2025 question paper

Computer Networks

28 questions

1. 1(a). Which layer of the OSI model is responsible for data representation and encryption? - (i) Session - (ii) Data Link - (iii) Presentation - (iv) Application20252m

   OSI Model

   Which layer of the OSI model is responsible for data representation and encryption?

   • (i) Session
   • (ii) Data Link
   • (iii) Presentation
   • (iv) Application

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   Worked Solution

   Answer: (iii) Presentation

   The Presentation layer (layer 6) handles data representation: translation between formats, encryption/decryption and compression.

2. 1(b). Which of the following topology provides the highest reliability? - (i) Mesh - (ii) Star - (iii) Ring - (iv) Bus20252m

   Network Topologies

   Which of the following topology provides the highest reliability?

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

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   Worked Solution

   Answer: (i) Mesh

   In a mesh every device has a dedicated link to every other device, so the failure of one link or node does not stop communication. Star depends on the central hub, and ring/bus can fail if the medium breaks.

3. 1(c). In Stop-and-Wait ARQ, the sender retransmits a frame when: - (i) ACK is received - (ii) Window is full - (iii) Timeout occurs - (iv) Buffer is empty20252m

   ARQ

   In Stop-and-Wait ARQ, the sender retransmits a frame when:

   • (i) ACK is received
   • (ii) Window is full
   • (iii) Timeout occurs
   • (iv) Buffer is empty

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   Worked Solution

   Answer: (iii) Timeout occurs

   The sender starts a timer after sending a frame. If no ACK arrives before the timer expires (frame or ACK lost/corrupted), it retransmits the same frame.

4. 1(d). Which access method listens before transmitting? - (i) Slotted-ALOHA - (ii) CSMA/CA - (iii) Token passing - (iv) ALOHA20252m

   Multiple Access

   Which access method listens before transmitting?

   • (i) Slotted-ALOHA
   • (ii) CSMA/CA
   • (iii) Token passing
   • (iv) ALOHA

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   Worked Solution

   Answer: (ii) CSMA/CA

   CSMA (Carrier Sense Multiple Access) methods sense the channel before transmitting. CSMA/CA (used in Wi-Fi) listens first and tries to avoid collisions. ALOHA and Slotted ALOHA transmit without sensing, and token passing uses a token instead.

5. 1(e). ARP protocol is used for: - (i) Mapping MAC to IP - (ii) Mapping port to process - (iii) Mapping IP to MAC - (iv) Mapping host to domain20252m

   ARP

   ARP protocol is used for:

   • (i) Mapping MAC to IP
   • (ii) Mapping port to process
   • (iii) Mapping IP to MAC
   • (iv) Mapping host to domain

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   Worked Solution

   Answer: (iii) Mapping IP to MAC

   ARP finds the MAC (physical) address corresponding to a known IP (logical) address on the same LAN. (Mapping MAC to IP is RARP; port to process is a transport-layer job; host to domain is DNS.)

6. 1(f). Which protocol is used for remote login? - (i) SMTP - (ii) HTTP - (iii) FTP - (iv) TELNET20252m

   Application Layer

   Which protocol is used for remote login?

   • (i) SMTP
   • (ii) HTTP
   • (iii) FTP
   • (iv) TELNET

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   Worked Solution

   Answer: (iv) TELNET

   TELNET gives a text-based remote login to another host (port 23). SMTP is for email, HTTP for the web and FTP for file transfer.

7. 1(g). The length of an IPv6 address is: - (i) 32 bits - (ii) 64 bits - (iii) 128 bits - (iv) 256 bits20252m

   IP Addressing

   The length of an IPv6 address is:

   • (i) 32 bits
   • (ii) 64 bits
   • (iii) 128 bits
   • (iv) 256 bits

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   Worked Solution

   Answer: (iii) 128 bits

   IPv6 addresses are 128 bits long (written as 8 groups of 16 bits in hexadecimal). IPv4 addresses are 32 bits.

8. 1(h). A firewall is used for: - (i) Error correction - (ii) Network security - (iii) Data encryption - (iv) Traffic shaping20252m

   Network Security

   A firewall is used for:

   • (i) Error correction
   • (ii) Network security
   • (iii) Data encryption
   • (iv) Traffic shaping

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   Worked Solution

   Answer: (ii) Network security

   A firewall filters incoming and outgoing traffic according to security rules, protecting a private network from unauthorized access.

