 errors disappear. 9 Octets 261 Octets Regenerator Payload AU-4 PTR Multiplex Figure 3.22 SDH frame for STM-1 interface. The frame size of an SDH frame is 260 (bytes) X 8 (bits) X 9 (rows) and the frame is transmitted 8000 s-1 , SO the payload bit rate is 149.76Mbps. The first column in the virtual container is the VC-4 path overhead (POH) and the header consists of regenerator and multiplex section overhead (SOH) and AU-4 pointer fields Another reason for the need for synchronisation is that there are equipments/systems other than the SDH and PDH in the network, such as base stations and other transmission equipments. The internal clock of these equipments is not very accurate, so external syn- chronisation is needed for movement of 'click free' calls through the network. The required accuracy for the GSM networks is of the order of 10-8. The quality of the clock is important in determining the quality level of synchronisation within the network. There are three clock levels recognised by the ITU-TG.803: PRC (primary reference clock) slave clock (synchronisation supply unit or SSU) SEC (SDH equipment clock). The primary reference clock usually provides the clock for the whole network, thereby acting as the master. The accuracy of the PRC is very high - usually of the order of 10-11 The specification of suitable clocks which can act as the PRC is in ITU-R recommendation DETAILED TRANSMISSION NETWORK PLANNING G.811. As the PRC is the master, it is advisable to have a redundant source (i.e. two PRCs, with one clock in standby mode). Caesium clocks are a typical example and are (usually SSU) highly accurate, but their high cost may be unacceptable in small networks. Slave clocks are used in conjunction with the master clock. These clocks are always locked into the clocks of higher accuracy (e.g. caesium) through the network. They are also known as refresher clocks' because they may be used in long chains to refresh the timings. They also have the capability to stand in for the master clock if the latter is lost temp