 the L-N plane. It can be used to define node structures where nodes are derived nodes, related over network interfaces. We also discussed the effect of using sub relations to define network partitions as nodes and nodes as node structures, only on different network levels. Both of these modeling techniques are used to define node structures. A more basic node concept, however, is the logical node, used to describe logi- cal networks. Logical networks are the result of a horizontal partitioning approach to network systems. This implies that a network can be sliced into a number of logical networks that rely on other logical networks for services, prima- rily for connectivity. Since this is more less the idea with the OSI RM layer con- cept, one may ask what the difference is between a layer and a logical network. An important aspect of logical networks is that a particular type of layer interface is specified as a standardized and configurable interface (see Fig. 1.26). Logical node is n1 Logical Route that is created by node is n2 the connectivity stratum Standardized and manageable Connectivity layer interface stratum is COS Abstract COS Logical network is nw= {n1,n2} Fig. 1.26 The definition of a logical network Here we have created a logical network out of logical nodes that comprise layer elements 4 through 7 of an OSI network. By defining these nodes we say that they can be replaced in the OSI network by other nodes (through a management activ- ity). Note that horizontal partitioning is applied on a given layer structure without changing anything of that structure. Since the only thing the logical nodes in the model see of the connectivity stratum (in this case OSI layers 1 to 3) is their inter- connection, we can produce a separate logical network definition that consists of the two nodes (n1 and n2), interconnected by a route symbol. 1 Introduction to Network System Modeling The benefit with horizontal partitioning is that we can describe and implement a complex network as a structure of less complex, con