f 2 Mbps planning would calculate the number of 2 Mbps ports that will be used at the BSC. The example shown in Figure 3.21 is for two sites that are connected between two BSCs. The sites are connected to each other by microwave radios. They are path-protected (i.e. in a loop) and thus 2 Mbps are terminating at cross-connect (here it is SXC or SDH cross-connect). Each site or base station is using one PCM and terminating at the ET ports at the BSC. Each of the PCM requires one port at the base station. In some cases, one PCM is shared between two or more base stations, as shown in Figure 3.20, but the number of ET ports required at the BSC would be still one. Transmission planning engineers should remember to reserve some ports for the 2 Mpbs connections between the BSC and Site 2 Figure 3.21 Example of 2 Mbps planning In big upcoming networks, the 2 Mbps plans keep on changing. Planning engineers need to keep a track of how the 2 Mbps paths are being routed as this directly effects the ET port allocation plans and the complete utilisation of the PCM signals. TRANSMISSION NETWORK PLANNING AND OPTIMISATION 3.4.4 Synchronisation Planning Another important aspect of detailed transmission planning is the synchronisation plans. Usually networks are designed using mixed technologies (SDH and PDH). In mixed net- works that do not have proper synchronisation planning, 'clicks' are heard owing to bit errors in the data service (see Figure 3.22). SDH networks themselves do not require any synchronisation for proper operation. The reason is that the SDH payloads are easily trans- ferred within the network while the frequency differences can be taken care of by a pointer mechanism. Pointers provide the mechanism for dynamically accommodating the variation in phase of the multiplex section into which they are to be multiplexed. Thus, when both PDH and SDH equipments are present in the network, degradation takes place due to the pointer movements. If the synchronisation planning is done properly, then these 'clicks' due to bit