A POOSL Model of the MASCARA Steady State Control

accept any responsibility regarding the contents of Master's Theses ii iii Summary The Parallel Object-Oriented Specification Language (POOSL) is meant for the specification of systems that are complex, reactive, concurrent, real-time and distributed. The SHESim tool can be used to construct POOSL models and run simulations on them. An abstract model of the Steady State Control part of the MASCARA protocol is developed in POOSL. The MASCARA protocol is a Medium Access Control protocol for wireless ATM. It is used within the European research project VIRES (Verifying Industrial REactive Systems). Since the MASCARA protocol as a whole is very large, a selection has been made. The selected parts of the MASCARA protocol are remodelled in POOSL on the basis of available informal specifications, source code written in SDL (Specification and Description Language, used within VIRES) and several Message Sequence Charts (MSCs). The original SDL source code is incomplete and contains modelling errors. Results of intermediate simulations of the POOSL model lead to suggestions for improvements. The development of the POOSL model also leads to extensive documentation on the modelled part of the MASCARA protocol. Furthermore, it serves as an example to examine the suitability of the POOSL language and the SHESim tool for the development of complex protocols. To some extent, the languages SDL and POOSL are compared and simulation as an approach is evaluated. Several abstractions are made to grasp the essential behaviour. The underlying communication structure (protocol layers) is omitted, so that peer processes directly communicate with each other, and the data part of the protocol is also completely left out. Decisions based on certain data values are replaced with probabilistic decision mechanisms. Assignments of frequency channels and MAC addresses are simplified. The simulation results indicate that the model seems to work correctly. However, error free simulation runs do not guarantee the correctness of the model. Rules of thumb are established regarding system parameters that indicate timer values. In spite of different terminology, specifications in both SDL and POOSL consist of a number of concurrent processes that communicate by exchanging information over channels. The SDL syntax and layout structure can be converted into POOSL syntax and layout structure. There are important semantic differences, however. The underlying communication mechanisms are completely different. SDL uses asynchronous buffered communication, whereas POOSL uses synchronising rendezvous communication. Furthermore, the grain of parallelism in SDL is situated at state transition level, whereas in POOSL …