es new internal interfaces. The result becomes a hierarchical discrimination structure. Each subflow created by a discriminating unit is used to support a hierarchical structure of units. Structures for different subflows that are created by the same discrimination unit are functionally completely independent from one another. Therefore, it is generally impossible to define a one-dimensional structure of layers (as e.g. the OSI seven-layer model) for an interface by applying discrimi- nation alone. The same is true for units, since a unit may support several (exter- nal) interfaces. Pure message discrimination is based on the fact that MD identifies one particu- lar parameter of a message as the Did, but leaves the rest of the (bits of) the mes- sage to be interpreted by the unit to which they are relayed. A more advanced form of discrimination is the operation discriminator, which discriminates communica- tion events in the external interface from communication events in the internal interfaces, see Fig. 2.26. A communication event refers a particular operation. When operation discrimi- nation is applied, the operation handler (OPH) terminates the external message for- mat (M) and extracts information about what operation is referred to (the message should include an operation identifier, OPid). The OPH then determines which of y1, y2 and y3 can handle that operation. Subsequently, the OPH creates a suitable internal message (m1, m2, etc.) that carries data that describes the event and relays the message to the internal unit. In Fig. 2.26 it is assumed that the OPH can reach all internal units over a relay chain. Relay chains are described in more detail in Sect. 5.6. 2.3 Architectural Principles A simpler form of discrimination is object discrimination. In this case, the dis- criminator searches for an object identifier (OBid) in the external message. It asso- ciates each value of OBid to a particular internal unit (the "object"), and relays the event data to that unit. Operation discrimination is, in that case