 within the single frame: Frame capacity = 270 bytes/row X 9 rows/frame X 8bits/byte = 19440 bits/frame The bit rate of the STM-1 signal is thus calculated using the formula Bit rate = frame rate X frame capacity As the frame period is 125 us, the frame rate is 8000 frames/second. Thus the bit rate is 8000 frames/second X 19 440 bits/frame = 155.52 Mbps This is the basic rate of the SDH. Other rates in the hierarchy are STM-4/16/64/256 whose rates are defined in Table 3.5. Frame Period = 125 usec Overhead Payload 9 Bytes 261 Bytes Figure 3.11 Transport frames in one frame period Digital Hierarchies - PDH and SDH Table 3.5 The SDH signal SDH signal Bit rate [Mbps] Number of total columns Number of payload columns STM-1 155.52 STM-4 622.08 STM-16 2488.32 STM-64 9953.28 16704 STM-256 39814.32 66816 Analogy of SDH Transmission Containers The transmission of SDH signals can be compared with the transmission of containers (Figure 3.13) on a conveyor belt. The payload is transported in containers of certain sizes. Since the payloads have different volumes, containers with different capacities have been defined. If the payload is too small, it is filled up with stuffing information. For transporting the information, the container needs a label (Figure 3.14). Overhead Payload 270 Bytes Figure 3.12 Transporting the STM-1 frame Transmission Network Planning and Optimisation Container Figure 3.13 A container Payload Blind information Label Figure 3.14 A container with a label The latter includes information on the container contents, monitoring data, etc. The receiver evaluates this information. The complete containers are then put on a kind of conveyor belt, as shown in Figure 3.15. This conveyor belt is divided into several frames of identical size. They are used to transport the containers. The position of the containers in the frame is arbitrary; i.e. a container does not have to start at the beginning of the frame. Groups of Containers Sometimes the information to be transmitted is small as compared to the container to 