t is com- mon for both physical nodes and an internal view for each physical node. Internal views are unique for each node. An internal view defines how the protocols (and medium) are terminated inside the physical node. The communication relations in an internal view are described as partial protocol stacks. A physical node consists of two logical units: a node platform and a node net- work function. Only the functions of the latter are visible to other physical nodes. A node platform offers node-wide services that must include communication serv- ices. A node platform should not be confused with a processing platform for soft- ware. A physical node integrates logical networks by introducing node platforms in every physical node. Logical nodes use the node platform for communication inside the physical node. A physical network model of a sysnet model consists of a hierarchical structure of subnetworks (substructures). Each subnetwork is described in a model fragment of its own. Subnetworks, physical nodes and network interfaces are identified according to physical network levels and according to a variant identification. Network service interfaces are implemented as network interfaces by creating applications on node platforms. Such an application consists of an event handler and a message handler, where the event handler implements the logical node. A message handler depends on the communication primitives that are offered by the node platform and on the language that is used to describe ASPs in network service interfaces. 5 Protocol Network Modelling Introduction A protocol network is a functional part of a logical network. A protocol network is defined in the D-dimension as a collaboration of protocol elements. Protocol ele- ments communicate according to the rules of a protocol (PR1, PR2 and PR3 in Fig. 5.1). Seen over a physical network, all protocol elements that can communicate by a set of mutual agreed protocols belong to the same protocol network. For a proto- col element to have an existence in the DC-