ugh the path layer network from the origin to the destination in the so-called SDH paths. This is realized by sending the VCs to the multiplex section layer. The adaptation function connecting the path layer with the multiplex section layer envelops the VCs adding header and footer. The synchronous transport signals (STSs) are multiplexed by the SDH multiplexer into the SDH Multiplex Section Frame, called STM- number, where the number is again related to the frame capacity. All the Multiplex Section Frames defined in the SDH and in the SONET standards are reported in Table 3.3. It is to be underlined that STSs of different capacity can be multiplexed into a single STM SO that SO generating a flexible multiplexing scheme. This is the so-called matrix multiplexing, whose scheme is represented in Figure 3.27. This is an important example of deterministic TDM protocol, for the huge diffusion of the SDH/SONET transport in data networks also (see POS, e.g.). For this reason the SDH frame is described in detail in Appendix A. Here we will underline mainly its functional characteristics. Different consecutive frames are arranged in a matrix and the multiplex section header is defined inside the matrix as shown in the figure. When a set of STSs have to be mul- tiplexed, they are located into the payload part of the matrix in such a way to have a Networks Fundamentals and Present Architectures Pointer Line OH STS-1(VC) (multiplex section) Reg. section SONET frame Row 1 Row 2 Row 3 Hearer FIGURE 3.27 Principle of tributary multiplexing in an SDH/SONET frame. smaller possible unusable area. The effect of this strategy is that the structure of the pay- load of a given STM can change its structure depending on the client STSs that was to be multiplexed. To take trace of the STSs position inside the payload, a zone of the multiplex section header is dedicated to a set of pointers. Each pointer is associated to an STS, as reported in the header itself, and points in the payload area to the STS start. In this way the demul- tiple