l such nonstandard bitrates will be handled by other means (e.g., ATM or IP). So far in this discussion, all the examples of a service rate signal being mapped into a synchronous container have assumed that the SDH network element makes no use of any structure that might be present in the service rate signal. In fact, this assumption is always true when the service rate signal is plesiochronous. However, because of the way the PSTN service is usually operated (with all of the switches running synchronously with one another), it was decided to endow SDH with a second class of mapping, known as byte-synchronous mapping. Currently, the only byte-synchronous mapping defined is for a 2-Mbit/s signal, which has a G.704 frame structure (i.e., is byte oriented) and is synchronous to the SDH network or, more precisely, synchronous with the network element that is mapping it into a synchronous container. In the course of a byte-synchronous mapping, the SDH network element locks onto the incoming G.704 frame alignment word and subsequently proceeds to load this, and every other byte in the G.704 frame, into predefined positions in the C12 (see Figure 10.18). This results in a situation in which any channel of a 30-channel group can be easily located when the location of the VC-12 POH has been established. The advantage of this SDH Standards 125/us CH1CH2 Individual 64 kbit/s Channels from a 30-Channel Group Fig. 10.18 Byte-synchronous mapping of a G.704 structured 2.048-Mbit/s signal into a VC-12, showing the fixed location of each of the 30 X 64-kbit/s channels. External External SDH MUX SDH MUX at Site A at Site B 2-Mbit/s 2-Mbit/s Plesioch STM-1 Plesioch Signal Signal PTP PTP PSTN 30 Frame Alignment and Plesiochronous Stuff/Destuff 30 PSTN Switch ch Operations at These Points ch Switch VC-12 VC-12 STM-1 PTP PTP Site A Site B No Frame Alignment Plus Easy Loading of VC-12s Fig. 10.19 Comparison of plesiochronous and byte-synchronous mappings of 30 channel groups between two PSTN switches. mapping becomes apparent when cons