s can be seen in NSS statistics mainly in the MSC/VLR. The main reports on location updates are provided by VLR measurement (Figure 4.38) and location update observation. All mea- surement data on location updates and IMSI attach/detach operations are collected in the VLR register units. The procedure used in VLR measurement (the register unit is VLRU) is the following. At the beginning of the measurement period, statistics process sends the start information to each active register unit. Then every three minutes, statistics process queries the data needed for averaging the number of registered subscribers from each active register unit. At the end of the measurement period, statistics process queries the full measurement data from each active register unit and saves the data to a file. When all data from each active register unit is received or the time supervision limit is reached, statistics process generates the report and sends it to the report forming service. If statistics process does not get measurement data from each active VLRU, this is mentioned in the VLR measurement report. In a similar way the HLR location update success rate can be evaluated and prepared for presentation. The results show the success rate of location updates from the HLR point of view. While these results tes- tify to the performance of the cellular network, they may, however, provide valuable inputs for evaluation of improvement in the network performance. Other KPIs listed above are evaluated further in this document. A list of the most important KPIs is given at the beginning of each module. The values may be evaluated in the network performance survey module or at initial phases of each module. The paging success rate may be evaluated as presented in the following subsection. Paging Success Rate VLR measurement provides the number of paging attempts through the GSM BSS network and Gs interface (SGSN) for GPRS attached mobile stations or mobile subscribers. In addition to a terminating call, these paging attempts are also updat