n two PSTN switches, or it could be a private leased line that is connecting two PBXs. Although the 2-Mbit/s circuit is bidirec- tional, the SDH operations are identical in both directions. Thus we will concentrate on just one direction of transmission, that from A to D. VC-12 Path Termination Points Fig. 10.7 Two-Mbit/s VC-12 Path plesiochronous circuit from A to D, SDH Network which is transported by an SDH network for part of its journey. SDH Standards At point B, where the 2-Mbit/s circuit meets the SDH network, the first operation is to take the incoming plesiochronous bitstream and selectively add "stuffing" bits to "pad out" this bitstream to the exact rate required to fill the appropriate synchronous container. In this case, the synchronous container size would be a C12, which is sufficiently large to accom- modate a 2-Mbit/s plesiochronous bitstream at the limits of its 10-ppm tolerance, together with some additional "fixed-stuffing" bytes. The stuffing of the plesiochronous bitstream should ideally be done relative to the clock to which the whole SDH network is beating; however, there is a chance that the SDH network element (e.g., multiplexer) that is perform- ing the stuffing operation is not quite synchronous with the rest of the network. In this case C12, which it creates, is similarly asynchronous. Path Overhead Information Having mapped the 2-Mbit/s bitstream into the C12, the next operation is to generate and attach the Path Overhead (POH) byte, which enables this C12 to be identified, monitored for errors, and routed through the SDH network. The addition of this POH byte to the C12 creates a virtual container 12 (VC-12). As mentioned in The Basis of SONET and SDH section, the idea is that the POH stays attached to its C12 all the way from the point where it was generated to the point at which the 2-Mbit/s payload exits the SDH network. These two points are the "path termination" points for this VC-12, with the continuous stream of VCs between them being referred to as the "path." Between the two