9. 1(i). The full form of OSI is: - (i) Open service Internet - (ii) Open service Implementation - (iii) Operating system interface - (iv) Open systems interconnection20252m

   OSI Model

   The full form of OSI is:

   • (i) Open service Internet
   • (ii) Open service Implementation
   • (iii) Operating system interface
   • (iv) Open systems interconnection

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   Worked Solution

   Answer: (iv) Open systems interconnection

   OSI stands for Open Systems Interconnection, the 7-layer reference model developed by ISO.

10. 1(j). Which of the following devices is not a networking device? - (i) Hub - (ii) Bridge - (iii) Switch - (iv) None of these20252m

    Networking Devices

    Which of the following devices is not a networking device?

    • (i) Hub
    • (ii) Bridge
    • (iii) Switch
    • (iv) None of these

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    Worked Solution

    Answer: (iv) None of these

    Hub, bridge and switch are all networking devices (physical layer, and data link layer devices respectively), so none of the options is the odd one out.

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12. 2(a). Write the advantages and disadvantages of Mesh, Star and Ring topologies.20257m

    Network Topologies

    Write the advantages and disadvantages of Mesh, Star and Ring topologies.

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    Worked Solution

    Mesh Topology

    Every device has a dedicated point-to-point link to every other device ($n(n-1)/2$ links).

    SVGVector Architecture Diagram (SVG)

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    Advantages

    • Dedicated links: no traffic problems
    • Very robust: one link failure does not affect others
    • Good privacy and security; easy fault identification

    Disadvantages

    • Very high cabling and port cost ($n-1$ ports per device)
    • Difficult installation and reconfiguration
    • Does not scale well

    Star Topology

    All devices connect to a central hub/switch.

    SVGVector Architecture Diagram (SVG)

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    Advantages

    • Cheap, easy to install and extend
    • Failure of one node/link does not affect others
    • Easy fault detection and management

    Disadvantages

    • Central hub is a single point of failure
    • Hub capacity limits network size; more cable than bus

    Ring Topology

    Each device connects to exactly two neighbours, forming a closed loop; data travels in one direction (often with token passing).

    SVGVector Architecture Diagram (SVG)

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    Advantages

    • Simple, needs only two connections per device
    • Easy to install; orderly access (no collisions with token passing)
    • Equal access for all stations

    Disadvantages

    • Failure of one node or link can break the whole ring (unless dual ring)
    • Adding or removing nodes disrupts the network
    • Unidirectional traffic can add delay

    Summary

    Basis	Mesh	Star	Ring
    Links (n nodes)	n(n-1)/2	n	n
    Cost	Very high	Low	Low
    Reliability	Very high	Depends on hub	Low (single ring)
    Fault finding	Easy	Easy	Moderate
    Scalability	Poor	Good	Moderate

13. 2(b). Discuss the responsibilities of the different layers present in the OSI model.20257m

    OSI Model

    Discuss the responsibilities of the different layers present in the OSI model.

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    Worked Solution

    OSI Reference Model: Layer Responsibilities

    SVGVector Architecture Diagram (SVG)

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    Layer	Responsibilities
    7 Application	Gives network services to user applications: email, file transfer, web access, directory services (HTTP, FTP, SMTP, DNS)
    6 Presentation	Translation between data formats, encryption/decryption, compression
    5 Session	Establishes, manages and terminates sessions; dialog control; synchronization with checkpoints
    4 Transport	Process-to-process (end-to-end) delivery, segmentation and reassembly, flow control, error control, connection control (TCP/UDP)
    3 Network	Logical addressing (IP), routing, path selection, packet forwarding, fragmentation
    2 Data Link	Framing, physical (MAC) addressing, error detection, flow control, access control to the medium
    1 Physical	Transmission of raw bits over the medium; signal encoding, data rates, voltage levels, topology, transmission mode

    Key points

    • Data units: Data, Segment, Packet, Frame, Bit as it goes down.
    • Each layer adds a header (data link also a trailer) on the sender side; the receiver removes them going up (encapsulation / decapsulation).
    • Layers 1-3 are network-support layers, layers 5-7 are user-support layers, and layer 4 links the two groups.
14. 3(a). The message 11001001 is to be transmitted using the CRC polynomial x^3 + 1 to protect it from errors. Write the transmitted message.20257m

    Error Detection / CRC

    The message 11001001 is to be transmitted using the CRC polynomial $x^3 + 1$ to protect it from errors. Write the transmitted message.

