CS-602 Computer Networks - Jun 2025 Question Paper

Under each question, How to answer lists the points an answer should cover, outlined by CampusPrep from the syllabus. Check the details against your textbook.

1. 7 Marks
   aDiscuss the operating principles of OSI reference model with the help of diagram showing headers and trailers added as the block of user data mover.
   How to answer

   • Define 7 layers of OSI reference model and peer-to-peer communication
   • Explain encapsulation with headers (and trailer at Data Link) as user data moves down
   • Describe decapsulation on receiver side
   • Provide diagram showing headers/trailers added per layer
2. 7 Marks
   bCompare connection oriented and connectionless service.
   How to answer

   • Define connection-oriented service with connection setup, transfer, teardown
   • Define connectionless service with independent datagrams
   • Compare on reliability, ordering, overhead, examples
   • Conclude with use cases (TCP vs UDP/IP)
3. 14 Marks
   What is the advantage of Go-back-n protocol? How is the parameter 'n' determined? Explain how the protocol works at transport layer in case of errors, lost packet or Acks?
   How to answer

   • Define Go-Back-N as sliding window ARQ with sender window size n
   • Explain how n is determined from sequence numbers and bandwidth-delay product
   • Describe sender and receiver operation, cumulative ACKs
   • Explain retransmission on timeout, lost packet or lost ACK
4. 7 Marks
   aWhat do you mean by Error detection? Write down the name of methods. An 8-bit with binary value 10101111 is to be encoded using an even-parity Hamming code. What is the binary value after encoding?
   How to answer

   • Define error detection vs error correction
   • List methods: parity, CRC, checksum, Hamming code
   • Calculate parity bits for 8-bit data 10101111 with even parity
   • Give final encoded binary value with parity bits placed
5. 7 Marks
   bDiscuss the different strategies used to avoid collisions in CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance).
   How to answer

   • Define CSMA/CA and why collision avoidance needed in wireless
   • Explain carrier sensing, interframe spacing, backoff
   • Explain RTS/CTS, virtual carrier sensing with NAV
   • List strategies to avoid collisions
6. 7 Marks
   aWhat is Pure ALOHA and slotted ALOHA? How the efficiency of slotted ALOHA is twice that of pure ALOHA? Derive it.
   How to answer

   • Define Pure ALOHA (unslotted) and Slotted ALOHA (slot = frame time)
   • State throughput formulas $S = G e^{-2G}$ and $S = G e^{-G}$
   • Derive vulnerable period 2T vs T
   • Show maxima 18.4% vs 36.8%, hence double efficiency
7. 7 Marks
   bExplain CSMA/CD protocol in detail.
   How to answer

   • Define CSMA/CD and persistence modes
   • Explain carrier sense, collision detection, jam signal
   • Explain binary exponential backoff algorithm
   • Give flow of transmission and efficiency considerations
8. 7 Marks
   aA TCP machine is sending windows of 65535 B over a 1 Gbps channel that has a 10 msec one way delay. What is the maximum throughput achievable?
   How to answer

   • Compute RTT = 2 x one-way delay = 20 msec
   • Compute window bits = 65535 x 8 and throughput = window / RTT
   • Compare with 1 Gbps channel capacity
   • State maximum achievable throughput in Mbps
9. 7 Marks
   bA TCP machine is sending windows of 65535 B over a 1 Gbps channel that has a 10 msec one way delay. What is the line efficiency?
   How to answer

   • Define line efficiency = achieved throughput / channel capacity
   • Use throughput from part (a)
   • Compute percentage efficiency
   • Comment on window limitation
10. 14 Marks
    Compare the TCP header and the UDP header. List the fields in the TCP header that are not part of the UDP header. Give reason for each missing field.
    How to answer

    • Draw/compare 20-byte TCP header vs 8-byte UDP header fields
    • List TCP-only fields: sequence, ACK, window, flags, urgent, options
    • Give reason for each missing field (reliability, flow control)
    • Conclude connection-oriented vs connectionless design
11. 4 Marks
    aAn organization is granted the block 211.17.180.0/24. The administrator wants to create 32 subnets. Find the subnet mask.
    How to answer

    • Determine bits needed for 32 subnets: 5 bits, new prefix /29
    • Write subnet mask 255.255.255.248
    • Explain borrowing from host portion of /24 block
    • State prefix length
12. 4 Marks
    bAn organization is granted the block 211.17.180.0/24. The administrator wants to create 32 subnets. Find the number of addresses in each subnet.
    How to answer

    • Compute host bits remaining = 3 bits
    • Number of addresses = 2^3 = 8
    • Distinguish usable vs total addresses
    • State result per subnet
13. 3 Marks
    cAn organization is granted the block 211.17.180.0/24. The administrator wants to create 32 subnets. Find the first and last address in first subnet.
    How to answer

    • Identify first subnet base 211.17.180.0/29
    • First address = network address
    • Last address = broadcast address 211.17.180.7
    • Show binary increment for subnet
14. 3 Marks
    dAn organization is granted the block 211.17.180.0/24. The administrator wants to create 32 subnets. Find the first and last address in last subnet.
    How to answer

    • Identify last subnet number 31, base 211.17.180.248/29
    • First address = 211.17.180.248
    • Last address = 211.17.180.255
    • Verify range size 8 addresses
15. 14 Marks
    Write a short notes (any three):

    1. i)TCP Header Format
    2. ii)TCP flow control
    3. iii)ICMP
    4. iv)Bellman Ford Algorithm
    How to answer

    • For each chosen note give definition and purpose
    • Explain key fields/mechanism with diagram where relevant
    • Give example or steps of operation
    • Mention applications/limitations