interfaces that are vis- ible on the network level, and those that are not, well separated. As a result of mod- ified standards, the network interfaces may be modified, retaining existing node platforms, or, as a result of new technologies, the platforms may be exchanged, retaining the network interfaces. Similarly, protocols may be changed while the implementation of network service interfaces are retained, and vice versa. We therefore must make a clear distinction between views of units in the different dimensions of functional space. This distinction is shown for the NNF in the intui- tive model in Fig. 2.35. Fig. 2.35 Functional space defines three orthogonal views on any unit The figure shows that: When viewed in the P-dimension, the NNF uses the services of a platform serv- ice interface. Such a view regards the NNF as a network application. We say that, in this view, the NNF exists in the P-dimension. When viewed in the D-dimension, the NNF cooperates with NNFs in other physical nodes according to the rules defined by protocols in network interfaces. Such a view regards the NNF as a protocol element. We say that, in this view, the NNF exists in the D-dimension. When viewed in the C-dimension, the NNF is a unit that interfaces physical media. This interface is the lowest type of network service interface that can exist in a sysnet model. We say that, in this view, the NNF exists in the C-dimen- sion. Views can be, and often are, combined in models of a sysnet in functional space. However, except for tutorial purposes, there is seldom any need for combining all three dimensions in the same model (as in Fig. 2.34). One-dimensional models are sometimes valuable, e.g. to show models of protocol networks (in the D-dimen- 2.3 Architectural Principles sion) and models of implemented systems (in the P-dimension). The most common model is probably the DC-plane model (often called "protocol architecture"), since it shows interfaces that are visible on the network level (network interfaces and net- work service interf