 Chapter 10 Synchronous Optical Networks and Synchronous Digital Hierarchy STM-1 STM-1 Site B Site C Site C STM-4 Section STM-1 Cross- Section Connect STM-1 Section STM-1 VC-4 Path Section VC-4 Path Site A Site D STM-1 STM-1 VC-4 PTP VC-4 PTP VC-12 VC-12 VC-12 Path 10.15 VC-12 paths between sites A and D, showing the VC-4 and STM sections involved. channels, data channels reserved for the PTO, and synchronization signaling channels. Even when these have been accommodated, there is substantial unallocated capacity that is being kept in reserve to service future network control requirements that have not yet been identified. Further Use of Pointers The use of STMs and their associated SOHs entails a few additional complications beyond those previously discussed. An STM might well be generated by a network element that did not have the privilege of also generating the particular VC-4 that it is attempting to load. This immediately introduces the possibility of a slight asynchronism between STM and VC-4, and, as with the VC-12, this problem is also solved by a slip mechanism plus pointer. The pointer bytes occupy defined positions within the SOH and indicate the offset, in bytes, between themselves and the first byte of the VC-4 POH. The main difference between this and the VC-12 pointer is that when a VC-4 slips its phase relative to the STM SOH, it does SO three bytes at a time, rather than the single-byte phase change experienced by the VC-12. A second difference between this case and that of a VC-12 is that the combination of a VC-4 plus its pointer is known as an Administrative Unit 4 (AU4), rather than a TU-4, when the pointer is located in an STM. The AU4 is used in all ETSI countries as the size of traffic block on which networks are planned and operated. There is also an AU3, which is used mainly in North America. This refers to an alternative construction of an STM, whereby the payload consists not of a single VC-4 but instead of a group of three VC-3s, together with their associated pointers. A further dif