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The programme explores how contemporary telephone exchanges use switching devices to route calls and in particular how the number of switches and interconnections can be optimised to produce the economic system.
Metadata describing this Open University video programme
Module code and title: TM361, "Graphs, networks and design"
Item code: TM361; 02
First transmission date: 18-03-1981
Published: 1981
Rights Statement:
Restrictions on use:
Duration: 00:24:00
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Producer: Ted Smith
Contributors: Ken Cattermole; Roy Nelson
Publisher: BBC Open University
Keyword(s): Cut-sets; Graphs; Networks; Nodes; Telecommunications; Vertices
Footage description: The programme opens in a central London telephone exchange with Roy Nelson listening to a recorded message from the weather bureau in San Francisco. With the aid of animated diagrams he then traces the development of the international telephone system from its beginning in New Haven, Connecticut in 1878. Prof. Ken Cattermole shows an animation in which a star graph Kln is represented as a digraph with each telephone subscriber connected both to an inlet and outlet of the exchange. He then distorts this to a more useful shape and replaces the exchange with a switch. Cattermole next demonstrates the operation of a crossbar switch. Roy Nelson briefly explains why a crossbar switch as demonstrated above could not be scaled up to deal with the traffic in an ordinary telephone exchange. Prof. Cattermole carries on by explaining how the introduction of multistage switching, with several small crossbar switches making up a stage, solves the problem of the physical size of individual co-ordinate switches. He uses several diagrams to illustrate his points. Cattermole goes on to explain how, with the use of graph theory, the problem of controlling switches in a multistage network can be worked out. He demonstrates this by looking at a three stage switch and its graph. Cattermole then looks at a more complex 5 stage switch which achieves even greater economy of crosspoints. Roy Nelson considers what happens in a network when more and more links become unavailable until finally the network becomes disconnected. This, he explains, is the phenomenon of blocking, whereby existing connections through the network can prevent subsequent connections being made. Nelson goes on, with the aid of animated diagrams, to show how one can choose between possible networks to find one which is least likely to become blocked. He explains that for channel regular networks it is only necessary to compare their respective channel graphs. Prof. Cattermole, with the aid of animated diagrams, explains how one constructs a graph of a network which is channel regular and has multiple outputs and inputs. Roy Nelson summarises the programme.
Production number: FOUT077P
Videofinder number: 1523
Available to public: no