network Fig. 2.53 Concepts that are independent of functional space. Integration levels 0 and The diagram in Fig. 2.54 shows relevant concepts and relations in the C-dimen- sion of functional space. Integrator(C) provides AssociationEndpoint Network connects 2 Protocol service Logical 1.. * element(C) node(C) network ConnectionEndpoint interface function(C) Service element provides Physical media isArequirementOn connection endpoint Abstract machine Fig. 2.54 Concepts in the C-dimension. Integration levels 2 and 3 The interfaces in the C-dimension between integrators and service elements are either connection endpoints or association endpoints, depending on if a connec- tion-oriented (CON) or a connection-less (CNL) network service interface is used. In both cases, it is a matter of identifying subflows over network service interfaces. In the former case, this identification is provided by the logical network that pro- vides the network service interface. In the latter case, the integrator must imple- ment a function (association handling) for identifying subflows (if that is of interest). From the service element point of view, this distinction only affects the way a service element addresses other protocol elements. In the CON-case, service elements must use SAPI for establishing connections. If the integrator provides associations, service elements can use a more function-oriented addressing method. This subject is discussed in Sect. 5.3.4. All concepts (except abstract machine) that are defined in Fig.2.54 are used when models are shown in the C-dimension only. Instead of giving units different names that depend on in which dimension they are viewed, a common name indexed with C (or D or P) is used. The only exception is the service element, which is a protocol element viewed in the C-dimension. The diagram in Fig. 2.55 shows relevant concepts and relations in the D-dimen- sion of functional space. 2.3 Architectural Principles reqOn Node network messageProtocol network function(D) function(C) Logical Logical 