Code: A-17 / T-17                                             Subject: TELECOMMUNICATION SYSTEMS

Time: 3 Hours                                                                                                     Max. Marks: 100

 

NOTE: There are 11 Questions in all.

 

·      Question 1 is compulsory and carries 16 marks. Answer to Q. 1. must be written in the space provided for it in the answer book supplied and nowhere else.

·      Answer any THREE Questions each from Part I and Part II. Each of these questions carries 14 marks.

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

 

Q.1       Choose the correct or best alternative in the following:                                           (2x8)

       

a.       The number of links in a fully connected network of 5 nodes is

                  

                   (A)  5.                                                 (B)  10.

(C)    20.                                              (D)  32.

 

             b.   The maximum data rate on a 3 KHz wide binary channel with a 15 dB SNR is about

 

(A)     6 Kbps.                                      (B)  12 Kbps.

(C)   15 Kbps.                                    (D)  45 Kbps.

          

             c.   In 6B-8B coding, the maximum number of 8-bit words that can have exactly four 1’s is

            

(A)  16.                                               (B)  48.

(C)  64.                                               (D)  More than 64.

 

             d.   During a two-hour busy period, 2400 calls arrive.  Each has an average holding time of two minutes.  The offered traffic is

 

(A)  24 E.                                            (B)  40 E.

(C)  48 E.                                            (D)  80 E.

 

             e.   In a certain system, the signal power is 20 mW while the noise power is         –0.5 dBm.  The SNR is

                  

(A)  10 dB.                                         (B)  12.5 dB.

(C)  13.5 dB.                                      (D)  19.5 dB.

 

             f.    TASI stands for

 

(A)  Time Assignment Speech Interpolation.          

(B)  Transmission And System Interface.

(C)  Transmission And System Interference.

(D)  Terminal Aided System Interaction.

 

             g.   The width of a B-channel in ISDN is

 

(A)     4 KHz.                                        (B)  16 KHz.

(C)  32 KHz.                                       (D)  64 KHz.

 

             h.   ADSL stands for

 

(A) Advanced Digital Subscriber Loop.    

(B) Asymmetrical Digital Subscriber Loop.

(C) Advanced Digital Session Layer.

(D) Asynchronous Digital Subscriber Loop.

 

PART I

Answer any THREE Questions. Each question carries 14 marks.

 

  Q.2     a.   List all types of transmission impairments and describe any two of these impairments.                      (7)

            

b.      What do you understand by Stored Program Control (SPC)?  Explain centralised SPC.                 (7)

 

  Q.3           Consider a three-stage space division switching network with first and third stages having square matrices.  Derive expressions for minimum number of switching elements and blocking probability.      (14)

                                                                                                                                                

  Q.4     a.   Derive an expression for the grade-of-service in a ‘Lost Calls Cleared’ (LCC) system with finite sources.                                                              (7)

            

             b.   A group of 20 servers carry a traffic of 10 E.  The average duration of a call is 3 minutes.  Find the average number of calls put through a single server and the group as a whole in one hour period.      (7)

 

  Q.5     a.   Consider 20 servers and 100 subscribers.  On an average 10 servers are busy.  The probability of all servers being busy is 0.2.  Find the grade-of-service for

                   (i)  Erlang traffic and                            (ii)  Engest traffic.                                    (7)

 

             b.   Find the probability of clipping for a TASI system having 50 speech channels supported by 25 speech circuits.  The activity factor is 0.4 / user and arrival is Poisson.                                            (7)

 

  Q.6     a.   Describe cellular telephony, using GSM.  Compare it with CDMA cellular systems.            (7)

 

             b.   Explain store-and-forward technique for data transmission giving its classification, merits and shortcomings.  Obtain an expression for transmission delay.                                                                       (7)

 

 

 

 

PART II

Answer any THREE Questions. Each question carries 14 marks.

 

  Q.7     a.   Explain wavelength division multiplexing and demultiplexing.                                (7)

 

             b.   What do you understand by chromatic dispersion?                                              (3)

 

             c.   Find the loss limit and chromatic dispersion limit of a single mode fiber optical transmission system operating at 1300 nm, fiber loss 0.35 dB/Km and providing a bandwidth of 417 Mbps.  The fiber has a BDP of 250 Gbps-Km.  A narrowband source is used giving 42 dB higher power than required at the receiver. Neglect other losses.                                        (4)

       

  Q.8           Find the system margin, the dispersion limited repeater spacing and the loss margin for an Fiber Optic Transmission system given

                   Data rate : 565 Mbps;  5B-6B coding;  RZ pulses;    DFB-LD with 0.4 nm FWHM; -5dBm output; SMF with dispersion coeff               17 ps/Km-nm; 0.2 dB/Km loss; Splice loss 0.2 dB/Km, Receiver sensitivity at 500-600 Mbps is –33 dBm and at 600-700 Mbps, it is                  –34 dBm. BDP 250 GHz-Km-nm.                                                                                           (14)

 

  Q.9     a.   What is ISDN?  Describe and list features and benefits of B and D channels.                      (8)

 

             b.   Find the distance limit imposed by the need to echo E bit in BRI S/T interface.  The minimum delay between a terminal transmitting a D bit and receiving it back in the following E bit is 7 bit duration.  The data rate is 192 Kbps.  The speed of transmission (propagation) is 25% of speed of light in vacuum.  Ignore other delays.                                                 (6)

 

Q.10           a.                                                        Write a note on ATM networks.                             (8)

       

             b.   An interactive computer user generates messages at an average rate of         2 messages / mt.  Each message is 40 characters long.  The line speed is 9600 bps.  Find the percentage utilisation of the line.          (6)

 

Q.11           a.                                                        Find the probability of maximum interference of a 32 channel CDMA system with 32 spreading codes.  Also find the signal-to-inference power ratio (SIR).  Assume that all channels operate at the same effective power level at the receiver and that all channel codes have a cross-correlation of 1 bit.                                                                                       (10)

 

             b.   Explain briefly SDH.                                                                                          (4)