 TUG-3 consists of a homogeneous assembly of TUG-2s or a TU-3 (Ref. 6). Container-n (n = 1-4). A container is the information structure that forms the network synchronous information payload for a virtual container. For each of the defined virtual containers there is a corresponding container. Adaptation functions have been defined for many common network rates into a limited number of standard containers (Refs. 6, 7). These include standard E-1/DS-1 rates defined in ITU-T Rec. G.702 (Ref. 8). 19.3.3.2 Frame Structure. The basic frame structure of SDH is illustrated in Figure 19.18. The three principal areas of the STM-1 frame are section overhead, AU pointers, and STM-1 payload. Section Overhead. Section overhead is contained in rows 1-3 and 5-9 of columns 1-9 X N of the STM-N shown in Figure 19.18. Administrative Unit (AU) Pointers. The AU-n pointer (like the SONET pointer) allows flexible and dynamic alignment of the VC-n within the AU-n frame. Dynamic alignment means that the VC-n floats within the AU-n frame. Thus the pointer is able to accom- modate differences, not only in the phases of the VC-n and the SOH but also in the frame rates. Row 4 of columns 1-9 X N in Figure 19.18 is available for AU pointers. The AU- 4 pointer is contained in octets H1, H2, and H3 as shown in Figure 19.19. The three individual AU-3 pointers are contained in three separate H1, H2, and H3 octets as shown in Figure 19.20. The pointer contained in H1 and H2 designates the location of the octet where the VC-n begins. The two octets (or bytes) allocated to the pointer function can be viewed as one word as illustrated in Figure 19.21. The last ten bits (bits 7-16) of the pointer word carry the pointer value. 270 X N /columns (bytes) 261 X N Section overhead Administrative unit pointer(s) STM-N payload Section overhead Figure 19.18 STM-N frame structure. 19.3 SYNCHRONOUS DIGITAL HIERARCHY Negative justification opportunity (3 bytes) Positive justification opportunity (3 bytes) H3 H3 H3 125 us H3 H3 H3 250 us 1* All 1s byte Y 1001SS11 