2021 question paper

Computer Networks

24 questions

Q1a. What layer in the TCP/IP stack is equivalent to the transport layer of the OSI model? (i) Application (ii) Host-to-host (iii) Internet (iv) Network access20212m
Module 1: Physical Layer & Data Communication
What layer in the TCP/IP stack is equivalent to the transport layer of the OSI model?

(i) Application
(ii) Host-to-host
(iii) Internet
(iv) Network access

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Worked Solution
Answer

(ii) Host-to-host layer

Explanation

The OSI model has seven layers, while the TCP/IP model is commonly represented using four layers: Application, Host-to-Host (Transport), Internet, and Network Access.

The Transport layer of the OSI model provides end-to-end communication between processes. It handles functions such as segmentation, flow control, error control, and reliable delivery when TCP is used.

In the TCP/IP model, these responsibilities belong to the Host-to-Host/Transport layer.

Option analysis

(i) Application: Corresponds broadly to the OSI Application, Presentation, and Session layers.

(ii) Host-to-host: Correct. It corresponds to the OSI Transport layer.

(iii) Internet: Corresponds to the OSI Network layer.

(iv) Network access: Covers functions associated broadly with the OSI Data Link and Physical layers.

Final answer: (ii) Host-to-host.
Q1b. Which topology covers security, robust and eliminating traffic factor? (i) Mesh (ii) Ring (iii) Star (iv) Bus20212m
Module 1: Physical Layer & Data Communication
Which topology covers security, robust and eliminating traffic factor?

(i) Mesh
(ii) Ring
(iii) Star
(iv) Bus

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Worked Solution
Answer

(i) Mesh topology

Explanation

A mesh topology connects nodes with multiple dedicated links. In a full mesh, every node has a direct connection to every other node.

This gives mesh topology strong reliability and robustness: if one link fails, traffic can often use another path. Dedicated links also reduce contention and unwanted traffic compared with a shared bus.

Option analysis

(i) Mesh: Correct. It is highly reliable, provides dedicated communication paths, and avoids the single shared communication medium of a bus.

(ii) Ring: Nodes share a logical ring; a link/node failure can disrupt communication unless redundancy is provided.

(iii) Star: Easy to manage, but the central device is a potential single point of failure.

(iv) Bus: Uses a shared medium, so collisions/traffic contention can occur.

Final answer: (i) Mesh.
Q1c. Internetworking protocol is known as (i) TCP (ii) IP (iii) ISP (iv) TCP/IP20212m
Module 3: Network Layer & Routing
Internetworking protocol is known as

(i) TCP
(ii) IP
(iii) ISP
(iv) TCP/IP

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Worked Solution
Answer

(ii) IP

Explanation

IP (Internet Protocol) is the Network-layer protocol responsible for logical addressing and forwarding datagrams between networks, making internetwork communication possible.

TCP: Transport-layer protocol providing reliable, connection-oriented delivery.

IP: Network-layer protocol providing logical addressing and datagram delivery across interconnected networks.

ISP: Internet Service Provider, not a protocol.

TCP/IP: A protocol suite, not the name of the individual internetworking protocol asked for here.

Final answer: (ii) IP.
Q1d. Which device uses logical addressing system? (i) Hub (ii) Switch (iii) Bridge (iv) Router20212m
Module 3: Network Layer & Routing
Which device uses logical addressing system?

(i) Hub
(ii) Switch
(iii) Bridge
(iv) Router

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Worked Solution
Answer

(iv) Router

Explanation

A logical address is an IP address. Routers operate at the Network layer and use logical IP addresses to make forwarding decisions between different networks.

Hub → Physical layer; does not use logical addressing for forwarding.

Switch → Data Link layer; normally forwards using MAC addresses.

Bridge → Data Link layer; uses MAC addresses.

Router → Network layer; uses IP/logical addresses.

Final answer: (iv) Router.
Q1e. Consider an instance of TCP's Additive Increase Multiplicative Decrease (AIMD) algorithm where the window size at the start of the slow start phase is 2 MSS and the threshold at the start of the first transmission is 8 MSS. Assume that a time-out occurs during the fifth transmission. Find the congestion window size at the end of the tenth transmission. (i) 8 MSS (ii) 14 MSS (iii) 7 MSS (iv) 12 MSS20212m
Module 4: Transport Layer & Congestion Control
Consider an instance of TCP's Additive Increase Multiplicative Decrease (AIMD) algorithm where the window size at the start of the slow start phase is 2 MSS and the threshold at the start of the first transmission is 8 MSS. Assume that a time-out occurs during the fifth transmission. Find the congestion window size at the end of the tenth transmission.

