The introduction of computers and software engineering in telephone switching systems has dictated the need for powerful design aids for such complex systems. Among these design aids simulators - real-time environment simulators and flat-level simulators - have been found particularly useful in stored program controlled switching systems design and evaluation. However, both types of simulators suffer from certain disadvantages. An alternative methodology to the simulation of stored program controlled switching systems is proposed in this research. The methodology is based on the development of a process-based multilevel hierarchically structured software simulator. This methodology eliminates the disadvantages of environment and flat-level simulators. It enables the modelling of the system in a 1 to 1 transformation process retaining the sub-systems interfaces and, hence, making it easier to see the resemblance between the model and modelled system and to incorporate design modifications and/or additions in the simulator. This methodology has been applied in building a simulation package for the System X family of exchanges. The Processor Utility Sub-system used to control the exchanges is first simulated, verified and validated. The application sub-systems models are then added one level higher_, resulting in an open-ended simulator having sub-systems models at different levels of detail and capable of simulating any member of the System X family of exchanges. The viability of the methodology is demonstrated by conducting experiments to tune the real-time operating system and by simulating a particular exchange - The Digital Main Network Switching Centre - in order to determine its performance characteristics.

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