 number of EGPRS users may be limited during the launch phase. In such cases the solution is to assign another PCM line to cater for the voice user of certain cells and a primary PCM can then be used for the DAP with more timeslots. This also results in a lower number of PCU units assigned in the BSC or a limit in the number of signal processing units. There are certain counters in the management systems (vendor dependent) that should be monitored before making changes. With careful assessment, the number of timeslots in the DAP and/or PCU units in the BSC should be increased. Quality There are few parameters that are measured for quality of microwave links. Some parameters in the ATM sections have been observed to find the quality of the network. However, these parameters define only the quality of the ATM network. To determine the performance of the access transmission network, the parameters should be defined with the network operator. For the access transmission network, especially GSM networks, there are not many parameters when compared with those in the radio network. However, as the main focus is on capacity and quality, the parameters defined, performance monitored and optimi- sation done revolve around these two aspects. Some basic parameters defined in ITU Recommendation G.826 on error performance and availability need to be understood. The Recommendation gives the basic error performance definition and related parameters. Transmission Network Optimisation Error Performance Events Errored Block (EB) This is a block in which one or more bits are in error. Errored Second (ES) This is defined as a 1 second period with one or more errored blocks or at least one defect. Severely Errored Second (SES) This is defined as a 1 second period containing > 30 % errored blocks or at least one defect. SES is a subset of ES. Error Performance Parameters Errored Second Ratio (ESR) This is the ratio of ES to total seconds in available time during a fixed measurement interval. Severely Errored Second Ratio (SESR) This is