 the same and can be referred back to the highly stable master clock (e.g. the PRC/SSU/SEC). Digital Hierarchies - PDH and SDH Advantages of the SDH The basic advantage of the synchronous nature has already been discussed in detail. There are other reasons why the SDH has overshadowed its plesiochronous counterpart: High transmission rates. Transmission rates of up to 10 Gbps can be achieved in modern SDH systems, making it the most suitable technology for backbones - the superhighways in today's telecommunica- tions networks. Simplified add/drop function. Compared to the older PDH system, low bit rate channels can be easily extracted from and inserted into the high speed bit streams in the SDH. It is now no longer necessary to apply the complex and costly procedure of de-multiplexing and then re-multiplexing the plesiochronous structure. Reliability. Modern SDH networks include various automatic backup circuits and repair mechanisms, which are designed to cope with system faults. As a result, failure of a link does not lead to failure of the entire network. Future platform for new services. The SDH is the ideal platform for a wide range of services in- cluding ISDN, mobile radio and data communications (LAN (local area network), WAN (wide area network), etc.). It is also able to handle more recent services such as video-on-demand and digital video broadcasting via the ATM. Interconnection. The SDH makes it much easier to set up gateways between different network providers and to SONET systems. The SDH interfaces are globally standardised, making it possible to combine network elements (NE) from different manufacturers into a single network, thus reducing equipment costs. The Layered SDH Model All the network technologies and architectures are based on the layered model SO that they can abstract the working of the entire structure in various parts. Hence the SDH system is modelled in a layered manner to separate the functionality of its various components, making understanding of the system easier. SDH networks a