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    Worked Solution

    Given

    • Message (dataword): 11001001
    • Generator polynomial: $x^3 + 1 \Rightarrow$ divisor 1001 (degree 3, so append 3 zeros)

    Step 1: Append zeros

    11001001 + 000 = 11001001000

    Step 2: Modulo-2 division by 1001

    Step	Current bits	XOR with	Remainder
    1	1100	1001	101
    2	101+1 = 1011	1001	010
    3	010+0 = 0100	0000	100
    4	100+0 = 1000	1001	001
    5	001+1 = 0011	0000	011
    6	011+0 = 0110	0000	110
    7	110+0 = 1100	1001	101
    8	101+0 = 1010	1001	011

    CRC (remainder) = 011

    Check using polynomials: since $x^3 \equiv 1 \pmod{x^3+1}$, the message $x^7+x^6+x^3+1$ times $x^3$ gives $x^{10}+x^9+x^6+x^3 \equiv x + 1 + 1 + 1 = x + 1$, i.e. 011. ✔

    Step 3: Transmitted message

    Dataword + CRC = 11001001 | 011  ->  11001001011
    

    Receiver check

    Dividing 11001001011 by 1001 gives remainder 000, so no error is detected.

15. 3(b). Differentiate between Pure ALOHA and Slotted ALOHA. How does Slotted ALOHA improve the efficiency of Pure ALOHA?20257m

    Multiple Access

    Differentiate between Pure ALOHA and Slotted ALOHA. How does Slotted ALOHA improve the efficiency of Pure ALOHA?

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    Worked Solution

    ALOHA

    ALOHA is a random access MAC protocol: a station transmits whenever it has data; if the frame collides it waits a random time and retransmits.

    Pure ALOHA vs Slotted ALOHA

    Basis	Pure ALOHA	Slotted ALOHA
    Time	Continuous; stations send any time	Divided into slots of frame time $T$; stations send only at slot start
    Synchronization	Not needed	Required (common clock)
    Vulnerable time	$2T$	$T$
    Throughput	$S = G e^{-2G}$	$S = G e^{-G}$
    Maximum efficiency	$1/(2e) \approx$ 18.4% at $G=0.5$	$1/e \approx$ 36.8% at $G=1$
    Collisions	Partial overlaps possible	Only full overlaps

    SVGVector Architecture Diagram (SVG)

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    How Slotted ALOHA improves efficiency

    1. Stations may start only at slot boundaries, so frames either collide completely or not at all.
    2. This halves the vulnerable period from $2T$ to $T$.
    3. Fewer collisions mean fewer retransmissions, so maximum throughput doubles from 18.4% to 36.8%.

    The cost is the need for clock synchronization and a slot-wait delay.

16. 4(a). How is a TCP connection established in the transport layer? Describe with a diagram.20257m

    TCP

    How is a TCP connection established in the transport layer? Describe with a diagram.

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    Worked Solution

    TCP Connection Establishment: Three-Way Handshake

    TCP is connection-oriented, so both sides synchronize sequence numbers before data transfer using three segments.

    SVGVector Architecture Diagram (SVG)

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    Steps

    1. SYN: the client (active open) sends a segment with SYN=1 and its initial sequence number $x$.
    2. SYN + ACK: the server (passive open, in LISTEN) replies with SYN=1, ACK=1, its own initial sequence number $y$ and acknowledgement $x+1$.
    3. ACK: the client sends ACK=1 with acknowledgement $y+1$. Both sides are now in ESTABLISHED and data transfer begins.

