wn internal structure of nodes. Vertical partitioning has until now been the only principle for node definitions in standards, primarily because it defines how terminals interface the network, how operator networks interwork, and how different types of nodes inside oper- ator networks interwork (which is needed SO that operators can buy their nodes from different suppliers). It was also regarded as a competitive advantage for suppliers that standards did not require any particular implementation of layer interfaces. A disadvantage that has become very obvious over time is, however, that nodes defined in this way become very complex. As a result, introducing new services and managing functionality in "vertical networks" is slow, tedious, and error prone. Horizontal partitioning implies that a particular type of layer interface is spec- ified as a network interface as well, exemplified in Fig. 3.1 for the layer inter- face between OSI layer 3 and 4. As a consequence, layer elements inside a node that are defined through vertical partitioning can be allocated to different inter- nal nodes in all nodes that are concerned. Note that horizontal partitioning, con- trary to vertical partitioning, does not cause any partitioning of the existing layer elements themselves, i.e., only layer interfaces are concerned by this approach. The benefit with horizontal partitioning is that we can describe and implement a complex network as a structure of less complex logical networks, which, however, requires that we define how layer interfaces are transformed into network inter- faces, and how horizontally partitioned nodes are interconnected. Let's make an example of the previous OSI RM model, by defining a logical network that com- prises OSI layers 4 through 7 only (see Fig. 3.2). When vertical partitioning is applied in the context of the OSI RM, network interfaces appear as vertical, which is why this method is regarded as "vertical partitioning." Fig. 3.1 shows, these network interfaces would appear as horizontal interfaces in