 existing transmission network is likely to need to be upgraded. GPRS: NETWORK PLANNING AND OPTIMISATION Second, the coding schemes in the GPRS network need to be reviewed. Usually CS-1 and CS-2 are used during the launch phase. If CS-3 and CS-4 are launched during optimisation or at a later phase, the Abis time slot allocation will be changed. 5.6.3 Core Network Optimisation Packet-core optimisation focuses on four issues: PCU/BSC and SGSN capacity, PDP functioning, and overall network security. Generally the SGSN capacity is not an issue. However, as port requirements for the Gb interface increase, then it is possible that a new SGSN will be required. However, based on the criteria and limitations mentioned earlier in the section on TNP in a GPRS network, there is a possibility that the number of PCUs will have to be increased. There may be problems related to PDP functionalities. This may be due to incompatibil- ities between different network elements or signalling such as HLR and SGSN or signalling between SGSN and GGSN (GTP signalling). Another problem can be congestion on the Gb interface due to a sudden increase in data traffic. The Gb interface might need to be readjusted for such a scenario. Security is another issue. Owing to variation in the types of data being exchanged through the GGSN, the measures against security infiltration may need to reviewed. EDGE: Network Planning and Optimisation 6.1 INTRODUCTION GPRS networks are able to handle higher bit rates than GSM networks, but the data rates still fall short of what is required to make existing GSM networks deliver services at a speed comparable to that promised by third-generation networks. The delay in the deployment of third-generation systems led to the emergence of a technology known as EDGE. This was capable of delivering services similar to those of third-generation networks, yet with implementation on the existing second-generation networks (e.g. GSM). EDGE stands for 'enhanced data rates for GSM evolution'. The enhancement from GSM was to 