ing of the real information, which is also called the payload. This is stored in the majority of the frame called the payload envelope. The SPE can further be classified into two parts: the path overhead and the payload. For monitoring and controlling the SDH network, additional information is transmitted together with the traffic data (payload). This additional information, called overhead, is divided into two main groups, i.e. the section overhead and the path overhead. The section overhead is a minicontainer holding different types of information required for transmission. The section overhead offers free capacity, which can be used for additional information. The section overhead always starts at the beginning of the transport frame. The transport overhead is basically divided into three parts: 1. Regenerator section overhead (RSOH) 2. Administrative unit (AU) pointer 3. Multiplexing section overhead (MSOH) PSTN/ISDN VC-12 Layer VC-4 Layer Multiplex Section Regenerator Section Physical Interface Figure 3.8 SDH layer model Digital Hierarchies - PDH and SDH Regenerator Section Overhead (RSOH) Section Overhead Payload (Data) AU-4 pointer Multiplexing Section Overhead (MSOH) Figure 3.9 SDH frame structure While the RSOH is terminated (i.e. disassembled, evaluated and newly generated) at each regenerator point, the MSOH passes the regenerator without being modified and is only terminated at the multiplexers (where the payload is assembled or disassembled). The section overhead also includes a pointer defining the position of the containers in the payload area. The pointer value, also referred to as offset, indicates the offset of the container with respect to a reference point of the frame. The pointer, however, is not part of the section overhead. Together with a container (C), the path overhead (POH) forms the virtual container (VC). The POH capacity depends on the path level. While the higher order POH is composed of 9 bytes (one row), only 4 bytes are available for the lower order POH. The higher order POH is 