izontal partitioning since logical network models cannot. To demonstrate that we show in Fig. 3.6 the effect of partitioning a 3 Node Structures derived OSI node that implements OSI layers 4-7 in both ways. In the vertical par- titioning approach we separate the layer 7 element in two interworking nodes. In the horizontal partitioning approach we separate layer elements 4-6 from 7 in sep- arate nodes. Vertical partitioning (4-7) Intralayer protocol Internal routes Interlayer (4-7) protocol Horizontal (4-6) partitioning (6/7) Fig. 3.6 Vertical and horizontal partitioning of the same derived node. DN=derived node Even if we examine an NI specification, it is not likely for us to realize that the protocol between the derived nodes (DN) is an intralayer protocol in the vertical case and an interlayer protocol in the horizontal case. It is therefore very impor- tant that network interfaces are identified with regard to whether they represent a vertical or horizontal partition, and which layers or layer interfaces they imple- ment. Figure 3.6 shows a possible way of identifying NIs. Network models that appear in standards always describe some kind of derived network. Nodes in such models are called either "logical node" or "network ele- ment" (the concept of "derived node" is based on AMLn; the hosting versus hosted node definition has not been observed in any standard SO far). The concept of "net- work interface" is commonly used in standards, in a similar sense as in AMLn. The way an NI should be identified (see Fig. 3.6), in order to be able to relate it to a layer structure in an unambiguous way, is not specified, however. In an attempt to produce something similar, network interfaces are commonly called either UNI (user--network interface) or NNI (node-node, node-network or network-network interface). Both UNIs and NNIs normally denote interfaces in vertical partitions (compare with Fig. 3.1). Thus, the distinction between vertical and horizontal parti- tions (as suggested by Fig. 3.6) is never reflected in standa