aces), and hides interfaces that are not (platform service inter- faces). A physical node can represent any type of network element in a telecom net- work, even a complete operator network. Thus, an NNF may be an extremely com- plex unit, which in its implemented form may comprise hundreds to thousands of hardware elements and software programs. An NNF is therefore an obvious target for applying message discrimination. The result of a first step discrimination of the NNF-protocols in Fig. 2.35, might be the internal structure of an NNF that is shown in Fig. 2.36. Node platform network service interface local global Fig. 2.36 An NNF is a structure of logical nodes The parts (A through E) of the NNF are called logical nodes. As for the NNF as a whole, a logical node has an existence in all three dimensions. A logical node defines protocols to logical nodes in other physical nodes in the D-dimension, and interfaces other logical nodes in the same physical node over network service interfaces in the C-dimension. In the P-dimension, a logical node is an application on the node platform. Note that the node platform is the mechanism by which we integrate several log- ical nodes into a physical node, e.g. by providing connectivity service over local SAPs. Message discrimination of the NNF protocol results in logical nodes becom- ing connected over network service interfaces that, as a minimum, must consist of a global SAP. Global SAPs are the type of SAPs (TE-SAP, A-SAP, N-SAP, etc.) identified in Sect. 2.2.1. 2 Foundations of Sysnet Modelling The notion of local and global SAPs is highly recursive, since physical net- works can be modelled on several network levels. This can be demonstrated by assuming that the node platform in Fig. 2.36 is not a single homogeneous platform, but a distributed system of processors that provide a common internal connectivity service to the logical nodes of the NNF. In Fig. 2.37, upper part, I have assumed that three different platforms (PL1, PL2 and PL3) are used to realize the node plat-