Code: AE28                                                                          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. Communication protocols always have which of the following?

(A)  A set of symbols                           (B)  Start of header

(C)   A special flag symbols                  (D)  BCC

b. Transmission media with maximum error rate is _____________.

(A) Coaxial cable                               (B) Twisted pair

(C) Satellite link                                   (D) Optical fiber

             c.   Which of the following is the escape character that identifies control characters in BiSync transparency mode?

(A) ESC                                             (B)  SYN

(C) DLE                                              (D)  RVI

             d.   Which of the following function is not provided as a part of the basic Ethernet design?

(A)  Access control                           

(B)  Addressing

(C)  Automatic retransmission of a message          

(D)  Multiple virtual network              

             e.   Gigabit Ethernet has a data rate of  __________.

(A) 100 Mbps                                    (B)  10 Gbps

(C)  1000 Mbps                                  (D)  10 Mbps

             f.    The address ftp.moscow.edu correspond to

(A) located in Russia                          (B)  an FTP server

(C) a military organization                    (D)  James Bond’s mail drop

             g.   The HDLC __________ field defines the beginning and end of a frame.

(A) flag                                              (B)  address

(C)  control                                         (D)  FCS

h.   The _________ is a product of the LLC sublayer.

(A) 802.3 frame                                 (B)  802.5 frame

(C)  PDU                                            (D)  preamble

             i.    The ____________ is a user-friendly unique name of a station on an internet.

(A)  domain name                                (B)  internet address

(C)  station address                             (D)  physical address

             j.    IEEE standard for token bus is _____________.

(A) 802.4 (B)  802.3

(C) 802.2 (D)  802.5

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

Q.2     a.   Match the following to one or more layers of the OSI model.                                  

(i)   Communicates directly with user’s application program.

(ii)  error correction and retransmission

(iii) mechanical, electrical and functional interface

(iv) responsibility for carrying frames between adjacent nodes                             (4)

             b.   Why do you think that an Ethernet frame should have a minimum data

                   size?                                                                                                                  (5)

     c.   Compare and contrast CSMA/CD with CSMA/CA.                                          (7)

Q.3     a.   In a Class A subnet , we know that IP address of one of the hosts and the mask as given below:

           IP Address             :   25.34.12.56                                                                         Mask   :   255.255.0.0

What is the first address (subnet address)?                                                         (4)

             b.   What is the maximum number of subnets in each case?

                   (i)   Class A; mask 255.255.192.0

                   (ii)  Class B; mask 255.255.192.0

                   (iii) Class C; mask 255.255.255.192

                   (iv) Class C; mask 255.255.255.240                                                                (4)

             c.   Show a routing table for a host that is connected to a LAN without being connected to the Internet.                                                                       (4)

             d.   Is the size of the RARP packet fixed? Explain.                                                    (4)

Q.4     a.   What would be the advantage of having a small MTU and large MTU?               (4)

             b.   Compare the TCP header and the UDP header. List the fields in the TCP

                   header  that are  missing from UDP  headers.  Give  the reason  for  their

                   absence.                                                                                                            (6)

c.   What do you mean by a synchronous TDM and a statistical TDM? Also define multiplexing.                                                                      (6)

Q.5     a.   Discuss the concept of switching as it relates to the problems involved in the connection of devices.                                                            (4)

     b.   Explain the following:-

(i) HDLC configurations and transfer modes.

(ii) HDLC frame format and control field formats.                            (8)

c. What does the number on an ACK frame mean for

(i)   stop-and-wait ARQ?

(ii)  go-back-n ARQ?

(iii) selective-repeat ARQ?                  

Also describe these above techniques in brief.                                                     (4)

  Q.6     a. Draw frame format of IEEE 802.3 MAC frame and IEEE 802.11 frame structure.              (6)

             b.   If a bridge sends data from an Ethernet network to a Token Ring network, how is a collision handled by the bridge and explain briefly various types of scheduling approaches used in Medium access control.   (7)

             c.   Explain the Leaky bucket algorithm to control congestion. Explain how the drawbacks of this are overcome in a token bucket algorithm.                 (3)

Q.7     a.   Use the following encryption algorithm to encrypt the message “GOOD DAY”:

(iii) Replace each character with its ASCII code

(iv) Add a 0 bit at the left to make each character 8 bits long

(v) Swap the first four bits with the last 4 bits

(vi) Replace every 4 bits with its hexadecimal equivalent.

                   What is the key in this method?                                                                          (6)

b.   Does RPF (Reverse Path Forwarding) actually create a shortest path tree? Explain.            (4)

             c.   Explain three types of Autonomous System (AS) used in Internet Routing protocols.  What are various interior and exterior routing protocols? (6)

Q.8     a.   Write short notes on (Any FOUR)

(i) M/M/1 Queues (Little’s Formula)

(ii) Datagram and Virtual Circuits

(iii) ATM adaptation Layer

(iv) Differences between B-ISDN and N-ISDN.

(v) Differences between IPv4 and IPv6.

(vi) Integrated services in the Internet.                                                         (12)

             b.   Describe Real Time Transport and Session Control protocols.                            (4)

Q.9     a.   What is the difference between a confirmed and unconfirmed services? Give an example for each.                                                              (4)

             b.   Explain the difference between Pure ALOHA and Slotted ALOHA. Which one of the two has better channel utilization?                               (5)

             c.    How DSL modem can support high speed digital communication over the existing telephone local loops?                                                               (3)

             d.   Give the differences between circuit switching and packet switching.                   (4)