cept). We interpret this as that an upper logical node is a hosted node and the lower one is a hosting node. The relation between these node types is a hosts relation. The hosts relations define a structure of logical networks. We therefore say that these relations exist in the logical-network dimension (NW). By using this terminology, the whole network is modeled as a logical-network structure, i.e., a structure of logical networks that are related by hosts relations 3 Node Structures between logical nodes. Thus, we actually dissolve the N dimension into an NO and an NW dimension. Standardized and manageable layer interface Hide nw1 and layer 4 Logical network is nw2= {n21,n22} NOnw2 hosts hosts Logical network is nw1= {n11,n12} NOnw1 Fig. 3.3 A possible logical-network structure in the OSI RM. NW=logical-network dimension Both vertical and horizontal partitioning is recursively applied in a number of refinement steps when a network system is developed. Each step refines the model to a lower network level. It is very important that each network level is modeled separately from the previous one, since protocols and layers on different network levels are independent of each other. We therefore show network levels in models by the substructure relation. For example, the complete node structure for the model in Fig. 3.1 would be modeled graphically as in Fig. 3.4. Note that there may be different numbers of lower network levels inside nodes of the same logical net- work. Network-level 1 node Substructure relation Network-level 3 node Network-level 2 node Network-level 2 node Fig. 3.4 Possible network levels in an OSI network 3.1 Introduction In this case we know that the internal nodes in terminals have been separated by vertical partitioning, since we reveal information about which layer elements are included. In general, however, a node structure model does not, and must not reveal such information. Therefore, a node-structure model over several network levels will not tell whether two nodes have been separated by