ning: Layer structure refinement: Node structure refinement: allo(_part_of Fig. 4.1 Separating connectivity functions from other functions Figure 4.1 shows three VP-refinement steps of L1 and nw1: L1 is first vertically refined into two layers, in order to separate functions that provide network connectivity (layer L11) from functions (L12) that handle the actual service that is offered over the control point. A particular distribution solution is then applied to define (in this example) nw1 as consisting of two nodes interconnected by a physical route. When we separate the layer structure from the node structure, the fact that the nodes will allocate parts of the layers is modeled as allo(-cates) (in this case allocates_part_of) relations, either as graphical pointers as in this figure, or as a 4.2 Vertical Partitioning table. Note that allo relations must be defined by the node structure model and not by the layer structure, since a given layer structure holds for an infinite number of node structures. This approach can then be repeated recursively inside an nw1 node. In Fig. 4.2 we have partitioned the L12 layer element inside n11 into two local layers in order to support vertical partitioning of this layer element. The distribution solution defines an internal network (nw2) in n11 that consists of two nodes, n21 and n22, interconnected by an internal physical route. Layer structure refinement: Node structure refinement: Fig. 4.2 Separating connectivity functions over several network levels Since the internal node structure of n11 is invisible from n12, and completely independent of any particular internal node structure in n12, nw2 must be defined as a node structure on a lower network level than nw1. Considering possible later refinements of the modeled network system, it is important that the independence between nw1 and nw2 is shown in the model, which is done by the sub relation. These examples have both focused on connectivity structures, i.e., to separate connectivity functions from other functions (pos