STS-3c, which has a line rate and payload that are identical to those of an STM-1. In contrast, SDH originally focused on the transport of Chapter 10 Synchronous Optical Networks and Synchronous Digital Hierarchy Synchronous Container (C-12) = 35 bytes/125 us Small amount of 2-Mbit/s Plesiochronous Bitstream capacity within the (32 bytes/125 us) C-12 unoccupled by 2-Mbit/s plesiochronous circuit Path Overhead Byte(s) for Synchronous Container 2-Mbit/s Plesiochronous Bitstream VC-12 Container + Overheads = Virtual Container (VC-12) = 36 Bytes/us Fig. 10.2 Example of the creation of a VC (2-Mbit/s channel being loaded into a VC-12). 2.048-Mbit/s, 34-Mbit/s, and 140-Mbit/s circuits, plus their North American counterparts at 1.5 Mbit/s, 6 Mbit/s, and 45 Mbit/s. The general way that any of these PDH rate circuits are transported by SONET or SDH is to map circuits into a synchronous container (see Figure 10.2). From here on, we adopt SDH to explain the principles of synchronous networking to avoid duplicating the terminology differences just described. A synchronous container can be viewed as a subdivision of the basic SDH frame struc- ture, and it consists of a predefined number of 64-kbit/s channels. The entire family of synchronous containers comprises only a few different types, each of which has been sized to accommodate one or more of the common plesiochronous transmission rates, without wasting too much bandwidth. When plesiochronous circuits are mapped into a synchro- nous container, the plesiochronous channel is synchronized to the frequency of the syn- chronous container, which is, in turn, synchronous to the basic SDH frame structure (see Figure 10.3). Before examining the question of exactly what the frame structure is synchronous with, it is worth digressing to discuss a further operation. This is the attachment of an overhead, known as the Path Overhead (POH), to each synchronous container (see Figure 10.2). The idea is that when a plesiochronous circuit has been loaded into a synchronous container, this 