orarily. The accuracy of slave clocks is of the order of 10 based on ITU-T recommendation G.812. The major advantage of such clocks is their low cost, but lifetime may be limited. The SDH equipment clock has an accuracy of the order of 10-4 based on ITU recommen- dation G.813. Usage of such clocks is recommended for SDH networks. Mixed networks (that have PDH equipments) should not use this clock for timing distribution as it may degrade the signal quality. Synchronisation Planning Principles As cellular networks are mixed networks, they generally follow the master-slave technique of distributing the clock signal. In this method, a higher-level clock synchronises the lower- level clock. The latter further distributes the timing signal as shown in Figure 3.23. The MSC receives the clock from the PRC and distributes it to the BSC. The BSC then further distributes it to the base stations, which further distribute the timing signal. Figure 3.23 Clock distribution in a typical cellular network Topology has an important place in synchronisation planning. Long chains of timing distribution should be avoided. There should not be more than 10 SSUs in the chain, and the number of SECs between two subsequent SSUs should not be more than 20. This implies that the maximum number of G.813 clocks in the synchronisation chain is 60, and the maximum number of G.812 clocks is 10. If there are loops in the network, then engineers should make sure that there is no loop of the timing signal, to avoid a timing loop. Synchronisation should be protected. There should be more than one source of the timing signal for each network element. The clocks should be derived from reliable sources such as SDH equipment, rather than from 2 Mbps signals from the leased lines whose accuracy is not known. TRANSMISSION NETWORK PLANNING AND OPTIMISATION Implementation of the Synchronisation The PRC is generally the source of the timing signal in a cellular network. The clock should be re-traceable back to the source. For loop protection, MCB and LCB bit