yer elements. Packaging layer elements of a given layer structure (thereby producing models of derived nodes) can be made in many ways, however. Furthermore, there are not only many types of connections around, we also know that one type of connection can be a bearer for other types of connections, such as a 64-kbps bit stream connec- tion that carries a voice connection, or a route in a switching network that defines a number of channels. Some principles for packaging on the network level are there- fore needed to support modeling of network structures. If we look in network standards as well as inside implementors solutions, we will find that almost all nodes are defined by applying one or both of two packing principles, commonly called vertical partitioning and horizontal partitioning. We can use the OSI RM model for defining the difference between these principles (see Fig. 3.1). Note, however, that OSI RM did not define any of these concepts. Network interface Vertical partitioning Node in operator network Operator network Terminal Network Horizontal partitioning Network interface Fig. 3.1 Partitioning strategies exemplified for the OSI RM 3 Node Structures The difference between these approaches lie in how the functionality of the overall network is partitioned. Vertical partitioning implies that a given interlayer structure is allocated to nodes SO that all layer interfaces become local interfaces inside the nodes. As a consequence, a layer becomes partitioned into layer elements that communicate by means of intralayer protocols between the nodes. Interfaces between nodes are commonly called network interface (NI). 1 As the model indicates, a net- work interface can be a very complex entity defined by a number of protocols and possibly a physical medium. Figure 3.1 shows how this approach can be recursively applied: the overall net- work is separated into nodes that are terminals and a node that represent the net- work between all terminals; this network is separated into operator networks, each with its o