operations of creating, multiplexing, and routing of VC-4s within the network, even if such multiplexing or routing happens to be completely inflexible (e.g., hardwired). When a network element (i.e., line terminal, multiplexer, or cross-connect) receives an STM, it imme- diately examines the relevant bytes of the SOH to determine whether any errors have been introduced into the payload (i.e., the VC-4). Unless this particular network element happens to contain the VC-4 path termination point, it subsequently calculates a new, replacement set of SOH bytes that are then attached to the VC-4 for onward transmission to the next network element (see Figure 10.15). This section-by-section monitoring gives the PTO a powerful tool for locating the source of any poor performance within the network and complements the capabilities of the VC POHs, which are solely concerned with end to end, rather than section-by-section (i.e., network element-to-network element), issues. It could be argued that the POH monitoring is redundant because the end-to-end path performance could be synthesized from the indi- vidual section indications. However, not only would this be difficult to do, especially if the VC-4 journey happened to traverse the networks of more than one PTO, but it would not necessarily detect all the errors in the VC-4 because it is quite possible for errors to be gener- ated within the confines of a transited network element-that is, within that portion of a network element between where the old SOH has been removed and a new one has been added (e.g., between X and Y of the MUX at site C in Figure 10.15). Both the VC POHs and STM SOHs have other duties in addition to the performance monitoring described above, but the SOHs shoulder by far the larger part of this extra burden-hence the reason for the much larger number of bytes in the SOH than, for example, in a VC-4 POH (see Figure 10.11). These additional duties include the STM align- ment function, the carriage of the network management channels, Engineer Orderwire