statistics such as the ES (error seconds) and SES (severe error seconds) should be analysed. These statistics can be collected using the NMS, as shown in Table 3.3. TRANSMISSION NETWORK OPTIMISATION MW Links Unavailability for 18GHz (Annual Rain Unavailability 0.005%) 0.0050006 0.0045006 0.0040006 Power level 0.0035006 0.0030006 Vertical 0.0025006 axis:Power 0.0020006 level(-dBm) Horizontal 0.0015000 axis:Hours 0.0010000 of the Day 0.0005000 0.0000000 Links Figure 3.27 Unavailability graph for four links and power level observation of an MW link for a 24-hour period Table 3.3 Statistics from NMS Avail_time Degraded_min Err_sec Err_sec_severe 1/10/1997 86304 1/11/1997 86640 1/12/1997 83059 1/13/1997 83640 1/14/1997 63598 1/15/1997 63766 In general, the dropping of calls in a mobile network is attributed to the radio network. In fact, in some cases, it may be due to problems with interference and synchronisation. The traffic lost in the form of dropped calls can be as high as a few thousand minutes for small 100-site network, as shown in Figure 3.28. This results in a huge revenue loss. With microwave transmission, the possibility becomes higher for interference being the cause of 'trouble' The trouble-shooting process starts with a search for the cause of dropped power levels at a hop (or hops in a region). The interference problem analysis in the links or region may include the study of existing frequency plans, or a practical study consisting of frequency scanning. The interference could be due to an internal or external source; an internal source being easier to deal with. Once the interference source has been identified, a solution has to be applied. This may include the changing the frequencies, reducing the power level of the interferer, antenna discrimination, etc. Synchronisation can be one of the reasons behind degraded network quality. This problem usually persists in mixed networks. The data collected would include mainly the synchroni- sation plans (on whose basis the implementation was done), and NMS 