ed as (local) networks of their own. Thus, documenting network levels in a network system model is necessary since it defines which structures are independent from each other, where interworking is an issue and who is responsible for which network level fragment. AMLn provides the general relation sub(-structure) for identifying network levels (see Fig. 1.13). A sub relation has no attributes or properties. Its only purpose is to define how a rela- tion on a higher network level is terminated on a lower one. 4 Logical nodes on Logical nodes on network level i network level i+1 substructure relation Fig. 1.13 Network levels and the substructure relation This model uses a sub relation for defining model fragments on different net- work levels. Note that both fragments will be described in the same way, i.e., by models in the L and N dimensions. The sub relation can also be used for fragment- ing strata into layer structures and derived network models into network partitions (i.e., terminals and different operator networks). 1.4.2 Modeling Layer Structures The L dimension defines the functional structure of a network system, including its behavior. Since we assume that network systems are layered, we say that the L dimension shows the layer structure of the system. Layer structures are of course based on the "layer" concept, originally defined in the OSI RM (see Figs. 1.8 and 1.9). However, the OSI RM layer concept is too primitive to be useful for modeling as it is (which partly explains why it is often misused). AMLn introduces four basic improvements of the OSI layer concept. They make it possible to model important structural distinctions in layer structures that The author has seen other modeling languages where the need for model fragmentation is satis- fied by references between pages that say "the description of this system continues at point X on page Y." Such a relation serves no purpose other than fragmenting a large description into multiple pages, and has nothing to do with the AMLn substructure relatio