for operations, administration, maintenance and provi- sioning (OAM&P) purposes. The new transmission hierarchy was named the synchronous optical network (SONET). The CCITT, now the International Telecommunication Union (ITU), also came into the matter and established an international standard based on the SONET specifications, known as the synchronous digital hierarchy (SDH), in 1988. The SONET is the subset of SDH as SONET is only used in North America and Japan, as compared to SDH, which is used in the rest of the world. The frame formats and thus the data rates of both systems are not the same but are compatible due to their synchronous nature. The hierarchies of both these systems along with their data rates are shown in the Table 3.4. What is 'Synchronous Digital Hierarchy'? The synchronous digital hierarchy refers to the group/layers of transmission rates or standards that can transport digital signals/data of different capacities through high bandwidth mediums like optical fibres or radio waves. The hierarchy starts from the basic SDH rate of 155.52 Mbps (which is greater than the highest transmission rate of the PDH system) and goes up to as high as 40 Gbps, which is more than sufficient for *bandwidth-hungry' applications like video conferencing. The further up in the hierarchy the more the level/order of multiplexing keeps on increasing, with the higher order signals being made by multiplexing the lower order signals in the time domain. Another aim for creating a new transmission hierarchy was the plesiochronous behaviour of the PDH. It is known that the data streams/tributaries in PDH systems were out of phase with each other, which made the processing of information and multiplexing more tedious. Bit stuffing/justification provided a solution along with the problem of extra bandwidth wastage and processing time. This demanded a synchronous system, giving the name 'synchronous digital hierarchy'. Thus, due to the synchronous nature of the SDH, the average frequency of all slave clocks in the system is