This paper proposes a new modeling method for the evaluation of dependability measures of highly available systems. The propose d method, which has been implemented in the tool OpenSESAME (Simple but Extensive Structured A availability Modeling Environment), combines the advantages of Boole an Methods and state space base d methods. The tool supports The modeler with a set of well-defined, structured, intuitive input diagrams and tables, which are automatically transformed into GSPNs (Generalized Stochastic Petri Nets) for evaluation. To show the usefulness of the proposed method, it is applied to A model of a typical CompactPCI-based high availability system as it can be found in the telecommunications area.