ministrative Unit 1244.160 6.784 Group STM-z = Synchronous 1866.240 48.384 Transport Module 2488.320 149.760 10.13 SDH multiplexing structure. Chapter 10 Synchronous Optical Networks and Synchronous Digital Hierarchy VC-12 VC-11 VC-12-52 VC-11 VC-12-38 Left, Back, Top Pointer Pointer Pointer Pointer Pointer Pointer Pointer Unused Bytes VC-4 POH Byte Column transmission VC-11 Payload sequence: left to right, VC-12 Payload back to front, VC-2 Payload VC-3 Payload to bottom 21 X TUG-2s Fig. 10.14 Partially filled VC-4 with TU structured payload. The TUG in this example is known as a TUG-2 because at 12 columns of 9 bytes it is large enough to accommodate a single TU-2. Beyond this, a TUG-3 has been defined. This is slightly larger than seven TUG-2s and is designed to accommodate a single 45-Mbit/s circuit after it has been mapped into its appropriate synchronous container (in this case, a VC-3). A 34-Mbit/s signal can also be mapped into a VC-3 in an operation that appears somewhat wasteful of bandwidth because the C3 has been sized for a 45-Mbit/s signal. It is interesting to note that, rather than use a more aptly sized container, this bandwidth sac- rifice was agreed to by European PTOs to reduce the potential network control problem posed by a larger variety of VCs. A TUG-3 is one-third of the payload capacity of a VC-4, and together with the TUG-2, it constitutes an extremely flexible mechanism for partitioning the payload bandwidth of a VC-4. The potentially complicated nature of this partitioning can be better understood by replacing the two-dimensional representation of the VC-4 payload structure by a three- dimensional one (see Figure 10.14). For this representation, the order of transmission is left to right, back to front, top to bottom. Figure 10.14 shows not only TU-12s and TU-3s, as we have already discussed, but also examples of other types of TUs, notably the TU-11, which is the TU used to accommo- date a 1.5-Mbit/s (DS1) signal in North America. This payload flexibility extends to alterna- tive mech