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Note for Digital Switching and Telecommunication Networks - DSTN by Divyansh Das

  • Digital Switching and Telecommunication Networks - DSTN
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Objective of the Module- I: • To learn about different basic components that are used in telephone exchanges • Difference between an automatic exchange and manual exchange • Learn about different types of electronic exchanges and about the software architecture used in an electronic exchange. • To differentiate between a single stage and a multistage network. • To understand the advantages of multistage network over a single stage network. • Design of multistage network to reduce blocking of calls. • Design of multistage network to reduce the number of switching matrices • To teach different types switching techniques that are used in exchanges such as time division time switching, time division space switching and combination of both types of switching MODULE-1: 1. INTRODUCTION: Telecommunication networks carry information signals among entities, which aregeographically far apart. An entity may be a computer or human being, a facsimile machine, atele-printer, a data terminal and so on. The entities are involved in the process of informationtransfer, which may be in the form of a telephone conversation (telephony) or a file transferbetween two computers or message transfer between two terminals etc. 2

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A switch transfers signals from one input port to an appropriate output. A basic problem is then how to transfer traffic to the correct output port .In the early telephone network, operator’s closed circuitsmanually. In modern circuit switches this is done electronically in digital switches. If no circuit is available when a call is made, it will be blocked (rejected). When a call is finished a connection teardown is required to make the circuit available for another user. Fundamentals of switching systems: Types of communication transmission mode: Simplex Half Duplex Talkie Full Duplex : : one way communication ex: Radio Two way communication shared by single channel ex: walkie : Two way communication simultaneously ex: Telephone Therefore, telephone comes under the Full Duplex type of communication. Point – Point Links/Fully Connected Network/Bell Proposed Network: Fig 1.1. Point to Point link [1] To connect ‘N’ Points the number of Links required is as below: Problem: Calculate the Number of links required to fully connect 5000 links and the numberof additional links required to fully connect 5001 links Solution: to connect 5000 points, numbers of links required are: 12497500 To connect 5001 points, number of links required: 12502500 Therefore, the additional links required to connect extra 1 point on a 5000 points network of fully connected are: 5000 From the above problem it is understood that it is highly impossible to connect large number points (telephones) as fully connected network/point-point network. To resolve this problem “Telephone Exchange” came into existence 2. TELECOMMUNICATION NETWORKS: 3

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Fig 1.2 Various telephone networks[6] (a) telephone network around 1890 , (b) telephone network around 1988 (a) telephone networks after 1990 with ISDN The electromechanical switching systems have been replaced by computer controlled switching systems referred to as stored program control (SPC). In SPC, switching is controlled by software program. The first computer controlled switch was introduced in 1960. Till 1965, computer controlled switching used transistors and printed circuit technology. Since 1965 switchingis based on microprocessors. 4

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