ission of bytes = 2 10.11 A VC-4 as a block of 261 X 9 bytes together with its incorporation within an STM signal. VC-12 slips phase relative to the VC-4, it slips within its own channel; that is, it does not overflow into another channel because this would obviously corrupt the data in the other channel. As mentioned earlier, the position of the pointer within this channel is constant; only the VC-12 part of the TU-12 is allowed to wander in phase. A group of three such TU-12 channels within a VC-4 is known as a Tributary Unit Group (TUG). As with the concept of a channel, a TUG is not a multiplexing level (such as a VC-12) but a group of defined byte positions within a VC-4. 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, instead of staying with that of a channel, is that, as mentioned in The Basis of SONET and SDH section, a C12 and hence a VC-12 is not the only size of synchronous container that has been defined (see Figure 10.13). For example, there exists a C2 that is designed to accommodate the North American 6.3-Mbit/s plesiochronous rate. In the event of a PTO needing to transport both 2-Mbit/s and 6.3-Mbit/s circuits, they can both be accommodated within the same VC-4 by assigning some TUGs to carry groups of three TU-12s, while other TUGs are assigned to each carry a single TU-2. SDH Standards Pointer for 4 Columns of Bytes Occupied by TU-12-38 VC-12-38 POH Byte for VC-12-38 Occupied by Pointer in Following Scanning Sequence for Transmission of Bytes = 2 Fig. 10.12 Internal structure of VC-4 showing fixed pointer locations and distribution of a single TU-12 over four separate columns. STM-N 140 Mbit/s TUG-3 45 Mbit/s Direct 34 Mbit/s TUG-2 6.3 Mbit/s Container = Container Capacity TU-12 VC-12 STM Rate 2 Mbit/s VC-x = Virtual Container TU-x = Tributary Unit Mbit/s C-x Mbit/s TUG-x = Tributary Unit Group 155.520 1.600 AU-x = Administrative Unit 662.080 TU-11 2.176 VC-11 1.5 Mbit/s AUG = Ad