B.

E (FT) END S E M E S T E R EXAMINATIONS - NOV / D E C 2023

Computer Science and Engineering

Vtli Semester
CS6111 & COMPUTER N E T W O R K S
(Regulation 2018 - RUSA)
Time: 3 Hours Answer A L L Questions Max. Marks 100

CO 1 Highlight the significance of the functions of each layer in the network

CO 2 Identify the devices a n d protocols to design a network and implement it
CO 3 Build network applications using the right set of protocols and estimate their
performance
CO 4 Trace packet flows a n d interpret packet formats
COS Apply addressing principles such as subnetting and V L S M for efficient routing
C06 Explain media a c c e s s and communication techniques

B L - Bloom's Taxonomy Levels

( L I - Remembering, L2 - Understanding, L3 - Applying, L4 - Analyzing, LS - Evaluating, L6 -Creating)

P A R T - A ( 1 0 x 2 = 20 Marks)

Q.No. Questions Marks CO BL

1 A 30 bytes long m e s s a g e is sent from one device to another, 2 2 L2
which is 1S m apart, through a link with bandwidth 10 bps. If the
data travels through the link at a speed of 3 m/s, calculate the
transmission delay and propagation delay.

2 How long does it take a packet of length 1000 bytes to propagate 2 3 LI

over a link of distance 2 5 0 0 k m , propagation speed 2.5x108m/s,
and transmission rate 2 Mbps?
3 A subnet has been a s s i g n e d a subnet mask of 2SS.2SS.2SS.192. 2 S L3
What is the maximum number of hosts that can belong to this
subnet?

4 What is the relationship between sequence number and window 2 3 L2

size in Selective Repeat Mechanism?
5 In what scenarios is U D P preferred over T C P , and vice v e r s a ? 2 4 L2

6 How does DHCP help in minimizing manual IP address 2 5 L2

configuration errors?
7 Name three error-detection strategies employed by link layer. 2 6 L1

8 The message 11001001 is to be transmitted using the C R C 2 4 L3

polynomial x^ + 1 to protect it from errors. What is the binary
message that should be transmitted?
 9 Determine tlie maximum lengtli of tlie cable (in km) for 2 2 L3
transmitting data at a rate of 500 Mbps in an Ethernet LAN with
frames of size 10,000 bits. Assume the signal speed in the cable
to be 2,00,000 km/s.

10 How are signal strength and transmission distance related to 2 6 L2

each other?

PART - B (8 X 8 = 64 m a r k s )
(Answer any 8 questions)
Q.No. Questions Marks CO BL
(a) Discuss in detail the different types of connections that are
11 3 L4
possible in the application layer protocol used by Google
Chrome. Will the working of the protocol differ when a
different browser, like Mozilla Firefox or Internet Explorer, is
used? Justify your answer. (4 marks) 8

(b) T C P congestion control decreases its transmission rate in the

face of increasing congestion (loss), while U D P sources need 4 L3
not, and it is possible for U D P sources to crowd out T C P
traffic. Does it mean that T C P is fairer than U D P ? Justify it.
(4 marks)

Design and describe an application-level protocol to be used

12 8 3 L4
between an automatic teller machine and a bank's centralized
computer. Your protocol should allow a user's card and
password to be verified, the account balance (which is
maintained at the centralized computer) to be queried, and an
account withdrawal to be made (that is, money disbursed to the
user). Your protocol entities should be able to handle the all-too-
common case in which there is not enough money in the account
to cover the withdrawal. Specify your protocol by listing the
messages exchanged and the action taken by the automatic
teller machine or the bank's centralized computer on
transmission and receipt of messages. Sketch the operation of
your protocol for the case of a simple withdrawal with no errors,
using a sequence diagram. Explicitly state the assumptions
made by your protocol about the underiying end-to-end transport
service.
(a) Consider distributing a file of F bits to N peers using a Client-
13 3 L5
Server and P 2 P architecture. Analyze and specify the
distribution time for both the architecture. (4 marks)
8
(b) A circuit in link is implemented with either frequency-division
multiplexing (FDM) or time-division multiplexing (TDM). What
1 L2
is the difference between FDM and TDM links? (4 marks)
 How to achieve reliable data transfer over a channel that can
#(4 L1
corrupt or lose p a c k e t s . ? Illustrate with the receiver side Finite
State Machine.
In the network of routers depicted in the figure given below, R I P
15
is used for routing.

