 Buffer fill continuously reducing Nearly empty 0 125 250 375 Time (us) 25 us Buffer fill drops below critical level VC-12-38 phase change (one byte of VC-12-38 is written into VC-4 twice) 250 us Phase change restores buffer fill 375 us Buffer fill reducing again Fig. 10.10 Pointer adjustment resulting from VC-12-38 running slower than the VC-4. Because of the importance of pointers for locating low-order VCs (e.g., VC-12s) within high-order VCs (e.g., VC-4s), the combination of a low-order VC plus its pointer is referred to as a Tributary Unit (TU). In this case, the combination of a VC-12 plus its pointer consti- tutes a TU-12. Channels and Tributary Unit Groups Having discussed the way in which up to 63 individual TU-12s are multiplexed into a VC-4, it is now necessary to examine more closely the internal structure of the VC-4. For this purpose, it is helpful to consider a VC-4 not as a linear string of bytes but instead as a two- dimensional block of bytes that is arranged as nine rows, each of 261 bytes. For transmission purposes, this block is serialized by scanning left to right, top bottom (see Figure 10.11). With this structure, a TU-12 can be seen to occupy four widely separated, nine byte columns, rather than the contiguous block of bytes, as suggested Figures 10.9 and 10.10 (see Figure 10.12). Because a VC-4 repeats every 125 us, this implies that a TU-12 contains 36 bytes per 125 us-that is, rather more than the 32 bytes nominally required by a plesiochronous 2.048- Mbit/s signal. This, however, is consistent with the need for a TU-12 to include a pointer, overflow byte positions, VC-12 overhead byte, plesiochronous stuffing, and SO forth. Each such group of four columns represents a separate channel within the VC-4, and when a Chapter 10 Synchronous Optical Networks and Synchronous Digital Hierarchy 261 Bytes 260 Bytes Higher-Order Virtual Container VC-4 POH Bytes Frame Start AU Pointer Administrative Unit AU-4 Frame Offset VC-4 Payload AU POINTER 9 Bytes 261 Bytes STM-N Scanning sequence for transm