under consideration by ITU-T. An STM comprises a single administrative unit group (AUG) together with the SOH. STM-N contains N AUGs together with SOH. Container, C-n (n = 1 to n = 4). This element is a defined unit of payload capacity, which is dimensioned to carry any of the bit rates currently defined in Table 19.2, and may also provide capacity for transport of broadband signals that are not yet defined by CCITT (ITU-T Organization) (Ref. 6). 139 264 kbits/s STM-N (Note) TUG-3 44 736 kbit/s 34 368 kbit/s (Note) 6312 kbit/s TUG-2 (Note) Pointer processing 2048 kbit/s Multiplexing TU-12 VC-12 (Note) Aligning Note: G.702 tributaries associated with containers C-x are shown. 1544 kbit/s TU-11 VC-11 Mapping Other signals (e.g. ATM) can also (Note) C-n Container-n be accommodated. Figure 19.16 Basic generalized SDH multiplexing structure. (From Ref. 7, Figure 1/G.709, ITU-T Rec. G.709.) 19.3 SYNCHRONOUS DIGITAL HIERARCHY Container-1 VC-1 POH Container-1 TU-1 PTR TU-1 PTR TU-1 PTR TUG-2 VC-3 POH TUG-2 TUG-2 AU-3 PTR AU-3 PTR AU-3PTR STM-N Logical association Note: Unshaded areas are phase aligned. Phase alignment between Physical association the unshaded and shaded areas is defined by the pointer (PTR) Pointer and is indicated by the arrow. Figure 19.17 SDH multiplexing method directly from container-1 using AU-3. (From Ref. 6, Figure 2-3/ G.708, ITU-T Rec. G.708.) Virtual Container-n (VC-n). A virtual container is the information structure used to support path layer connection in the SDH. It consists of information payload and POH information fields organized in a block frame that repeats every 125 usec or 500 usec. Alignment information to identify VC-n frame start is provided by the server network layer. Two types of virtual container have been identified: 1. Lower-Order Virtual Container-n, VC-n (n = 1, 2). This element comprises a single C-n (n = 1, 2), plus the basic virtual container POH appropriate to that level. 2. Higher-Order Virtual Container-n, to VC-n (n = 3, 4). This element comprises a single C-n (n 