undreds of sites, the SDH backbone is advisable SO that capacity problems do not happen in a crucial part of the network. Some networks that had a PDH backbone in the initial stage outgrew the capacity and hence faced huge amounts of drop calls. In that case, the capacity upgrade should be done immediately to prevent loss of revenue. Transmission Network Planning and Optimisation Daily Outage Graph Trend Figure 3.78 Example of recording PCM failures at the BSC Performance problems related to the microwave link can be twofold: one is related to the performance of a single link and the other to collective performance of the links. The former may be related to errors in link design and atmospheric changes while the latter may be due to errors in system design, e.g. bad frequency planning. In either case, it is good to review both the original plans - design and implemented. Link budget calculations should be checked and compared to commissioning results in terms of transmitted power, received power, losses and availability. If possible, the performance of the 'problem link' should be recorded for a 24 h period (shown in Figure 3.79) for certain periods of time (ranging from a week to a few months). Especially in cases where phenomena like ducting or layering take place, i.e. microwave links in a valley, near large water bodies, etc., the measurement over a substantial period of time is helpful. Based on the results, changes to the microwave antennas with respect to transmitted power, antenna heights or orientation should be made. When talking of system performance with respect to a microwave link, frequency planning and inter- ference are the two major aspects to be looked into. Frequency planning in bigger networks with less frequency spots may be a cause for concern, especially in the roll-out phase. When re-examining the frequency plans, the rules of thumb that were seen in the sections above should be followed. Techniques Figure 3.79 Typical performance of the link recorded on a 24 h basis Transmission Network O