Subject Name DIGITAL SWITCHING SYSTEMS Subject Code 10

• Subject Name: DIGITAL SWITCHING SYSTEMS • Subject Code: 10 EC 82 • Prepared By: Farha Kowser, Aparna P • Department: Electronics and Communication Engineering • Date: 06/4/2015

UNIT 4 SWITCHING SYSTEMS 06/4/2015

Introduction: A basic requirement for switching systems is to be able to design switching networks having a greater number of outlets than the switches from which they are built. If the traffic offered A increases the number of trunks also increases. As a result the number of crosspoint s increases hence cost will increase. So we can go for Grading or Multistage networks 06/4/2015

Grading: This a technique with the help of which we can reduce the number of trunks used by making some trunks common to all the groups. Principle of Grading: If the traffic offered to two groups of incoming trunks is random. Efficiency can be improved by making few trunks common to both the groups. In a group, if the switches search for free outlets sequentially , the later choice trunks carry least traffic. Hence it is desirable to connect the later choice trunks to both the groups in common 06/4/2015

Example for Grading: In the above example, even though the network shown in fig 2 consists of 16 trunks, it can handle same amount of traffic as network of 20 trunks as shown in fig 1 Fig 1: 20 trunks connected in two separate groups. a) Full Diagram b) Grading Diagram 06/4/2015 Fig 2: 16 trunks interconnected to two groups. a) Full Diagram b) Grading Diagram

Link Systems or Multistage Networks: Single stage networks suffer from a lot of disadvantages which can be overcome by adopting multistage networks. We have • Two stage network • Three Stage network • Four stage network Two stage networks: A two stage switching network is shown in figure. It consists of two sets of switches primary and secondary switches. The link which connects primary switches to secondary switches are called as A links 06/4/2015

Three stage networks: A three stage switching network is shown in figure. It consists of three sets of switches primary, secondary and tertiary switches. The links which connects primary switches to secondary switches are called as A links and The links which connects secondary switches to tertiary switches are called as B links. The comparison of two stage, three stage and four stage switching networks is given in the table 06/4/2015

Fig: Comparison of Switching networks 06/4/2015

Grade of Service: In telecommunication engineering, and in particular teletraffic engineering, the quality of voice service is specified by two measures: the grade of service (Go. S) and the quality of service (Qo. S). Grade of service is the probability of a call in a circuit group being blocked or delayed for more than a specified interval, expressed as a vulgar fraction or decimal fraction. This is always with reference to the busy hour when the traffic intensity is the greatest. Grade of service may be viewed independently from the perspective of incoming versus outgoing calls, and is not necessarily equal in each direction or between different source-destination pairs. GOS of a link system depends on the way it is used • Mode 1: Connection is required to one particular free outgoing trunk • Mode 2: Connection is required to one particular outgoing trunk, but any free trunk on that route may be used. • Mode 3: Connection may be made to any free outgoing trunk. 06/4/2015

Hence we have B 1 - GOS in mode 1 B 2 - GOS in mode 2 B 3 - GOS in mode 3 Two stage networks: B 1 =1 -(1 -a)(1 -b) B 2 = [ a + b(1 -a)]g B 3 =E 1, N (A) Three stage networks: B 1 =[a + (1 -a) b] g 2 B 2 = [ B 1 + c(1 -B 1)] g 3 Four stage networks: B 1 =[1 – (1 -a)(1 -b)(1 -c)] g 2 06/4/2015

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