ad to drop calls. Owing to its importance, the handover success rate is often specified as a key performance indicator. It must be noted, however, that a handover failure does not automatically mean a dropped call or bad quality. Most networks have a fallback mechanism where the call continues on the old cell and a new handover attempt is made after some waiting time. Handovers can also be triggered by other variables, such as capacity, call quality and mobile speed. A typical handover success rate for mature 2G networks is of the order of 95% as measured from network statistics. Call Set-up Time The call set-up time is the time it takes for the user to get into a call. Again different operators worldwide define this differently. A common requirement is that call set-up time is the time measured from when the user presses the 'dial' button to when the ring tone is heard. Call set-up time can be measured by both drive tests or by the system itself. Owing to the current trend of having different vendors supplying different sub-systems (e.g. the radio and the switching parts supplied by different vendors), drive testing seems to be the test relied on for this. To improve accuracy of measurement and to remove ambiguities, the call set-up time defined as 'the interval between the dial command to the mobile and the receiving of the layer-three message 'alerting' for 2G GSM networks. Similar definitions can be arrived at for other 2G and 3G systems. Current trends seem to indicate a call set-up time of 4 seconds for a mobile to PSTN call and about 8 seconds for a mobile-to-mobile call. The mobile-to-mobile call takes longer since the called or 'B party' mobile has to be authenticated by the network. SERVICE-SPECIFIC KPIs The call set-up time depends heavily on the type and amount of authentication used on the network. Call set-up times are faster by at least 2-3 seconds where authentication is not used. D.3 SERVICE-SPECIFIC KPIs Voice Call Quality It was noted in the early days of 2G networks that the quality of the bea