rriers of the abstract service primitives of the abstract protocol, and if the network service interface is for- mally specified by operations and an abstract protocol, MEHs can be code-gen- erated. Methods for doing that are discussed in Sect. 5.4. An additional requirement is that SAPs of a CSP are made available that can transport messages between the two physical nodes. The type of CSP will affect the design of the MEH. It is therefore essential that the abstract description of the network service interface and the message protocol description are kept apart. This allows for reuse of a network service interface specification, in case another CSP or a vertical integration solution is chosen. As a matter of fact, if the abstract technique for network service interface specification had been used in the past, it would have been fairly simple to make the transition from horizon- tal to vertical integration. 2. By transforming a network service interface into a network interface, a single physical node is refined into a system of two physical nodes. The (external) 4 Physical Network Modelling behaviour of the new system is the same as for the original single physical node. The new network interface will be an internal interface inside the new system. We also say that the internal network interface exists in the D-dimension of the internal functional space, while the original network service interface exists in the C-dimension of the functional space where the original physical node was defined. Thus, the original node and the two new nodes exist in different func- tional spaces and different physical network levels. These spaces and levels are orthogonal, since external network interfaces are independent of internal net- work interfaces. The orthogonal properties of this transformation is a very important architec- tural quality. If that property is not preserved, the architecture tends to increas- ingly degenerate. Preserving orthogonal properties keeps physical network levels separate. This is accomplished by sp