e data elements (bit- or octet- strings), called service data units (SDU), are transported between SAPs. The properties of the flow of SDUs is specified in terms of quality of service (QoS) parameters. The entities communicate over SAPs by a set of standardized abstract service primitives (ASP), called DATA request or DATA indication, where SDU is a parameter of the primitives. In the interface between OSI layer entities in the same layer but in different physical nodes, protocol data units (PDU) are exchanged (PDU is synonymous with message). A PDU is a data element that is described as a structured data type. The different views languages have on the interface concept represent different abstractions of interfaces: UML does not include data flow aspects at all. OSI regards data flow aspects in interfaces between entities in different layers. ROOM combines both message aspects and some data flow aspects (through ports) in the same interface concept. Data flow properties (QoS) are, however, not part of the (ROOM) interface definition. The difference between SM&AML and other modelling languages in this respect is that SM&AML recognizes all views as relevant abstractions, and there- fore provides a method for interface refinement, which allows interfaces to be spec- ified on different levels of abstraction. SM&AML shares the view (expressed in OSI RM and partly in ROOM) that it is important to separate the aspects of an interface into the data flow over the inter- face and the information that is carried by that data flow. Data flows and informa- tion are just two different aspects of an interface, as shown in Fig. 2.15. 2.3 Architectural Principles interface information data flow Fig. 2.15 Refinement of an interface into an information flow and a data flow There are always two aspects of the data flow to consider: QoS and the connec- tivity relation between units. SM&AML distinguishes between three types of con- nectivity relations: mechanical connections (cables), physical media connections and logical connection