SIMULATION OF A MULTIPROCESSOR COMPUTER SYSTEM

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.The General Electric Company Ltd, GEC Hirst Research Cent, Wemble

Reliable file storage in a distributed computing system

This thesis considers the problem of how to construct a file-store which is reliable in terms of high accessibility of data and low likelihood of data loss. In particular the problem is discussed in the context of a local area network computer architecture. A novel approach is taken to the implementation of the naming network for files, which controls multiple copy redundancy. The naming network is a single hierarchy which incorporates redundancy in the access paths to files as well as in the files themselves, thus improving accessibility as well as reducing likelihood of file loss. A prototype file-store was developed and implemented; to facilitate this the author had to develop a simple distributed operating system which evolved as an interesting research project in its own right. A distributed name-server algorithm was developed, and interesting insight gained into the design of local area network computer systems. The property of local area networks important to the file-store is scope for dynamic redundancy; the file-store is constructed out of a number of independent file-servers. Inconsistencies between multiple copies of a file are resolved automatically. Levels of redundancy and data location are controlled through the naming network, allowing replication of files to any degree thought necessary, bounded by the number of independent storage volumes in the system. Deadlock avoidance and automatic reconfiguration on hardware component failure are included. Some simple combinatorial mathematics is included to highlight the reliability of multiple independent copies of a file; the need for a more quantitative approach is indicated