 to monitor the network's performance besides drive testing. Monitoring the network is a key element to achieve the best quality of service. QoS monitoring involves permanent observation, qualification and adjustment of various network parameters. The objective of this subsection is to present and detail all aspects related to statistics extraction, handling and exploitation. Use of Statistics The notion of statistics in mobile networks refers to a general set of metrics that helps the operator in three main directions: First, assess the performance of the network and benchmark it to other networks. Second, analyse faults and check improvements. Finally, dimension extensions for the network. These metrics are directly generated through real subscriber traffic. Each event that occurs in the network (originating/terminating call, handover failure, location update, etc. ) is reported to the NMS. With a mechanism, the operator does not have to invest in any simulation tool to model the performance of the network towards a certain subscriber's profile. In dealing with statistics, two elements should be distinguished: Pure counters (elementary performance indicators, or PIs), which are incremental values of events generally with no significant relevance if handled individually. They provide data on a specific aspect (number of calls, for example) but, practically speaking, it is hard to interpret their values. Radio Network Planning and Optimisation Key performance indicators (KPIs), which are computed formulas based on PIs, better translating the subscriber's experience. Network Assessment Most operators have chosen relevant KPIs to monitor. They have also established targets to be met in order to achieve the wanted level of end user quality of service. The idea is to check whether the relevant KPI meets the target terms. If not, then troubleshooting starts to identify the faulty element or the network bottleneck. Operators use KPI targets to benchmark their network with others and to benchmark different suppliers in 