e over a local SAP that allows implementations of logical nodes to communicate with other units in the same physical node. We say that implementations of logical nodes and those units that provide platform services exist in the processing dimen- sion. The processing dimension is first revealed when one looks inside a physical node, as shown in Fig. 2.34. Physical node Platform service interface Processing dimension (P) Fig. 2.34 The processing dimension (P) exists inside a physical node The P-dimension separates a physical node into two functional elements, a node platform and a node network function (NNF). The NNF has two important properties: - It is the part of the total functionality of a physical node that will be visible to other physical nodes (through network interfaces). A physical media connec- tion endpoint of an NNF can be connected to an NNF in another physical node. - An NNF is also a unit that executes on a node platform, and uses the services of that platform through platform service interfaces. One of those services must be a connectivity service that is offered to different (internal) parts of the NNF over local SAPs. The node platform (and thereby the platform service interface) is, by definition, not visible to other physical nodes since the platform does not have any proto- col relations on the particular network level, where this physical node is identi- fied. If it had, the corresponding function would instead be part of the NNF. By this restriction, there cannot exist any perception of platforms in a physical network model. However, the platform middleware, operating system and the processor used are what determine the characteristics (response time, transfer rate, congestion probability, transfer delays and reliability) of the connectivity services provided by a telecom system to its end-users. This is the only way in which the platform of a physical node is visible on the physical network level. 2 Foundations of Sysnet Modelling From an engineering point of view, it is important to keep 