plane of functional space, it must be allocated in a logical node that comprises a permanent integration function, gener- ally modelled as an integrator unit, see the example in Fig. 5.1. The protocol networks: Protocol element The logical network: Logical node Service element Integrator Integrator Integrator Fig. 5.1 Integration of two protocol networks in the same logical network When a protocol element is integrated in a logical node, it becomes a service element. A service element is a unit in the substructure of a logical node that has interfaces in the C-dimension and that "implements" the requirements that are defined by the corresponding protocol. Thus, the protocol element is the D- dimension view, the service element the C-dimension view of the same thing. A protocol element can have a temporary or a permanent existence. For a proto- col element to exist in the D-dimension, the corresponding service element must have an interface to the integrator. T. Muth, Modeling Telecom Networks and Systems Architecture © Springer-Verlag Berlin Heidelberg 2001 5 Protocol Network Modelling Integrators in different logical nodes cooperate by a protocol that in some way integrates all protocols that are used by the service elements in the logical node. The protocol between logical nodes, therefore, comprises all protocols that are supported by service elements, and by the integrators of the logical nodes. Depending on the type of integration that the logical node provides, protocols can be message or ASP oriented. The former type of protocol is called message protocol, the latter abstract protocol. Note that a model fragment that shows a protocol network in isolation does not specify where in a sysnet the protocol elements exist. To define that, protocol ele- ments must be integrated in logical nodes, which are the type of units that are addressable in the DC-plane of functional space (since they are connected over SAPs to transport networks). As a consequence, a protocol network does not show a distributed system, but