igure 2.14 gives two examples of this dualism between interface and unit. A collaboration The collaboration defined defined by interfaces. as a unit structure All interfaces related A collaboration The collaboration defined defined by interfaces. as a unit structure Not all interfaces related Interface interactions units Fig. 2.14 How interfaces and units are related To the left in the upper part of the Fig. 2.14 we have a structure of three inter- faces, all related. Two interfaces are related if there is at least one interaction in one interface that may cause an interaction in the other. To the right, I have defined a unit that operates in all three interfaces and handles all relations between those interfaces, which is just one of a number of possible solutions. Below, another interface structure, also of three interfaces is shown, but here only two of them are related. One of a number of possible unit-structures is shown to the right. Thus, the primary structure of a collaboration is the structure of interfaces. Although, in a complex structure, there might be many ways to assign units to interfaces, it must be done within the framework of the given structure of inter- faces. In effect, units become defined as black boxes on all structural levels, and in all types of model fragments. We can only describe them through their external behaviour, considering all interfaces they support. 2 Foundations of Sysnet Modelling 2.3.3 Interface Refinement Most modelling methods and languages use the term "interface" to describe some communication aspect between two or more entities. There are, however, consider- able differences with respect to which aspect. The entities in ROOM, see Selic et al. (1994), are actors. Actors exchange (ROOM) messages over interfaces. The entities in UML are (UML) objects, see OMG UML v.1.3 (1999). Object interfaces are defined by e.g. a set of (UML) operations. An interface between OSI layer entities in different layers in the same physical node is called service access point (SAP). Discret