d capacities of containers and their hierarchy, making a complete STM-n signal. The discussion starts with a simple container and then moves on to the next hierarchies. Containers Containers are fixed size blocks that contain the incoming signals. As the signals are smaller than the capacity of the container they need to be stuffed with extra bits. This filling of the source signal with additional bits is called mapping. Containers are denoted as C-nx (where n = 1 to 4 and x = 1 or 2), where n is the PDH level and x indicates the bit rate (i.e. X = 1 => 1.544 Mbps and X = 2 => 2.048 Mbps). Different container sizes (e.g. C-11, C-12, C-2, C-3, C-4) are available for the different source signal bit rates. C-2 is used for a 6 Mbps signal, C-3 for a 34 Mbps signal and C-4 for 140 Mbps signal (see Figure 3.18). Virtual Containers The path overhead (POH) is used to establish a path in the network by monitoring the addressing. When this component is added to the container, it becomes a virtual container; i.e. when POH is added to C-n, it is referred to as VC-n (where n = 1 to 4), as shown in Figure 3.19. Administrative Unit The administrative unit (AU) is a chunk of bandwidth used for communicating in a telecommunication network. Addition of the administrative pointer in the virtual container makes the VC the administrative Digital Hierarchies - PDH and SDH Hierarchy Bit rate assignment 1.5 Mbps + stuffing bits 2.0 Mbps + stuffing bits 6.0 Mbps + stuffing bits 34 Mbps + stuffing bits 140 Mbps + stuffing bits Figure 3.18 Containers with different capacities unit; e.g. addition of the AU pointer to the VC-3/VC-4 makes it the AU-3/AU-4. The payload of an STM-1 signal consists of one AU-4 or three AU-3s, as shown in Figure 3.20. Tributary Units The next block is the tributary unit (TV), denoted as TU-nx (where n = 1 to 3 and X = 1 or 2). The virtual containers VC-11, VC-12 and VC-2 are completed to make a tributary unit by adding the pointer. The pointer specifies the phase of the VC, which is said to be mapped with respect