d via objects in the traf- fic system that are called managed objects (MO). Communication between the management system and MOs may also be object oriented. Interfaces that define how management plane functions access resources in the traffic plane are interfaces between layers in different planes. We therefore denote them MR rela- tions in Fig. 1.12. Management functions are frequently integrated in the traffic system in the sense that they rely on connectivity services defined in the traffic plane. Such functions operate more or less autonomously from other functions of the man- agement plane. The relation between these functions and the traffic plane are normal connectivity layer interfaces, used for management purposes however. We therefore denote them Mc relations in Fig. 1.12. 1.4 Modeling Network Systems in AMLn 1.4.1.3 Network Levels By modeling a network system separately in the L and N dimensions, and by defin- ing the distinction between managed and management planes, we achieve a consid- erable reduction in model complexity. Further reduction of model complexity is needed, however. Any of these views can be fragmented in smaller pieces, primarily applied to node structures (i.e., the N dimension). Fragmentation is also needed in order to allow for piecewise development of a model, and by different modelers. Such frag- mentations must be done SO that the internal structure of a fragment is not revealed outside it, which implies that interfaces between fragments must be defined by architectural relations such as layer interfaces, routes, and network interfaces. Since different specification bodies may define structures on different levels, it is also very important that a fragmented view shows to which level of fragmenta- tion a particular functional element or node belongs. We call such levels network levels. Typically: 1. A standardization body will define the top network level, where a whole net- work (including terminals) is regarded as a single node, and only the services to network users are defin