2017 question paper

Computer Networks

23 questions

1. Q1a. Write any two functions of the session layer in OSI model.20172m

   Module 1: Physical Layer & Data Communication

   Write any two functions of the session layer in OSI model.

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   Worked Solution

   Answer

   Any two important functions of the Session Layer are:

   1. Dialog control: It manages the dialogue between two communicating applications, including controlling which side can transmit at a particular time. It can support half-duplex and full-duplex communication.

   2. Synchronization/checkpointing: The Session layer can insert synchronization points (checkpoints) into a long data transfer. If communication is interrupted, the transfer can resume from a checkpoint instead of restarting from the beginning.

   Other valid functions

   The Session layer is also associated with session establishment, session management, and session termination between applications.

   Exam-ready answer

   The Session layer establishes, manages, and terminates communication sessions between applications. Two functions are dialog control and synchronization/checkpointing.

   For 2 marks: Write any two functions with one-line explanations.

2. Q1b. What is Hamming code?20172m

   Module 2: Data Link Layer & MAC Sublayer

   What is Hamming code?

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   Worked Solution

   Answer

   Hamming code is an error-detecting and error-correcting block code that adds parity bits to data so that certain bit errors can be detected and corrected at the receiver.

   For a data word containing $m$ data bits, the number of parity bits $r$ is chosen so that:
   $2^r \ge m+r+1$.

   The parity bits are placed at positions that are powers of 2: $1,2,4,8,\ldots$. Their parity checks allow the receiver to determine the position of a single-bit error.

   Exam-ready answer

   Hamming code is a forward error-correction technique that adds redundant parity bits to a data word. It can generally correct a single-bit error and detect certain multiple-bit errors depending on the variant.

   For 2 marks: Definition + single-bit error-correction property is sufficient.

3. Q1c. What is the significance of the twisting in twisted-pair cable?20172m

   Module 1: Physical Layer & Data Communication

   What is the significance of the twisting in twisted-pair cable?

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   Answer

   Twisting the conductors in a twisted-pair cable helps reduce electromagnetic interference (EMI) and crosstalk.

   Each pair is twisted so that external interference affects the conductors in a more balanced way. The twisting also reduces the amount of electromagnetic radiation coupled between adjacent pairs.

   Exam-ready

   The twisting of wires reduces noise, electromagnetic interference, and crosstalk, improving the reliability of data transmission.

   For 2 marks: Mention reduction of EMI/crosstalk and improved signal quality.

4. Q1d. What do you mean by piggybacking?20172m

   Module 2: Data Link Layer & MAC Sublayer

   What do you mean by piggybacking?

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   Worked Solution

   Answer

   Piggybacking is a technique in bidirectional data communication where an acknowledgement (ACK) is attached to an outgoing data frame instead of sending a separate ACK frame.

   Example

   If A sends a data frame to B and B also has data to send to A, B can include the acknowledgement for A's frame in B's outgoing data frame.

   Advantage

   Piggybacking reduces the number of separate control frames and therefore improves bandwidth efficiency.

   Limitation

   The receiver may need to wait briefly for outgoing data before sending the ACK, so a maximum waiting/delay limit is normally used.

   Exam-ready: Piggybacking means combining an ACK with an outgoing data frame to reduce overhead.

5. Q1e. What is the purpose of the jam signal in CSMA/CD?20172m

   Module 2: Data Link Layer & MAC Sublayer

   What is the purpose of the jam signal in CSMA/CD?

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   Answer

   The jam signal in CSMA/CD is sent after a collision is detected so that all stations on the shared Ethernet medium become aware that a collision has occurred.

   Why it is needed

   A station detecting a collision transmits the jam signal, ensuring the collision is long enough to be noticed by other transmitting stations. The affected stations then stop the current transmission and apply the collision backoff procedure before trying again.

   Exam-ready: The jam signal informs other stations of a collision and ensures the collision is propagated throughout the shared medium.

6. Q1f. What are the advantages of dividing an Ethernet LAN with a bridge?20172m

   Module 2: Data Link Layer & MAC Sublayer

   What are the advantages of dividing an Ethernet LAN with a bridge?

