ks usually comprise a combination of plesiochronous and asynchronous equipments. For operators, the SDH is clearly the best choice for new transport networks. It is suitable for all types of networks: from core networks to regional transport networks, for city access and residential access networks, for cellular transport networks, etc. However, from the perspective of the network design engineer, a cellular network is a mixed network in terms of synchronisation. This kind of scenario, where both the PDH and SDH exist in the same network, will be there for quite some time to come. However, before the principles of synchronisation are discussed, it is important to understand why synchronisation is needed in mobile networks. Consider a cross-connect equipment having different bit rates at the incoming and the outgoing interfaces. The buffers in these equipments take care of the speed differences, but due to the difference between the incoming and outgoing traffic, there is a possibility of 'slips' happening. The bit errors due to these 'slips' result in 'clicks' taking place during the call. In the worst cases, the calls may even drop. Synchronised payloads can then easily be transferred within the network even if no synchronisation is applied, as the pointer mechanism would takes care of frequency differences. However, synchronisation is required due to the presence of the PDH interfaces. The performance of the SDH de-synchronisers and PDH access equipment may be degraded by pointer movements. With proper synchronisation the pointer events are Synchronisation Planning rare and do not cause bit errors in data services or clicks in speech. Also, the frequency reference with the equipments, e.g. the BTS, is not accurate enough. By providing external synchronisation, a more accurate frequency reference is provided for these equipments. ITU-T Recommendation G.803 outlines the architecture of the transport network based on the SDH and the synchronisation networks. Highlights of this recommendation are the reference cloc