ed. This network level is obviously valid for any kind of internal structure of the node. We call this level "network level 0" since it is a model with a collapsed N dimension. The specification of services are associ- ated to layer interfaces (i.e., the L dimension) that are accessible in terminals. Realization of such interfaces is part of the network level 0 model, but normally not specified by the standardization body. 2. On next network level (level 1), the standardization body describes the structure of different types of terminals connected to the network, where the latter is defined as a single black box. All access network interfaces are specified on this level. 3. The standardization body may stop there as far as terminals are concerned, and leave to implementors to define the internal structure of terminals (on network level 2), including how to realize network level 0 layer interfaces. 4. On network level 2, however, the standardization body will most likely define an internal structure of the network defined on network level 1 in terms of oper- ator networks, each having its own terminals. These networks operate autono- mously but need to interwork with each other. The network level 2 model shows interworking networks as black boxes, defining everything that is included in network interfaces between such black boxes. 5. At least one of these networks will be of the actual network system type. For this network the standardization body will most likely define an internal structure of types of derived nodes, such as access nodes, switches, service controllers, media gateways, etc. This model is then a network level 3 model. 6. Standardization bodies normally stop there. However, implementors will no doubt develop at least one lower network level (level 4), where they describe an 1 Introduction to Network System Modeling internal structure of the derived nodes defined on network level 3. This structure may be a node structure as well, since nodes that are defined on network level 3 are commonly construct