Optical Network Design and Implementation

SDH Framing

SDH adopts a frame rate of 8000 frames per second. The STM-1 frame is the basic transmission format for SDH. As illustrated in Figure 6-5, each two-dimensional frame has 9 rows and 270 columns of bytes with a total of 2430 bytes (9 rows * 270 columns = 2430 bytes). The STM-1 frame consists of an transport overhead plus a virtual container (VC-4) capacity. The first 9 columns of each frame make up the transport overhead, and the remaining 261 columns make up the virtual container (VC-4). The VC-4 plus the nine pointer bytes (H1 * 3, H2 * 3, H3 * 3) in row 4 is known as the administrative unit (AU-4). The first column of the VC-4 is the POH, as indicated in Figure 6-5. Actually, the POH floats within the STM-1 frame and can occupy any ...