ave been defined for all the common plesiochronous bitrates, together with a few others, notably the 125-Mbit/s FDDI signal. This latter mapping is somewhat wasteful of transmission bandwidth because it loads the 125-Mbit/s FDDI signal into a VC-4, which has a payload capacity of around 149 Mbit/s. It might be thought that this degree of inefficiency is more or less inevitable for any further type of signal whose bitrate does not correspond roughly with that of one of the existing synchronous containers. In fact, this is not necessarily SO because ITU-T G.707 contains a provision for the concatenation of TU-2s, AU3s, and AU4s. As a hypothetical example, to illustrate the use of concatenated TU-2s, an incoming service signal at, say, 16 Mbit/s is mapped into a group of three VC-2s, known collectively as a VC-2-3c. (see Figure 10.17). Chapter 10 Synchronous Optical Networks and Synchronous Digital Hierarchy TU-2 Pointers (all have the same value) Note that the 12 columns of each TU-2 are actually dispersed throughout the VC-4 rather than existing as the contiguous Mapping Function block shown here 16-Mbit/s Signal Fig. 10.17 Loading of a hypothetical 16-Mbit/s signal into three concatenated VC-2s (VC-2-3c) within a VC-4. These three VC-2s are loaded into the VC-4 with identical pointer values and are sub- sequently transported, as a group, across the entire SDH network. The best current example of the use of this technique is in the area of video transmission, where a TV signal is digitally encoded at around 32 Mbit/s and subsequently loaded into a concatenated group of five VC-2s (VC-2-5c). This mapping allows up to four such video signals to be transported in a single VC-4. If, instead, the normal mapping into a VC-3 had been used, then only three video signals could have been accommodated within one VC-4-hence a useful increase in efficiency by using concatenation. Despite this, the concept of concatenating VCs as a mecha- nism for increasing SDH's flexibility has never really caught on, and it is likely that al