ance Indicators Key performance indicators (KPIs) can be defined as a set of results that measure performance in the busy hour or average period on the entire network. The KPI is the result of a formula that is applied to performance indicators (PIs). PIs can be extracted at an area, cell, TRX or adjacency level. Hundreds of KPIs exist. They use counters from one or several measurements and can be mapped directly to one counter or a formula of several counters. The period of observation refers to the duration of collected samples: hour, day, week, month, etc. The area indicates the location and the sites where the statistics are collected. Formulas Formula means a mathematical combination of counters that results in a meaningful indicator. Defining a formula using several PIs helps to identify a KPI. As explained before, the KPI gives more flexibility and clarity to the operator in interpreting the network's behaviour. For one KPI there can be several different formulas available, depending on three elements: the capabilities of the equipment to provide detailed level of statistics; the track request of the operator; the area where the formula is calculated (cell or group of cells). Formulas, once chosen, should stay unchanged in order to observe the evolution of network performance in the time. In a multivendor environment, the operator sets a performance strategy and defines formulas for each KPI. Then as each equipment triggers its own counters, formula terms are mapped to the corresponding counter for each vendor. Detailed examples of KPIs are presented later in the chapter. To approach this notion of using different formulas for the same KPI, an example will be considered. Example: Drop Call Rate (DCR) From a user's perspective, the dropped call refers to the case where the voice communication is interrupted by the network. In this scenario the formula is logically: DCR % = 100 X Nbr drops / (Nbr calls + Nbr Inc-HO) where Nbr drops = number of call drops that occurred during the conversation phase Nbr calls 