consumption etc., but still preserv- loads the 125Mbit/s FDDI signal into a VC4 which has a payload ing the ability to detect traffic errors and access some management capacity of around 149Mbit/s. It might be thought that this degree channels. On the other hand, the line terminal equipments process of inefficiency is more or less inevitable for any further type of both the RSOH and MSOH. (See Figure 42.17.) signal whose bit rate does not correspond roughly with that of one of the existing Synchronous Containers. In fact, this is not necess- Finally, as with VCs, STMs come in various sizes. As mentioned arily so, because CCITT G.708 and G.709 contain provision for the earlier, the smallest size is termed an STM-1, and can accommodate concatenation of both TU2s and AU3s and AU4s. a single VC4. However, larger sizes exist whose bit rates are integer As a hypothetical example, to illustrate the use of concatenated multiples of the basic STM-1 rate. CCITT G.707 currently recog- TU2s, an incoming service signal at, say 16Mbit/s, is mapped into nises the STM-4 and STM-16, but STM-12 based network elements a group of three VC2s, known collectively as a VC2-3c. (See Figure are also being designed, and it is likely that STM-64 will become a 42.18.) These three VC2s are loaded into the VC4 with identical de facto standard in the near future. For all these higher rate STMs, pointer values, and are subsequently transported, as a group, across the construction mechanism is the same: The payload is produced the entire SDH network. The best current example of the use of this by straight byte interleaving of the tributary VC4s, while the SOH technique is in the area of video transmission, where a TV signal is 42/16 The Synchronous Digital Hierarchy (SDH) INDIVIDUAL 64 kbit/s channels from a 30 channel group Figure 42.19 Byte synchronous mapping of a G.704 structured 2.048Mbit/s signal into a VC12, showing the fixed location of each of the 30 X 64kbit/s channels digitally encoded at around 32Mbit/s, and subsequently loaded into e