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   Answer

   Dividing an Ethernet LAN using a bridge provides several advantages:

   1. Reduces collision domain size: Each bridged segment is a separate collision domain, reducing contention.
   2. Improves performance: Local traffic can remain within its segment instead of consuming bandwidth on every segment.
   3. Filters frames: The bridge learns MAC addresses and forwards frames only when necessary.
   4. Extends the LAN: Different physical Ethernet segments can be interconnected.
   5. Reduces congestion: Separating traffic between segments can reduce the load on each segment.

   Exam-ready: A bridge divides a LAN into smaller collision domains, filters unnecessary traffic using MAC addresses, and improves overall network performance.

7. Q1g. What does the term 'best effort' mean in context with IP?20172m

   Module 3: Network Layer & Routing

   What does the term 'best effort' mean in context with IP?

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8. Q1h. Why is there no need of CSMA/CD in a full-duplex switched Ethernet?20172m

   Module 2: Data Link Layer & MAC Sublayer

   Why is there no need of CSMA/CD in a full-duplex switched Ethernet?

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9. Q1i. How does redundancy facilitate error detection?20172m

   Module 2: Data Link Layer & MAC Sublayer

   How does redundancy facilitate error detection?

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10. Q2a. Describe the architecture and working of ring topology. What are its advantages and disadvantages?20177m

    Module 1: Physical Layer & Data Communication

    Describe the architecture and working of ring topology. What are its advantages and disadvantages?

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12. Q2b. Compare and contrast between the OSI model and the TCP/IP model.20177m

    Module 1: Physical Layer & Data Communication

    Compare and contrast between the OSI model and the TCP/IP model.

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13. Q3a. Describe the working of fiber-optic cables. Briefly explain different propagation modes used in fiber-optic cables.20177m

    Module 1: Physical Layer & Data Communication

    Describe the working of fiber-optic cables. Briefly explain different propagation modes used in fiber-optic cables.

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14. Q3b. Explain Go-Back-N ARQ as a sliding window mechanism for error control.20177m

    Module 2: Data Link Layer & MAC Sublayer

    Explain Go-Back-N ARQ as a sliding window mechanism for error control.

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15. Q4a. Describe how the two-dimensional parity check is able to detect errors. Consider '1100101100110000110100101101' as the data that is to be sent over the network.20177m

    Module 2: Data Link Layer & MAC Sublayer

    Describe how the two-dimensional parity check is able to detect errors. Consider '1100101100110000110100101101' as the data that is to be sent over the network.

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16. Q4b. Draw a flowchart and explain the working procedure of the pure ALOHA protocol.20177m

    Module 2: Data Link Layer & MAC Sublayer

    Draw a flowchart and explain the working procedure of the pure ALOHA protocol.

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17. Q5a. Explain in detail Fast Ethernet. Describe the structure of the physical layer and various implementation types at this layer.201714m

    Module 1: Physical Layer & Data Communication

    Explain in detail Fast Ethernet. Describe the structure of the physical layer and various implementation types at this layer.

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18. Q6a. With the help of a neat and labelled diagram, explain the message format of ICMP. Explain various types of ICMP error messages and query messages.201714m

    Module 3: Network Layer & Routing

    With the help of a neat and labelled diagram, explain the message format of ICMP. Explain various types of ICMP error messages and query messages.

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19. Q7a. What do you mean by layer 2 switching? Explain in detail the functioning of layer 2 switches.20177m

    Module 2: Data Link Layer & MAC Sublayer

    What do you mean by layer 2 switching? Explain in detail the functioning of layer 2 switches.

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20. Q7b. Discuss the functions and the design issues of the network layer in the OSI model.20177m

    Module 3: Network Layer & Routing

    Discuss the functions and the design issues of the network layer in the OSI model.

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21. Q8a. Describe bridge in terms of networking device. What are different types of network bridge?20177m

    Module 2: Data Link Layer & MAC Sublayer

    Describe bridge in terms of networking device. What are different types of network bridge?

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23. Q8b. Differentiate between Static Routing and Dynamic Routing.20177m

    Module 3: Network Layer & Routing

    Differentiate between Static Routing and Dynamic Routing.

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24. Q9a. Explain the following terms: (a) Border Gateway Protocol20177m

    Module 3: Network Layer & Routing

    Explain the following terms: (a) Border Gateway Protocol

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25. Q9b. Explain the following terms: (b) User Datagram Protocol20177m

    Module 4: Transport Layer & Congestion Control

    Explain the following terms: (b) User Datagram Protocol

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