h the pricing policy. GSM standards offer the possibility of monitoring the network by computing statistics. Depending on the type of network management system, either in the BSC or in the BTS, each cell reports thousands of statistics about all relevant behaviours (number of attempts, failures, successes, during call, handover, setup, etc.). These statistics are reported to the Network Management System (NMS) as counters. To facilitate interpretation of the behaviour, a set of key performance indicators (KPIs) is defined out of formulas using pure counters. Each operator chooses its own KPIs and sets, according to specific criteria, some objectives to be met in order to achieve a good end user perception of the service offered and also in order to benchmark one network with other operators. Another aspect that is important in the optimisation phase deals with drive tests. In fact, while statistics give a general idea of the cell's behaviour at a certain period, field measurements give a one instant scenario of one area's behaviour during a call. Different tools can be used to perform drive tests. Each specific tool is able to report measurements such as MOS (mean opinion score), FER (frame erasure rate) and BER (bit error rate) in addition to standard reporting at the signal level, quality and site information (cell identity, BCCH, mobile allocation list, best neighbours, etc.). Statistics and drive tests are the main methods used to monitor the network's performance. However, other specific methods can also be used. Tracing catches one object's behaviour (TRX, cell, BTS or BSC) during a certain period and regarding a specific event (SDCCH allocation, conversation phase of a voice call, etc.) or a set of successful events (IMSI attach, paging, call setup, location update, etc.). Alarm monitoring, transmission network auditing and network switching subsystem (NSS) performance follow-up are also important in the sense that they give an idea of hardware problems or parameter errors, which definitely affect the end 