Code: D-20                                                    Subject: ELECTRONIC SWITCHING SYSTEMS

Time: 3 Hours                                                                                                     Max. Marks: 100


NOTE: There are 9 Questions in all.

·      Question 1 is compulsory and carries 20 marks. Answer to Q. 1. must be written in the space provided for it in the answer book supplied and nowhere else.

·      Out of the remaining EIGHT Questions answer any FIVE Questions. Each question carries 16 marks.

·      Any required data not explicitly given, may be suitably assumed and stated.



Q.1       Choose the correct or the best alternative in the following:                                   (2x10)


a.       A Master group consists of


                   (A)  12 voice channels.                        (B) 24 voice channels

(C)    60 voice channels.                       (D) 300 voice channels


b.      Direct inward dialing is used as a feature in


                   (A)  PSTN.                                         (B)  PBX.

(C)     EPABX.                                      (D)  VPN.


             c.   Trunks are the lines that run between


(A)    Subscribers and exchange             (B) switching system and power plant

(C) local area network.                        (D) switching stations.


             d.   Traffic Capacity is given by


(A)    Switching capacity  Theoretical maximum load       

(B)    Switching capacity / Theoretical maximum load

(C)  Theoretical maximum load / Switching capacity         

(D)  Theoretical maximum load  Switching capacity


             e.   In a time multiplexed space switching system, one speech sample appears every


(A)     125 microsec                               (B)  20 msec

(C)  125 msec                                     (D)  1 sec


             f.    ISDN handles data pertaining to


(A) All digital services                          (B)  Speech and Video

(C) Computer data only                       (D)  Speech only


             g.   A star connected intermediate exchange is known as a


(A)    Repeater exchange                       (B) Hub exchange

(C) Private branch exchange                (D) Tandem exchange


             h.   Time synchronization is necessary in


(A) FDM.                                           (B) TDM.

(C) WDM.                                          (D) Quadrature multiplexing


             i.    In a frame transmission, CRC stands for


(A)   Code Renewable Check               (B) Cyclic Redundancy Check

(C) Control and Refresh Code             (D) Cyclic Refreshing of CPU


             j.    In a LAN network every system is identified by


(A)  Name                                           (B) MAC address

(C)  IP Address                                  (D) Serial number given by




Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.


  Q.2     a.   What are the different tones used in  Strowger telephony?  Explain with the help of waveforms and the timings.                                                                                                              (8)


             b.   Using a combination of uniselectors and two motion selectors, draw a schematic of thousand line exchange and explain its working.                        (8)


  Q.3     a.   Name the switching schemes used in a digital exchange. How call processing takes place?             (8)


             b.   How speech is transmitted in a digital switching environment using PCM/TDM?                  (8)       


  Q.4     a.   List the major differences in  two stage, three stage and n-stage networks. Compare their blocking characteristics.                                     (8)


             b.   A three stage network is designed with the following parameters:

                   M=N=512, p= q = 16 and .  Calculate the blocking probability of the network, if s=16. Symbols carry their usual meanings.                    (8)


  Q.5     a.   How does one arrive at the probability of availability of free lines during the busy hour?  How can this be improved?                                           (8)


             b.   Discuss grade of service.  During busy hour, 1500 calls were offered to a group of trunks and 8 calls were lost.  The average call duration was 120 seconds. Calculate the traffic offered, traffic lost, Grade of service and total duration of congestion.                                                                         (8)


  Q.6     a.   What are the different types of distributing frames used in exchanges? Explain their importance.                                                                  (8)





             b.   In a 10000 line exchange, 0000 to 2999 is allotted to x group of subscribers, out of which 40% are active during busy hour.  The remaining numbers are domestic numbers out of which 20% are active.  Each group consists of 100 subscribers.  Determine the number of final selectors required.                    (8)       


  Q.7     a.   What is meant by common control. Explain all the categories that are served by Common Control switching.                                                           (8)


             b.   Explain the working of broad band ISDN.                                                         (8)


  Q.8     a.   What is the need of an hybrid in telephone networks? How does it  work?          (8)


             b.   What layers are covered under end to end layer connectivity? Explain briefly the function of each one of them.                                                   (8)


  Q.9           Write short notes on:


(i)                  Stored Program Control.

(ii)                Congestion.

(iii)               Common channel signalling.

(iv)              PSTN.                                                                                (4  4)