, made separately by each internal unit. Discrimination of abstract data is a form of discrimination that lies between message and operation discrimination. It is defined in the OSI archi- tecture: the OSI presentation layer handles external abstract data and relays them to OSI application entities. E-set::={e1_set,e2_set,e3_set} M::={...} e3_set e2_set E-set M::={...} M::={...) (operation discriminator) m1.:=(...) e1_set Fig. 2.26 A substructure created by operation discrimination. E-set is the set of events defined in the external protocol. el 1_set, etc., are parts of E_set All four forms of discrimination exist in different network concepts. In support for modelling discrimination, SM&AML provides a generic message handling model, see Sect. 5.4, that defines the relations between different types of protocols. Thus, there are many ways in which the protocol of an interface to a unit can be terminated in a substructure. An original discrimination approach may evolve to a more translation-oriented approach, and functions may be added to the interface unit. As a result, it may become more and more difficult to control the mapping between external protocols and internal interfaces. Therefore, it is wise to keep translation and discrimination functions separate (although they may be performed by the same interface unit) and not allocate other types of functions in interface units. Over time there will also be a need to support older versions of substructures. On each substructure level there will, after some time, exist several alternative sub- structures. It is therefore important that not only the structure levels, but also alter- native substructures within each level, be kept apart. 2 Foundations of Sysnet Modelling 2.3.5 Interface Transformation The effect of substructuring is that a simpler model with few interfaces becomes a more complex model with many more interfaces, see Fig. 2.27. separate protocol and data flow The interface collapse expand transform -{A,B,C}- refine protocol by message discrimination {