d) as the target for mod- elling with SM&AML. We defined such systems as consisting of units where con- siderations regarding if a unit is software or hardware or both is of no interest. Implemented systems are described by software- and hardware-oriented languages. Those languages use their own concepts for specifying interfaces, components, structures and behaviour. Since sysnet models can specify levels of details that match most software-oriented modelling languages, sysnet models can be mapped more or less automatically on models of implemented systems. The purpose with sysnet modelling might be just to give an overview over major system structures and functions, as we did in Sect. 2.2. However, it might as well be to produce a model that can act as a detailed specification for the design of implemented systems. The level of details of the sysnet model must be defined sep- arately by each development process that applies SM, but AML must be able to support all levels of details, including levels that match the level of details of imple- mented systems. To support such specification levels, the model must also specify interfaces and protocols (message protocols, abstract protocols and primitive serv- ices). Message protocol specification is an important aspect of a language for systems modelling. An important part is to specify the messages. Messages must be 2.5 Views and Visualization Modes described in a way that is viewed as independent of any specific implementation language, because we want to be open to the use of different implementation lan- guages in a network. A language for message specification will therefore be regarded as an abstract language. Note that in some methodologies, protocol spec- ification is regarded as an implementation issue. Since, however, a protocol specifi- cation does not prescribe a specific way of implementing the network elements that handle the protocol, this view is not shared by SM&AML. As part of the OSI concept, the Abstract Syntax Notation One (ASN.1) was developed. T