e and data, as well as cell reselection algorithms, media access control, location areas, routing areas, handover criteria, number of subscribers (voice and data), processing and switching capacity, number of PDCHs supported simultaneously, etc. It would also be useful to have the data from any previous optimisation that was done for the GSM network. Performance Data Performance data collected usually relate to reliability and average throughput. From a user's perspective, reliability can be understood as success in call establishment and call release. The average throughput can be understood as the throughput with a desired quality as perceived by the subscribers. Key Performance Indicators Chapter 2 covered the voice KPIs of the radio network. Here we look at some of the data KPIs. In a GPRS network there are a variety of services that can be present, with each service having a unique performance requirement. The quality of each service can usually be negotiated between the mobile station and the network, and should be maintained during the duration of the call. As mentioned above, reliability and average throughput are the two main criteria behind the process of defining the KPIs. Apart from these two, other important functions on which KPIs are based are load, utilisation, blocking and delay. As reliability is about call establishment and release, the key performance indicators are blocking in the uplink and downlink directions both for PDTCH and TBF. For the average throughput, the uplink and downlink average throughput could be measured. For enhanced reliability, re-transmissions are done on the air interface (i.e. in the RLC layer). PDTCH channels carry the user data, re-transmission and control information. For loading, data Erlangs is used as a key performance indicator and is based on the number of radio blocks per duration of busy hour. The amount of hardware used for data services can be indicated by the utilisation factor, which describes the average amount of GPRS time slots that are used for PS ser