lement which is mapping it into a Synchronous Container. In the a concatenated group of five VC2s (VC2-5c). This mapping allows course of a byte synchronous mapping, the SDH network element up to four such video signals to be transported in a single VC4. If, locks onto the incoming G.704 frame alignment word, and sub- instead, the normal mapping into a VC3 had been used, then only sequently proceeds to load this, and every other byte in the G.704 three video signals could have been accommodated within one frame, into predefined positions in the C12. (See Figure 42.19.) VC4, hence a useful increase in efficiency by using concatenation. This results in a situation whereby any channel of a 30 channel So far, in this discussion, all the examples of a service rate signal group can be easily located once the location of the VC12 POH has being mapped into a Synchronous Container have assumed that the been established. The advantage of this mapping becomes apparent SDH network element makes no use of any structure that might be when considering two PSTN switches which are interconnected by present in the service rate signal. In fact, this assumption is always several groups of 30 channels, that are transported over an SDH true when the service rate signal is plesiochronous, however, be- network. (See Figure 42.20.) It is easy for such a PSTN switch to cause of the way the PSTN service is usually operated, with all of operate as part of the SDH network by generating and byte syn- the switches running synchronously with one another, it was de- chronously loading its own VC12s, because it already knows the cided to endow SDH with a second class of mapping, known as byte location of all 30 channels. It obviously does not need to search for synchronous mapping. Currently, the only byte synchronous map- frame alignment, as this too is already known. At the receiving ping defined, is for a 2Mbit/s signal which has a G.704 frame PSTN switch, because the VC12s were mapped byte synchronously, structure (i.e. is byte orientated) an