    Points to note

    • The SYN segment consumes one sequence number.
    • The handshake also negotiates parameters such as window size and MSS.
    • Using random initial sequence numbers protects against old duplicate segments and spoofing.
17. 4(b). Why did the IPv6 addressing system come into picture when IPv4 was already there?20257m

    IPv6

    Why did the IPv6 addressing system come into picture when IPv4 was already there?

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    Worked Solution

    Why IPv6 was introduced

    1. Address exhaustion (main reason)

    • IPv4 has 32-bit addresses, only about $2^{32} \approx 4.3$ billion, which are nearly exhausted because of the growth of the Internet, mobile devices and IoT.
    • IPv6 uses 128-bit addresses ($\approx 3.4 \times 10^{38}$), a practically unlimited space.

    2. Limitations of IPv4

    • Workarounds like NAT and CIDR break end-to-end connectivity and add complexity.
    • Complex header with options; routers must fragment and recompute the header checksum at every hop.
    • Security (IPsec) was optional.
    • Poor support for QoS, real-time traffic, autoconfiguration and mobility.

    3. Improvements in IPv6

    Feature	IPv4	IPv6
    Address size	32 bits	128 bits
    Header	Variable, 20-60 bytes, with checksum	Fixed 40 bytes, no checksum
    Fragmentation	By routers and source	Only by the source
    Configuration	Manual / DHCP	Auto-configuration (SLAAC) plus DHCPv6
    Security	IPsec optional	IPsec built in
    Broadcast	Yes	No (multicast and anycast)
    QoS	Limited (ToS)	Traffic class and flow label

    Hence IPv6 solves the address shortage and gives a simpler, faster, more secure and scalable network layer.

18. 5(a). State the major difference between distance vector routing and link state routing. How do these routing techniques work?20257m

    Routing

    State the major difference between distance vector routing and link state routing. How do these routing techniques work?

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    Worked Solution

    Distance Vector vs Link State Routing

    SVGVector Architecture Diagram (SVG)

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    Basis	Distance Vector	Link State
    Algorithm	Bellman-Ford	Dijkstra (shortest path first)
    Information shared	Whole routing table (vector of distances)	Link-state of own links (LSA)
    Shared with	Neighbours only	All routers (flooding)
    Knowledge of topology	Only distance and next hop	Complete map of the network
    Updates	Periodic (e.g. 30 s in RIP)	Event-triggered, on topology change
    Convergence	Slow; count-to-infinity problem	Fast
    Resource use	Low CPU and memory	High CPU and memory
    Examples	RIP, IGRP	OSPF, IS-IS

    Working of Distance Vector

    1. Each router starts knowing only the distance (cost) to its directly connected neighbours.
    2. Periodically it sends its distance vector (destination, cost) to its neighbours.
    3. On receiving a vector, a router updates its table with the Bellman-Ford equation: $d_x(y) = \min_v ( c(x,v) + d_v(y) )$.
    4. This repeats until tables become stable (convergence). Problems: slow convergence and count-to-infinity, reduced by split horizon and poison reverse.

    Working of Link State

    1. Each router discovers its neighbours (Hello packets) and measures link costs.
    2. It builds a Link State Advertisement (LSA) listing its links and costs.
    3. The LSA is flooded to all routers, so every router has the same link-state database (full topology).
    4. Each router runs Dijkstra's algorithm to build a shortest-path tree with itself as root and fills its routing table from it.
19. 5(b). Explain how the Leaky Bucket algorithm is used in traffic shaping and rate-limiting.20257m

    Congestion Control

    Explain how the Leaky Bucket algorithm is used in traffic shaping and rate-limiting.

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    Worked Solution

    Leaky Bucket Algorithm

    A traffic shaping algorithm that turns bursty traffic into a steady, constant-rate flow, like water leaking from a bucket through a small hole at a fixed rate.

    SVGVector Architecture Diagram (SVG)

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    Working

    1. Packets arriving from the host are placed in a finite-size queue (bucket).
    2. Packets leave the bucket at a fixed rate $\rho$, whatever the arrival rate.
    3. If the bucket is full, new packets are dropped (or marked).
    4. For variable-size packets, a fixed number of bytes per clock tick is released.

    Use in traffic shaping

    • Smooths bursts so the network sees a predictable, uniform flow, reducing congestion at routers.
    • Applied at the network edge so traffic conforms to the agreed profile (SLA).

