concerned with end to end, rather than section by section (i.e. As with the VC12, the VC4 POH is capable of indicating that errors network element to network element) issues. It could be argued that have been introduced into the VC4 payload during its journey, but the POH monitoring is redundant, because the end to end path it is not capable of identifying which network element was respon- performance could be synthesised from the individual section indi- sible. This problem is solved by the addition of yet another set of cations. However, not only would this be difficult to do, especially overhead bytes to the VC4, known as the Section Overhead bytes if the VC4 journey happened to traverse the networks of more than The SDH standards 42/13 STM-N 140 Mbit/s TUG-3 45 Mbit/s DIRECT 34 Mbit/s TUG-2 6.3 Mbit/s Container TU-12 VC-12 Capacity 2 Mbit/s STM Rate Container Mbit/s Cx Mbit/s C - x Virtual Container 155.520 1.600 TU-11 VC-11 1.5 Mbit/s Tributary Unit 662.080 2.176 TUG X Tributary Unit Group 1244.160 6.784 Administrative Unit 1866.240 48.384 Administrative Unit Group STM Z Synchronous Transport Module 2488.320 149.760 Figure 42.14 SDH multiplexing structure one PTO, but it would not necessarily detect all the errors in the Although both the VC POHs and STM SOHs have other duties in VC4, because it is quite possible for errors to be generated within addition to the performance monitoring described above, the SOHs the confines of a transited network element, i.e. within that portion shoulder by far the larger part of the burden, hence the reason for of a network element between where the old SOH has been the much larger number of bytes in the SOH than, for instance, in a removed, and a new one has been added, e.g. between X and Y of VC4 POH. (See Figure 42.12.) These additional duties include STM the Mux at site C in Figure 42.16. alignment function, the carriage of the network management chan- VC12#52 VC12#38 pointer pointer pointer pointer pointer pointer pointer Unused Bytes VC4 POH byte column Byte transmiss