 to the TUs. In TU-11, TU-12 and TU-2, there is only space for one pointer byte (shown in Figure 3.21). Administrative Unit Group On multiplexing the AU-n into STM-n, an administrative unit group (AUG) is formed by three AU-3s or one AU-4. The three AU-3s are interleaved byte by byte. The AUG represents an information structure composed of nine rows each consisting of 261 columns plus 9 bytes in row 4 for the AU pointers (shown in Figure 3.22). SDH Multiplexing The basic elements given in the previous section are used in multiplexing signals of various capacities and are sent through the same common channel simultaneously. What actually happens while packing a signal into an STM-n signal will become clearer once the stages of hierarchy are gone through using a particular signal capacity. Using these objects, the multiplexing of smaller signals is done stepwise. Consider a DS-4 signal (140 Mbps); the STM-1 can be generated in a sequence of steps. Firstly, a 140 Mbps signal is added with additional bits, making it fit in the C-4 exactly. The container is then added with the POH, converting Transmission Network Planning and Optimisation VC-12 VC-11 C-11+POH VC-11 C-12+POH VC-12 C-2+POH C-3+POH C-4+POH Figure 3.19 Virtual containers with different capacities AU-4 pointer AU-3 pointer VC - 4 + AU - 4 Pointer AU - 4 VC - 3 + AU - 3 Pointer AU - 3 Figure 3.20 Administrative unit Digital Hierarchies - PDH and SDH TUG-3 TUG-2 Stuffing Information 4 X TU-11 TUG-2 3 X TU-12 TUG-2 1 X TU-2 TUG-2 7 X TUG-2 + stuff. Info TUG-3 1 X TU-3 + stuff. Info TUG-3 Figure 3.21 Tributary unit group it into a VC-4. The next stage is to convert it into the AU by addition of the administrative unit pointer (AU-4). The addition of section overhead finally makes it a complete STM-1, to be transmitted over the network. A very low speed signal can also be converted into an STM-1, but the intermediary multiplexing stages increase in order to maintain the proper hierarchy. Using this multiplexing hierarchy, any signal with less capacity or more c