ally focus on the intralayer protocols that are the results of vertical partitioning. Sometimes they provide ASPs, some- times also behavior descriptions of layer interfaces, but mostly nothing of the kind. Some layers will be refined to strata. Vertical partitioning turns layer elements into intralayer structures and adds extra layers for supporting such structures. Applying horizontal partitioning to a given layer structure implies: Group of layers are allocated to different logical networks. Nodes of different networks thereby become related by hosts relations. Layer interfaces are standardized and may be refined to interlayer protocols. Horizontal partitioning does not affect existing layer structures. It may, however, lead to the appearance of new layers to support layer interface communication. When refining a layer structure, both vertical and horizontal partitioning may be applied recursively. Major refinements of network structures must be distinguished 4 Modeling Vertical and Horizontal Partitions as models on different network levels. Adjacent network levels are identified by sub(-structure) relations. Vertical Partitioning Vertical partitioning (VP) of network systems follow the OSI RM paradigm, which places layer interfaces as local (non-standardized) interfaces in nodes. No logical- network structure is defined, i.e. the concept of hosting network and hosted net- works, and thereby the hosts relation, is neither defined nor modeled. As a result, VP is only applicable to a single logical network. Normally, a model created by VP only shows how distribution of layer elements is supported by connectivity layers or strata. Figure 4.1 shows a simple example, where the layer L1 in the network nw1 is partitioned into two layers, L11 and L12. L1 offers some type of control-point service and is also assumed to interface a physical-media stratum. Note, however, that the reasoning is the same for a layer that offers connectivity services, or is based on some other type of connectivity stratum. Vertical partitio