good and excellent voice quality). On the market some tools exist that have algorithms designed to measure speech quality objectively and deliver measures that are mapped into the MOS scale. Generally, audio quality depends not only on the radio link quality and codec capacity but also on the type of handset used, background noise, speech content and echo problems. Radio Network Planning and Optimisation Table 2.20 Correlation between the BER and FER BER and FER are very bad Quality is confirmed to be bad Either BER or FER are very bad No conclusion can be made BER and FER are good Quality is good Correlation Practically speaking, the best KPI to monitor speech quality is the one that gives a combined indication on BER and FER. A certain correlation has to be considered between these two KPIs (see Table 2.20). Tracing The optimisation process is an ongoing operation, which becomes more and more complex as the network matures and becomes larger. The first steps of optimisation consist mainly of standard cleaning operations such as frequency correction, adjacency handling and site engineering review. At a certain phase, fine- tuning of parameters and a complex troubleshooting cell-by-cell start become necessary. The principle behind tracing relies on analysing the flow of messages. This assumes certain a knowledge of signalling charts and call phases (explained later in the section). Several types of tracing exist; they can be summarisd into two categories: online tracing like interface tracing; offline tracing more commonly called observation. Abis/ Interface Trace This type of tracing consists of plugging a special tool on to the wanted interface in order to intercept all layer 3 messages. It is a continuous tracing in the time. However, a review of the trace for analysis purposes is possible offline once completed. Some filtering is possible at the beginning in the setup parameter definition phase in order to reduce the size of the output log file. Figure 2.47 presents an example of the Abis trace model. The lef