(i) 8 MSS
(ii) 14 MSS
(iii) 7 MSS
(iv) 12 MSS

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Worked Solution
Answer

(iii) 7 MSS

Step-by-step solution

Initial values:

$cwnd=2$ MSS

$ssthresh=8$ MSS

During slow start, the window grows rapidly until the threshold:

Transmission cwnd (MSS) Phase

1 2 → 4 Slow start

2 4 → 8 Slow start; reaches threshold

3 9 Congestion avoidance

4 10 Congestion avoidance

5 11 Congestion avoidance; timeout occurs

On a timeout, TCP sets the new threshold approximately to half of the current congestion window:

$ssthresh=11/2\approx5$ MSS

and resets $cwnd$ to 1 MSS.

The next transmissions then grow as:

Transmission cwnd (MSS)

6 2

7 4

8 5

9 6

10 7

Therefore, the congestion window at the end of the tenth transmission is 7 MSS.

Final answer: (iii) 7 MSS.
Q1f. Which transmission media has the highest transmission speed in a network? (i) Coaxial cable (ii) Twisted pair cable (iii) Optical fiber (iv) Electrical cable20212m
Module 1: Physical Layer & Data Communication
Which transmission media has the highest transmission speed in a network?

(i) Coaxial cable
(ii) Twisted pair cable
(iii) Optical fiber
(iv) Electrical cable

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Worked Solution
Answer

(iii) Optical fiber

Explanation

Optical fiber transmits data using pulses of light through glass or plastic fiber. It supports very high bandwidth and transmission speeds compared with traditional copper media.

Advantages include:

Very high bandwidth.

Low attenuation over long distances.

Immunity to electromagnetic interference.

Suitable for high-speed backbone and long-distance communication.

Therefore, among the given options, optical fiber has the highest transmission speed.

Final answer: (iii) Optical fiber.
Q1g. What is the maximum size of data that the application layer can pass onto the TCP layer below? (i) Any size (ii) 2^16 bytes-size of TCP header (iii) 2^16 bytes (iv) 1500 bytes20212m
Module 5: Application Layer & Security
What is the maximum size of data that the application layer can pass onto the TCP layer below?

(i) Any size
(ii) 2^16 bytes-size of TCP header
(iii) 2^16 bytes
(iv) 1500 bytes

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Q1h. The maximum window size for data transmission using the selective reject protocol with n-bit frame sequence numbers is (i) 2^n (ii) 2^(n-1) (iii) 2^n - 1 (iv) 2^(n-2)20212m
Module 2: Data Link Layer & MAC Sublayer
The maximum window size for data transmission using the selective reject protocol with n-bit frame sequence numbers is

(i) 2^n
(ii) 2^(n-1)
(iii) 2^n - 1
(iv) 2^(n-2)

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Q1i. Station A needs to send a message consisting of 9 packets to Station B using a sliding window (window size 3) and go-back-n error control strategy. All packets are ready and immediately available for transmission. If every 5th packet that A transmits gets lost (but no packets from B ever get lost), then what is the number of packets that A will transmit for sending the message to B? (i) 12 (ii) 14 (iii) 16 (iv) 1820212m
Module 2: Data Link Layer & MAC Sublayer
Station A needs to send a message consisting of 9 packets to Station B using a sliding window (window size 3) and go-back-n error control strategy. All packets are ready and immediately available for transmission. If every 5th packet that A transmits gets lost (but no packets from B ever get lost), then what is the number of packets that A will transmit for sending the message to B?

(i) 12
(ii) 14
(iii) 16
(iv) 18

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Q1j. The message 11001001 is to be transmitted using the CRC polynomial x^3 + 1 to protect it from errors. The message that should be transmitted is (i) 11001001000 (ii) 11001001011 (iii) 11001010 (iv) 11001001001120212m
Module 2: Data Link Layer & MAC Sublayer
The message 11001001 is to be transmitted using the CRC polynomial x^3 + 1 to protect it from errors. The message that should be transmitted is

(i) 11001001000
(ii) 11001001011
(iii) 11001010
(iv) 110010010011

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Q2a. What are the duties of physical layer? Differentiate between fiber optic cable and twisted pair cable.20217m
Module 1: Physical Layer & Data Communication
What are the duties of physical layer? Differentiate between fiber optic cable and twisted pair cable.

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Q2b. Explain how sliding window protocol works at transport layer.20217m
Module 4: Transport Layer & Congestion Control
Explain how sliding window protocol works at transport layer.

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Q3a. Consider the delay of pure ALOHA versus slotted ALOHA at low load. Which one is less? Explain your answer.20217m
Module 2: Data Link Layer & MAC Sublayer
Consider the delay of pure ALOHA versus slotted ALOHA at low load. Which one is less? Explain your answer.

