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. A fully connected network of six nodes needs

(A) 6 links. (B) 15 links.

(C) 30 links. (D) 36 links.

b. The maximum bit rate on a 5 KHz wide channel with a 30 dB SNR is about

(A) 10 Kbps. (B) 30 Kbps.

(C) 50 Kbps. (D) 150 Kbps.

c. In 5B-6B coded transmission, the maximum run length of 0-bits is

(A) 4. (B) 5.

(C) 6. (D) 11.

d. In a busy hour, 300 calls arrive. The average duration of a call is 3 minutes. The offered traffic, in Erlangs, is

(A) 90. (B) 15.

(C) 9. (D) 3.

e. The ideal Grade of Service (GOS) in telephone systems is

(A) 0. (B) 0.5.

(C) 1. (D) 100.

f. The operating state diagrams of a modem has states at coordinates (1, 1), (1 ,-1), (-1, -1) and (-1, 1). The bit rate corresponding to a 2400 baud modem is

(A) 9600. (B) 4800.

(C) 2400. (D) 1200.

g. PSTN stands for

(A) Public Switched Telephone Network.

(B) Private System Transmission Network.

(C) Private Subscriber Telephone Network.

(D) Public Switched Transmission Network.

h. A drawback of Avalanche Photo Diodes (APD’s) as optical-to-electrical converters is

(A) not operating in 1300-1550 nm range.

(B) having lower sensitivity.

(C) having no internal amplification.

(D) being temperature sensitive.

PART I

Answer any THREE Questions. Each question carries 14 marks.

Q.2 a. What do you understand by pair-gain systems? Explain and compare concentrators and multiplexers. (7)

b. Draw a comparison between single stage and multistage space division switching networks. Give your answer pointwise in a tabular form. (7)

Q.3 a. Describe TS switching and find its implementation complexity. (7)

b. Find the implementation complexity of a TS switch where the number of TDM lines is 100 and each input line contains a single DS1 signal (32 channels). Assume a one stage matrix for space stage. The number of bits per channel is eight. (7)

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

b. A total of 40 subscribers initiate calls in a 20 minute period. The total duration of calls is 4800 sec. Find the load offered to the network and the average subscriber traffic. (7)

Q.5 a. A small community with 400 subscribers is to be serviced with a community dial office switch. Assume that the average subscriber originates 0.1 erlang of traffic. Also assume that 20% of the originations are local (intracommunity) calls and that 80% are transit calls to the serving central office. How many erlangs of traffic are offered to the community-dial-office-to-central-office trunk group? How many trunks are needed for 0.5% blocking of the transit traffic? (7)

b. Draw a cell structure and allocate frequencies such that there is a separation of at least two cells using the same frequency. Ensure minimum number of frequencies. (7)

Q.6 a. Describe CDMA cellular system and explain channel establishment. (7)

b. A TASI system has 10 channels and 20 sources connected to it. Find the probability of clipping if the activity factor for each source is 0.4. (7)

PART II

Answer any THREE Questions. Each question carries 14 marks.

Q.7 a. Describe 5B-6B coding scheme and give its salient features. (7)

b. A graded, multimode fiber exhibits 100 psec/km nm of chromatic dispersion at 850nm. Determine the BDP of a NRZ system using an LED having 40 nm of spectral width. (7)

Q.8 a. Write a short note on data networks. (7)

b. For a certain EPABX, 180 outgoing calls are initiated every hour. Equal number of calls also come in. Average duration of each call is 200 sec. The required grade-of-service is 0.05. Find the number of lines required between EPABX and the main exchange. (7)

Q.9 Describe ISDN giving benefits / features of B and D channels. Also give basic rate and primary rate access architecture. Describe ISDN DSL Transmitter / receiver with the help of a block diagram. (14)

Q.10 a. Explain circuit switching technique for data transmission and list its merits and shortcomings. Also derive an expression for the total time for a given data transfer. (7)

b. A 50 Km long circuit switched connection involves 5 nodes, each node taking 2 sec for establishing and 0.2 sec for releasing a connection. Data to be transferred is 30,000 bits long and the rate is 2400 bps. The signal propagation speed is 200 m /. Find the total data transfer time. (7)

Q.11 a. Distinguish between hard hand off & soft hand off in cellular systems. (4)

b. What is the effective signal-to-interference ratio of a single CDMA uplink channel operating at a distance that is twice as far from the base station as 62 other channels? Assume the following:-

(a) Code length of 64, cross correlations of 1, and all transmitters

operate at identical power levels.

(b) Assume all interferers are active.

voice activity. (10)