re restores buffer fill 375 /u S Buffer fill reducing again Figure 42.11 Pointer adjustment resulting from VC12#38 running slower than the VC4 The SDH standards 42/11 261 bytes 260 bytes Higher Order Virtual Container VC4 POH Bytes Frame Start AU Pointer Administrative Unit Frame Offset VC4 Payload Scanning sequence for transmission AU POINTER of bytes 9 bytes 261 bytes STM-N Figure 42.12 Representation of a VC4 as a block of 261 X 9 bytes together with its incorporation within an STM signal 42.4.3 Channels and Tributary Unit Groups defined byte positions within a VC4. Some of these positions are reserved for TU pointers, while the others are for the rest of the TUs i.e. the VCs. The reason for introducing the concept of a TUG, Having discussed the way in which up to 63 individual TU12s are instead of sticking with that of a channel, is that, as mentioned in multiplexed into a VC4, it is now necessary to examine more closely Section 42.3, a C12 and hence VC12, is not the only size of the internal structure of the VC4. For this purpose, it is helpful to Synchronous Container which has been defined. (See Figure 42.14.) consider a VC4 not as a linear string of bytes, but instead, as a two For example, there exists a C2 which is designed to accommodate dimensional block of bytes which is arranged as 9 rows, each of 261 the North American 6.3Mbit/s plesiochronous rate. In the event of bytes. For transmission purposes, this block is serialised by scan- a PTO needing to transport both 2Mbit/s circuits and 6.3Mbit/s ning left to right, top to bottom. (See Figure 42.12.) With this ones, they can both be accommodated within the same VC4 by structure, a TU12 can be seen to occupy 4, widely separated, 9 byte assigning some TUGs to carry groups of three TU12s while other columns, rather than the contiguous block of bytes, as suggested by TUGs are assigned to each carry a single TU2. Figures 42.10 and 42.11. (See Figure 42.13.) Since a VC4 repeats every 125us, this implies that a TU12 contains 36 bytes per 125us The TUG in thi