Lead Research Organisation: Imperial College London
Department Name: Dept of Computing


Quantitative methods are vital for the design of efficient systems in ICT, communication networks and other logistical areas such as business processes and healthcare systems. However, the resulting models need to be both accessible to the designer, rather than only to the performance specialist, and efficient. A sufficiently expressive formalism is needed that can specify models at a high level of description and also facilitate separable and hence efficient mathematical solutions. Stochastic process algebra (SPA) is a formalism that has the potential to meet these requirements. One approach to tackling the state space explosion problem common to all compositional modelling techniques is through the exploitation of, so called, product-form solutions. Essentially, a product-form is a decomposed solution where the overall steady state distribution of the system can be found by multiplying the marginal distributions of the components. product-form solutions can generally be defined by properties of the reversed process and Harrison's seminal result, known as the Reversed Compound Agent Theorem (RCAT) [15], gives a method for generating the reversed process of a Markovian process algebra model at the component level (under simply specified conditions), without recourse to the underlying continuous time Markov chain. This has led to new understanding of a range of product-form results that were previous considered separately, as well as to new product-forms. It also enables a mechanical derivation of decomposed solutions, not only of models with an exact product-form, but also potentially bounded approximations for models which almost have a product-form in a certain (quantitative) sense. This application is closely related to the EPSRC funded SPARTACOS grant (EP/D047587/1), currently held by Harrison. This current application will complement and extend work in SPARTACOS by considering models which 'almost' have a product-form solution, in a sense to be specified precisely, and models which are subject to a non-product-form decomposition, which has not been part of the SPARTACOS pro ject. (SPARTACOS itself also considers response time distributions, higher moments, discrete time and fluid models.) The principal part of this pro ject will be in facilitating an extended research visit to Imperial College by Dr Thomas.


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Description The primary goal of this project was to explore the practicality and effectiveness of a compositional approach to modelling the performance of computer networks and systems, in the mold of more traditional engineering as well as Software Engineering itself. Some significant extensions of the existing state-of-the-art in product-form theory were found, leading to a notion of semi-product-forms. Following on from the PI's RCAT theorem's approach, the analysis was based on the notion of reversed processes, and a number of successful research papers resulted and are attached. The second goal was to strengthen the collaboration between the PI's research group at Imperial and Nigel Thomas's in Newcastle and to write a new research proposal. The former of these was certainly accomplished and the second has been delayed due mainly to various personal circumstances. However, a new proposal is under construction.
Exploitation Route The grant achieved its intended purpose of strengthening the collaboration between my research group at Imperial and Nigel Thomas's in Newcastle. Indeed, the project output was impressive given the duration of only 6 months, testifying to the quality of the collaboration that already existed.

The findings naturally lead to new research proposals, and APROPOS, funded by the EPSRC, was a concrete example of a successful one. The collaboration continues, a new proposal is underway and in fact the results would be appropriate for any other interested research group to take up. One example who has developed the ideas in his own way is Dr Andrea Marin of the University of Venice (see the project "APROPOS").
Sectors Digital/Communication/Information Technologies (including Software),Education