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"Side 1:- This side of the cassette contains just one band, associated with Unit 9. Peter Strain-Clarke discusses the problems that arise when the definition of "communication" given... in the Unit is applied to simple concurrency models. This band is intended for listening whilst looking at the frames in the Unit. The first model shown is that of asynchronous behaviour, with no intercommunication at all. This develops into a simple model with a single directed channel from sender to receiver. Although this is very simplistic, it still involves a certain amount of organisation before there can be any meaningful communication, otherwise there may be "deadlock" or "livelock". The discussion of these ideas introduces the concepts of "input" and "output", and of "types" of messages that can be transmitted. The next model takes two-way communication at its simplest level, and looks at the inherent problems in planning and executing this effectively in order to avoid deadlock and get meaningful communication between the two parties. Doing this involves the use of "protocols". As a development of this simple model, Peter Strain-Clarke takes a look at telephone networks. His new model has just two people, each having an answering machine which they can control. Looking at this model introduces ideas of "parallel processing" as a way to avoid deadlock problems, although there are still inherent problems due to the asynchronous behaviour of each end of the network. Finally he looks at the problem of three-way conversations, where each person has available two one-way channels. This time there are several alternative protocols which can be applied to ensure no deadlock and the rest of the band looks at how such a protocol could be developed and at the idea of "alternatives"; alternatives are typical of many real-time concurrent systems. Students will be meeting these latter ideas again in a case study. Side 2: There is only one band on this side of the cassette, associated with Unit 12. By applying them to a familiar problem David Crowe reviews the ideas of process analysis, formal specification and OCCAM which are introduced earlier in the Unit. Having been introduced to the famous Tower of Hanoi problem students are challenged to develop and describe a solution for the problem involving just three discs which can readily be generalized for any number of discs. It turns out that the simplest algorithm alternately keeps the smallest disc moving while doing whatever is possible on the other moves but the snag is that this does not exploit concurrency; the band goes on to find a sequential algorithm that can be described by a small number of communicating processes. Each tower can be thought of as a separate process, and so effectively we now have three such processes operating in parallel. As the processes involve the exchange of discs between towers, the problem can be modelled as three concurrent processes connected by two-way channels. At this stage the towers are numbered (0, 1 and 2) and the channels are labelled with directions (clockwise and anticlockwise). Five parameters are introduced corresponding to the tower index and to the operation of the communication channels, which are ordered systematically. Process analysis yields a CSP specification which is gradually refined and given the corresponding OCCAM code. Code is also developed for initialisation. The next stage is developing a new sequential algorithm, it's CSP specification and the related OCCAM code plus the necessary initialisation. Finally this solution is checked out and it is found that as it stands it may lead to deadlock. Students will be shown how to overcome this problem in their follow-up work."
Metadata describing this Open University audio programme
Module code and title: M355, Topics in software engineering
Item code: M355; AC2
Rights Statement: Rights owned or controlled by The Open University
Restrictions on use: This material can be used in accordance with The Open University conditions of use. A link to the conditions can be found at the bottom of all OU Digital Archive web pages.
Duration: 00:57:27
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Track listing:
track listing for this programme
Side 1 Track 1 Unit 9: Concurrent Processes
Side 2 Track 2 Unit 12: Concurrency Case Studies
Publisher: BBC Open University
Master spool number: AC1179
Production number: AC1179
Available to public: no