ks and clock distribution. ITU G.803 recognises a hierarchy of clocks that can be used as a reference clock source. They are the primary reference clock (PRC), the Slave clock and the SDH equipment clock (SEC). The PRC has the highest accuracy and hence is at the top of the hierarchy level. It can be derived from an autonomous system or nonautonomous system, with the autonomous system deriving the accuracy from primary atomic standards such as caesium (using a caesium beam atomic oscillator). The PRC must maintain frequency accuracy of 1 X 10- or better, with verification to universal time coordinated (UTC). The accuracy requirements of the PRC are given in ITU-T Recommendation G.811. These clocks are highly accurate but are also expensive. Slave clocks such as the synchronisation supply unit (SSU) is a stand-alone synchronisation supply equipment (SASE). The accuracy of the SSU is specified in ITU-T Recommendation G.812. They are always locked to the higher level clock references via the network. These are generally used in long chains to refresh the clock signal. The traditional technology for implementing the SSU is the rubidium atomic oscillator. They have a lower cost than caesium and have excellent short time stability. However, their lifetime is limited. Modern quartz technology allows low cost oscillators to be built. Although their accuracy may not be as good as that of rubidium oscillators, they not only satisfy G.812 but are also cheaper and have a longer lifetime. The SEC is a built-in clock and comes under the slave category (i.e. needs a master clock). Its accuracy as specified by ITU-T Recommendation G.812 is of the level 4.6 X 10-6. In cellular networks, the master-slave method is used to distribute the clock in the system, as shown in Figure 3.70. In this system, a higher level clock is used to synchronise a lower level clock. The lower level clock can then further distribute the timing signal in the network. The following are synchronising elements in the cellular network: The MSC can be supplie