Code: AE-28                                                                        Subject: COMPUTER NETWORKS

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 best alternative in the following:                                         (2x10)

a. Which topology requires a central controller or hub?

(A)  Mesh                                           (B)  Star

(C)   Bus                                             (D)  Ring

b. The physical layer is concerned with the transmission of __________ over the physical medium.

(A) Programs                                     (B) Dialogs

(C) Protocols                                      (D) Bits

             c.   Which multiplexing technique shifts each signal to a different carrier frequency?

(A) FDM                                           (B)  TDM 

(C) Both (A) & (B) (D)  None of these.

             d.   The local loop uses __________  cable that connects the subscriber telephone to the nearest switching office.

(A)  Twisted pair                                (B)  Coaxial

(C)  Fiber-optic                                  (D) (B) or (C)

             e.   In ________ ARQ, if ACK is not received, the damaged or lost frame is retransmitted.

(A) Stop-and-wait                              (B)  Go-back-N

(C)  Selective repeat                            (D)  Both (A) & (B)

             f.    When a primary device asks a secondary device if it has data to send, this is called __________

(A) Polling                                          (B)  Selecting

(C)  Reserving                                     (D)  Backing off

             g.   An 80 station traditional Ethernet is divided into four equal sized collision domains.  This means that a maximum of __________ stations contend for medium at any one time.

(A) 320                                              (B)  80

(C)  76                                                (D)  20

             h.   Which of the following is defined in 802.11

(A) LLC                                             (B)  PPP

(C)  DCF                                            (D)  None of the above

             i.    Which layer in ATM protocol reformats the data received from other networks?

(A)  Physical                                        (B)  ATM

(C)  ATM adaptation                           (D)  Cell transformation

             j.    Class __________ has the greatest number of hosts per given network address.

(A) A                                                 (B)  B

(C) C                                                  (D) D

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

Q.2     a.   What are the essential elements or functions of a network architecture to achieve good and reliable communications?  How are these functions organized into a layered structure?                    (6+1)

             b.   Explain the encapsulation of PDUs in TCP/IP, and addressing with suitable diagrams.                     (5)

     c.   Internet users are assumed to double every 18 months.  If the number of users is 7 million in the year 1996, what will be the expected number of Internet users in the year 2008?                                  (4)

Q.3     a.   24 voice channels are to be multiplexed and transmitted over twisted pair.  What is the bandwidth required for FDM?  Assuming a bandwidth efficiency of 1 bps/Hz. What is the bandwidth required for TDM using PCM?                                                 (6)

             b.   Make a critical comparison of communication switching techniques with the help of event timing diagram.                                                               (8)

             c.   Define SDU and PDU.                                                                                       (2)

Q.4     a.   Explain stop-and-wait ARQ protocol with suitable diagrams when                          

                   (i)   Frame is lost                                  (ii)  ACK is lost.                                     (6)

b.   A channel has bit rate of 4 Kbps and a propagation delay of 20 msec.  For what range of frame size does stop-and-wait protocol gives an efficiency of at least 50%?                                                (6)

             c.  What do you mean by multiple access communication?  What is the role of MAC protocols?                                                                     (3+1)

Q.5     a.   Differentiate between random access and scheduling approach to MAC.  Discuss the basic reservation system of scheduling.                           (4+4)

     b.   Explain Ethernet protocol with special reference to frame format.                        (8)

  Q.6     a. What is the need for ICMP?  Explain with the format of ICMP.                      (2+4)

             b.   Discuss the usefulness of extension headers in IPv6 with suitable figures.              (6)

             c.   Convert IP address whose hexadecimal representation is C22F1582 to dotted decimal notation?  To what class does this address belong?            (4)

Q.7     a.   What is the function of signalling?  Discuss PNNI signalling with an example and suitable diagram.                                                              (1+8)

b.   Explain why each specific set of traffic descriptors and QoS parameters were selected for each of ATM service categories.                               (7)

Q.8     a.   What are block ciphers?  Discuss DES encryption algorithm with a block diagram.             (1+8)

             b.   What are the key elements of a network management system?  Explain with an example of distributed network management configuration.      (2+5)

Q.9           Write explanatory note on any TWO of the following:

(i) Integrated services model.

(ii) MIME

(iii) CSMA/CA operation. (2 x 8=16)