ks, such as Gi. However, one aspect does change with 3G networks, and that is the timing of the opti- misation. With GSM networks, core network optimisation did not usually begin instantly after the network launch. It took some time (a few months to a few years) before it was realised that core network optimisation was needed, whereas the radio network optimisation cycle started almost in parallel with network planning. In some cases of 2.5G networks, and almost all cases of 3G networks, core network optimisation will be an integral part of network optimisation. The reason for this is that quality of service in 3G is considered from an end-to-end perspective, rather than from an individual network subsystem perspective (see Figure 9.11). As quality of the packet data is SO important, the following section focuses on QoS of packet data and ways to improve it. End-to-End QoS Radio Transmission CS-Core PS-Core Network Network Network Network Figure 9.11 E2E QoS structure 9.5 END-TO-END QUALITY OF SERVICE Mobile subscribers' perspective of the quality of the network is what matters, especially in third-generation networks where QoS is the backbone of design and implementation. All the three network subsystems (radio/transmission/core) contribute to the quality of services delivered. For a given application, a group of parameters are identified and are called QoS param- eters. One important aspect of optimisation is to keep this parameter group as precise and small as possible, because having too many parameters leads to increased complexity in the network and in the optimisation process. Each parameter group has control mechanisms, because the quality of any given application should be negotiable. As we have seen, four traffic classes are defined in UMTS networks: conversational, streaming, interactive and background. Various applications fall under each category, but 3G CORE NETWORK PLANNING AND OPTIMISATION technically speaking these four classes can be differentiated by two factors: maximum permissible delay maxim