core network optimisation is typically a part of the network operation process. It consists of a follow-up of how the planned services are actually provided to ensure efficient utilisation of existing network resources. It also provides a follow-up of the business plan in order to meet the operator's projected service quality, subscriber and service penetration forecast. 4.7.1 Key Performance Indicators The key performance indicators (KPIs) provide information on how the network resources are utilised, whether there is underutilised capacity or bottlenecks and when resources get used up. An initial as- sessment of the CS core network's KPIs is done during the network audit phase. The initial values of the KPIs are determined before the optimisation process begins. Other activities include determining available optimisation tools, available measurements, measurement periods, access to network elements for the optimisation team and reporting procedures to be used during the project. The KPIs to be used in the project are defined during the project definition phase. The performance of the network is evaluated based on the agreed KPIs. The KPIs may comprise a network element's measurement counter informa- tion, external test equipment measurements or other easily accessible data that give relevant information on the quality of service and performance of the network. Some of the measured values may be used directly as KPIs but others require postprocessing to derive the desired performance indicators. The existing documents are reviewed in this module. A follow-up procedure to be included in the project plan is defined for the selection of measurements needed for checking the success of the optimisation project and to define a regular audit plan to be followed in the regular network operation. The main KPIs for th