Industry is facing a crisis in the design of complex hardware/software systems. Due to the increasing complexity, the gap between the generation of a product idea and the realisation of a working system is expanding rapidly. To manage complexity and to shorten design cycles, industry is forced to look at system level languages towards specification and design. In this paper we report on the system level modelling language called POOSL. The language is very expressive and is able to model dynamic hard real-time behaviour as well as static (architecture and topology) structure in an object-oriented fashion. The language integrates a process part, based on the process algebra CCS, with a data part, based on the concepts of traditional object-oriented programming languages. Unlike many modelling languages today, POOSL is equipped with a complete mathematical semantics. Currently a number of automated software tools (model editing, simulator and compiler tools) are available. It will be shown how the language and tools allow the estimation of a performance parameter of a datalink protocol.