located in the first column (9 bytes) of VC-3 or VC-4. It is formed on generation of the VC-3 (VC-4) and remains unchanged until the VC is disassembled in order to be able to monitor the complete path. The SPE is completed with a POH, which is used to monitor and control the correct addressing as well as to identify the payload contents, whereas the payload is the revenue producing traffic being transported and routed over the SDH network. The SDH frame may contain more than one low speed digital signal such as 2/34/140 Mbps or some part of the high speed signal. A general SDH frame format can be represented as in Figure 3.10, where N is the level of hierarchy of the SDH signal. The number of columns remains nine in every level. The building block of the SDH hierarchy is the STM-1 (synchronous transport module, level 1). This is the lowest level SDH signal where N = 1; thus the total number of rows is 9 and the total number of columns is 270, out of which 9 are used for transport overhead and the remaining 261 columns include the synchronous payload envelope. At the macro level SDH data are transported frame by frame, with the basic frame being the STM-1. Considering bitwise data transport, the MSB (most significant bit) of each byte is transported to the frame first, followed by the others. Then 1 byte is stored in every cell of the 9 X 270 matrix, which are transported to the frame till one frame fills up. All bytes of row 1 are first transmitted starting from column 1 and move off to the first byte in row 2 and SO on till the end of the frame. This frame is then transported in the STM-1 container whose frame period is 125 us (Figure 3.11). Figure 3.12 depicts the order of transporting nine rows one by one. Transmission Network Planning and Optimisation Transport Synchronous Payload Overhead Envelope 270 columns 9 columns 261 columns Additional 9 rows transport Payload area capacity Nx 9 columns Nx261 columns Figure 3.10 Transport frame of the STM-1 signal The frame capacity is thus the number of bits contained