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Q3b. A pure ALOHA network transmits 200-bit frames on a shared channel of 200 kbps. What is the throughput, if the system (all stations together) produces (i) 1000 frames per second and (ii) 500 frames per second?20217m
Module 2: Data Link Layer & MAC Sublayer
A pure ALOHA network transmits 200-bit frames on a shared channel of 200 kbps. What is the throughput, if the system (all stations together) produces (i) 1000 frames per second and (ii) 500 frames per second?

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Q4a. A client uses TCP to send data to a server. The data are 16 bytes. Calculate the efficiency of this transmission at the TCP level (ratio of useful bytes to total bytes). Calculate the efficiency of transmission at the IP level. Assume no options for the IP header. Calculate the efficiency of transmission at the data link layer. Assume no options for the IP header and use Ethernet at the data link layer.20217m
Module 4: Transport Layer & Congestion Control
A client uses TCP to send data to a server. The data are 16 bytes. Calculate the efficiency of this transmission at the TCP level (ratio of useful bytes to total bytes). Calculate the efficiency of transmission at the IP level. Assume no options for the IP header. Calculate the efficiency of transmission at the data link layer. Assume no options for the IP header and use Ethernet at the data link layer.

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Q4b. Describe the services provided by network layer. Explain ICMP.20217m
Module 3: Network Layer & Routing
Describe the services provided by network layer. Explain ICMP.

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Q5a. Discuss link state routing with an example.20217m
Module 3: Network Layer & Routing
Discuss link state routing with an example.

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Q5b. An ISP is granted a block of addresses starting with 120.60.4.0/22. The ISP wants to distribute these blocks to 100 organizations with each organization receiving just eight addresses. Design the sub-blocks and give the slash notation for each sub-block. Find out how many addresses are still available after these allocations.20217m
Module 3: Network Layer & Routing
An ISP is granted a block of addresses starting with 120.60.4.0/22. The ISP wants to distribute these blocks to 100 organizations with each organization receiving just eight addresses. Design the sub-blocks and give the slash notation for each sub-block. Find out how many addresses are still available after these allocations.

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Q6a. Explain in detail about TCP/IP protocol suite with neat diagram.20217m
Module 1: Physical Layer & Data Communication
Explain in detail about TCP/IP protocol suite with neat diagram.

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Q6b. Differentiate between forwarding and routing.20217m
Module 3: Network Layer & Routing
Differentiate between forwarding and routing.

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Q7a. Compare and contrast the protocol field at the network layer with the port numbers at the transport layer. What is their common purpose? Why do we need two port-number fields but only one protocol field? Why is the size of the protocol field only half the size of each port number?20217m
Module 4: Transport Layer & Congestion Control
Compare and contrast the protocol field at the network layer with the port numbers at the transport layer. What is their common purpose? Why do we need two port-number fields but only one protocol field? Why is the size of the protocol field only half the size of each port number?

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Q7b. With a neat diagram, explain distance vector routing protocol.20217m
Module 3: Network Layer & Routing
With a neat diagram, explain distance vector routing protocol.

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Q8. Draw and describe the TCP header. Assume that a host receives a UDP segment with 01011101 11110010 (we separated the values of each byte with a space for clarity) as the checksum. The host adds the 16-bit words over all necessary fields excluding the checksum and obtains the value 00110010 00001101. Is the segment considered correctly received or not? What does the receiver do?202114m
Module 4: Transport Layer & Congestion Control
Draw and describe the TCP header. Assume that a host receives a UDP segment with 01011101 11110010 (we separated the values of each byte with a space for clarity) as the checksum. The host adds the 16-bit words over all necessary fields excluding the checksum and obtains the value 00110010 00001101. Is the segment considered correctly received or not? What does the receiver do?

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Q9. Differentiate between POP3 and IMAP4. Suppose Alice, with a Web-based e-mail account (such as Hotmail or Gmail), sends a message to Bob, who accesses his mail from his mail server using POP3. Discuss how the message gets from Alice's host to Bob's host. Be sure to list the series of application-layer protocols that are used to move the message between the two hosts.202114m
Module 5: Application Layer & Security
Differentiate between POP3 and IMAP4. Suppose Alice, with a Web-based e-mail account (such as Hotmail or Gmail), sends a message to Bob, who accesses his mail from his mail server using POP3. Discuss how the message gets from Alice's host to Bob's host. Be sure to list the series of application-layer protocols that are used to move the message between the two hosts.

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Transmission	cwnd (MSS)	Phase
1	2 → 4	Slow start
2	4 → 8	Slow start; reaches threshold
3	9	Congestion avoidance
4	10	Congestion avoidance
5	11	Congestion avoidance; timeout occurs

Transmission	cwnd (MSS)
6	2
7	4
8	5
9	6
10	7

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Computer Networks

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Option analysis

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Option analysis

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Step-by-step solution

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