L3

The following s e q u e n c e of events occur:

• A, B, C , D and E construct initial routing table
• A receives a triggered update from B
• B receives a triggered update from E
• C receives a triggered update from A
• D receives a triggered update from C
• E receives a triggered update from B
Depict how the routing tables of the respective nodes change on
occurrence of each event in the same order as mentioned above.
Indicate clearly which node's (A/B/C/D/E) routing table is
updated and how.

Consider the following network. With the indicated link costs, use
16
Dijkstra's algorithm to compute the shortest path from x to all
network nodes. S h o w how the algorithm works by computing a L4
table for the following figure.

(a) Suppose that the links and routers in the network never fail
17 L5
and that routing paths used between all source/destination
pairs remains constant. In this scenario, does a virtual circuit
or datagram architecture have more control traffic overhead?
W h y ? Explain in detail. (4 marks)

(b) T C P congestion control is often referred to a s an additive-

increase, multiplicative-decrease form of congestion. Justify. L2
(4 marks)
 In a particular router, the following sequence of events occur: 3 L4
18 8
• packet 1 with weight 4 arrives
• packet 2 with weight 2 arrives
• packet 3 with weight 4 arrives
• a packet (say, A) is processed and forwarded by the router
• packet 4 with weight 1 arrives
• packet 5 with weight 1 arrives
• packet 6 with weight 4 arrives
• packet 7 with weight 4 arrives
• packet 8 with weight 4 arrives
• packet 9 with weight 2 arrives
• a packet (say, B) is processed and forwarded by the router
• packet 10 with weight 3 arrives
No more packets arrive and the remaining packets are
processed and forwarded.
What is the packet number of A and B (mentioned above), and
the order in which the remaining packets will be processed when
each of F I F O queuing. Priority scheduling and Weighted Fair
Queuing is used by the router?

(a) Why are policy considerations as important for intra-AS

19 2 L3
protocols, such as O S P F and R I P , as they are for an inter-
Autonomous System routing protocol like B G P ? (4 marks)

(b) Suppose four active nodes - nodes A, B, C and D are

competing for access to a channel using slotted A L O H A .
Assume each node has an infinite number of packets to send.
E a c h node attempts to transmit in each slot with probability
p. The first slot is numbered slot 1, the second slot is
numbered slot 2, and so on. (4 marks)
i. What is the probability that node A succeeds for the first 8 6 L4
time in slot 5 ?
ii. What is the probability that some node (either A, B, C
or D) succeeds in slot 4 ?
iii. What is the probability that the first success occurs in
slot 3?
iv. What is the efficiency of this four-node system?

(a) Detail and Discuss CSMA/CD algorithm and derive the

20 4 L2
efficiency for the same. (4 marks)

(b) When a company wants to manufacture adapters, it 8

purchases a chunk of the address space consisting of 2^''
addresses for a nominal fee. I E E E allocates the chunk of 2^"
addresses by fixing the first 24 bits of a MAC address. How 6 L5
many adapters can such a company manufacture? (4 marks)
 Compare and discuss at least with four performance metrics of
21 8 2 L2
the following wireless technologies with simple diagrams: Ad hoc
Networks, Wireless LAN, Cellular Technologies, and L T E / 5 G .

(a) Sketch the IPv4 datagram format and highlight their

22
functionalities. (4 marks)
8 4 L3, L4
(b) When NAT traversal problem occurs? Discuss at least three
solutions to address the same. (4 marks)

P A R T - C ( 2 x 8 = 16 m a r k s )

Q. No. Questions Marks CO BL

23 Consider two hosts, A and S , connected by a single link of rate 8 4 L5
R bps. Suppose that the two hosts are separated by m meters,
and suppose the propagation speed along the link is s
meters/sec. Host A is to send a packet of size L bits to Host S .
a. E x p r e s s the propagation delay, dprop, in terms of m and s.
b. Determine the transmission time of the packet, dtrans, in terms
of L and R.
c. Ignoring processing and queuing delays, obtain an expression
for the end-to-end delay.
d. Suppose Host A begins to transmit the packet at time t = 0. At
time f = dtrans, where is the last bit of the packet?
e. Suppose cfprop is greater than cftrans. At time i = dtrans, where is
the first bit of the packet?
f. Suppose dprop is less than cftrans- At time t = c/trans, where is the
first bit of the packet?
g. Suppose s = 2.5 x 108, L = 120 bits, and R = 56 kbps. Find
the distance m so that dprop equals c/trans.
24 Construct LAN (wired & wireless) architecture for your campus 8 1 L2
with the maximum possible interfaces, technologies, network
elements, middleware and protocol involved in it. Discuss their
characteristics and specifications.
(Note: Highlight the category of switches involved, specification
of various physical media used, IP address of the nodes, etc.)