s example is known as a TUG2, because at 12 i.e. rather more than the 32 bytes nominally required by a plesio- columns of 9 bytes, it is large enough to accommodate a single TU2. chronous 2.048 Mbit/s signal. This, however, is consistent with the Beyond this, a TUG3 has also been defined. This is slightly larger need for a TU12 to include a pointer, overflow byte positions, VC12 than 7 TUG2s, and designed to accommodate a single 45Mbit/s overhead byte, plesiochronous stuffing etc. Each such group of four circuit, after this has been mapped into its appropriate Synchronous columns represents a separate channel within the VC4 and when a Container (in this case, a VC3). A 34Mbit/s signal can also be VC12 slips phase relative to the VC4 it slips within its own channel mapped into a VC3, in an operation that appears somewhat wasteful i.e. it does not overflow into another channel, as this would ob- on bandwidth, as the C3 has been sized for a 45Mbit/s signal. It is viously corrupt the data in the other channel. As mentioned earlier, interesting to note that, rather than use a more aptly sized container, the position of the pointer within this channel is constant, only the this bandwidth sacrifice was agreed to by European PTOs in order VC12 part of the TU12 is allowed to wander in phase. to reduce the potential network control problem posed by a larger A group of three such TU12 channels within a VC4 is known as a variety of VCs. Tributary Unit Group (TUG). As with the concept of a channel, a A TUG3 is one third of the payload capacity of a VC4, and TUG is not a multiplexing level (such as a VC12) but a group of together with the TUG2, it constitutes an extremely flexible mech- 42/12 The Synchronous Digital Hierarchy (SDH) Pointer for 4 columns of bytes occupied by TU12#38 VC12#38 POH byte for VC12#38 Scanning sequence for transmission of bytes occupied by pointer in following Figure 42.13 Internal structure of VC4 showing fixed pointer locations and distribution of a single TU12 over 4 separate columns anism for part