egration hierarchy. Each level defines its own notations Integrates relations imply more than just aggregation: Many units from different (or the same) higher-level model fragments may coexist in a single unit in a lower-level model fragment. This requires that the lower level model fragment implements some type of discrimination function (as was discussed in Sect. 2.3.4). 2.3 Architectural Principles Units on the lower level may identify each other by addresses (SAPI), while units on the higher level address each other by names. This requires that the lower-level model fragment implements (or has access to) some type of name- address-translation function. Units on the higher level may communicate by communication events, while units on the lower level must communicate by messages (PDUs). This requires that the lower-level model fragment implements some type of message handling functions. Thus, to create a model fragment on a lower level implies defining integrates relations and specifying integration functions (discrimination, name-address trans- lation, etc.) to be implemented on the lower level. Integration functions provide transparency of properties for model fragments on the higher level, and are com- mon for the whole or part of a network on a lower level. The integration functions that may exist on each level are defined by SM&AML in generic models. For example, SM&AML defines generic models for discrimi- nation structures in the units of a logical network, for message handling in the units of a protocol network and for connection and association handling in the units of a logical network. Figure 2.45 shows an example: the generic model for a logical node that performs association handling. The integration functions are handled by the integrator. The model is explained in detail in Sect. 5.3.4. Protocol Logical node element, type B Protocol element, type A Association Integrator protocol x0 AEid x1 AEid x2 AEid Discrimination Aid UserData Discriminator protocol AEid Association endpoint identifier Aid Ass