re subdivided into various layers that are directly related to the network topology. The lowest layer is the physical layer, which represents the transmission medium. This is usually a fibre optic path or a satellite or radio wave link. Then follows the regenerator section, which is the basic segment of the SDH network. It is the smallest entity that can be managed by the system. Each repeater monitors defects such as the loss of signal, loss of frame, etc. Part of the regenerator section overhead (RSOH) is fully calculated by passing the signal through a regenerator. The next section is the multiplex section, which covers the part of SDH link between multiplexers. The multiplex section overhead (MSOH) is used for this section to detect any errored blocks and defects and generates a special alarm in the forward and backward directions. The basic architecture of the SDH path is shown in Figure 3.7 (where MUX is the multiplexer and IP the Internet Protocol). The two virtual container (VC) layers represent a part of the mapping process. Mapping of the signals is done to pack the various incoming tributaries like ATM and PDH into the SDH transport modules. VC-4 mapping is used for packing high capacity signals like the ATM signals, whereas VC-12 mapping is for low capacity signals like the PDH signals. This layered model can be represented as shown in Figure 3.8. SDH Frame Format The SDH signal is transmitted in the form of frames, with each frame of each signal containing a specific amount of information. The higher the hierarchy of the signal, the higher the speed of the signal and the higher will be its information carrying capacity. The SDH frame in general consists of two basic areas, the transport overhead and the synchronous payload envelope (SPE). The transport overhead comprise Transmission Network Planning and Optimisation SDH Regenerator Cross Connect Regenerator section Regenerator section Multiplex Section Multiplex Section VC-4 Path Figure 3.7 SDH architecture basics the starting bits used for housekeep