are called VP layer trails. The VP adaptation functions encompass: multiplexing and demultiplexing the virtual connections of the VP layer handling the O&M flows for the VP layer. The trail termination functions of the VP layer handle the end-to-end O&M flows. Those VCs that carry O&M flows also terminate in the VC layer. OSS for Telecom Networks TTP object represents trail termination function CTP object 1 represents CTP object n represents an adaptation function an adaptation function supports supports Figure 3.2 Relationship between TTP and CTP objects. 3 Table 3.2 Sameness of TTP and CTP objects at various layers. Layer Now, the VC layer is the client of the VP layer (although the "C" in VC is for chan- nel, not client!). The virtual connections of the VC layer are also called trails. VCs carry their own O&M flows. Trail termination points (TTPs) represent trail termination functions in the ATM functional architecture. On the other hand, connection termination point (CTP) objects represent the adaptation functions - whether in the VC layer or VP layer. The adaptation functions multiplex and demultiplex trails in the VC layer. Trail termination functions support adaptation functions and so TTP objects contain CTP objects (see Figure 3.2). The TTP objects of the VC and VP layers are similar and the CTP objects of those layers are also similar. On the other hand, the TTP objects of the TC layer are different (see Table 3.2). TTP objects in the TC layer raise an alarm if the trail termination functions that they model cannot extract cells from the incoming physical transmission. To acti- vate or deactivate the scrambling of ATM cells on a physical link, the operations sys- tem of an ATM switch must vary the appropriate attribute for the link in its TTP object in the TC layer. There are often limits on the VC/VP identifier of ATM connections. The capacity that is available for new connections is also limited. ATM access profiles describe either: the profile of the VPs that are supported by a trail on the TC layer,