    Use in rate-limiting

    • The output rate $\rho$ is the maximum allowed rate; the bucket size limits the burst absorbed.
    • Excess traffic beyond bucket capacity is discarded, which enforces the limit (e.g. API/ISP rate limiting, policing in routers and gateways).

    Note

    The leaky bucket never allows output above $\rho$, even after idle periods. The Token Bucket algorithm allows controlled bursts.

20. 6(a). Compare TCP and UDP. When is each used?20257m

    Transport Layer

    Compare TCP and UDP. When is each used?

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    TCP vs UDP

    Basis	TCP	UDP
    Full form	Transmission Control Protocol	User Datagram Protocol
    Connection	Connection-oriented (3-way handshake)	Connectionless
    Reliability	Reliable: ACKs, retransmission, ordering	Unreliable: no ACK, no ordering
    Flow and congestion control	Yes (sliding window, congestion control)	No
    Header size	20-60 bytes	8 bytes
    Speed	Slower, more overhead	Faster, low overhead
    Data unit	Byte stream (segments)	Datagrams (message boundaries kept)
    Communication	Point-to-point only	Unicast, multicast and broadcast

    When each is used

    TCP: when reliable, ordered delivery matters.

    • Web browsing (HTTP/HTTPS), email (SMTP, IMAP), file transfer (FTP), remote login (SSH, TELNET)

    UDP: when speed and low latency matter more than perfect delivery, or for short request-response messages.

    • DNS queries, DHCP, streaming video/audio, VoIP, online gaming, SNMP, TFTP
21. 6(b). A block of addresses is granted to a small organization. One of the addresses is 205.16.37.39/28. Write the first address, last address and total number of addresses present in the block.20257m

    IP Addressing

    A block of addresses is granted to a small organization. One of the addresses is 205.16.37.39/28. Write the first address, last address and total number of addresses present in the block.

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    Worked Solution

    Given

    Address: 205.16.37.39/28

    • Prefix length $n = 28$, so host bits $= 32 - 28 = 4$
    • Subnet mask: 255.255.255.240
    • Block size $= 2^4 = 16$

    Binary of the last octet

    $39 = 0010\,0111$. The first 4 bits (0010) are network bits and the last 4 bits are host bits.

    	Last octet (binary)	Address
    Given	0010 0111	205.16.37.39
    First address (host bits all 0)	0010 0000	205.16.37.32
    Last address (host bits all 1)	0010 1111	205.16.37.47

    Total number of addresses

    $$2^{32-28} = 2^4 = \mathbf{16}$$

    Answer

    • First address: 205.16.37.32/28
    • Last address: 205.16.37.47/28
    • Total addresses: 16 (usable hosts: 14, from .33 to .46)
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23. 7(a). What is DNS? Explain with a suitable example how the query resolving process is done.20257m

    DNS

    What is DNS? Explain with a suitable example how the query resolving process is done.

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    Worked Solution

    What is DNS?

    The Domain Name System is a distributed, hierarchical database that translates human-readable domain names (e.g. www.example.com) into IP addresses. It uses UDP/TCP port 53 and a hierarchy of root, TLD and authoritative name servers, plus a local (recursive) resolver.

    Query resolution example: www.example.com

    SVGVector Architecture Diagram (SVG)

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    1. The browser asks the local DNS resolver for the IP of www.example.com (recursive query).
    2. If not cached, the resolver asks a root server, which refers it to the .com TLD server.
    3. The TLD server refers it to the authoritative server for example.com.
    4. The authoritative server returns the IP address (an A record).
    5. The resolver caches the answer (for its TTL) and returns it to the client, which connects to that IP.

    The resolver-to-server steps are iterative (each server returns an answer or a referral). Common record types: A, AAAA, CNAME, MX, NS.

24. 7(b). What are the different techniques to improve Quality of Service?20257m

    Quality of Service

    What are the different techniques to improve Quality of Service?

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    Worked Solution

    Quality of Service (QoS)

    QoS is the ability of a network to give required bandwidth, delay, jitter and loss guarantees to different kinds of traffic. Main techniques:

    1. Scheduling

    Decides the order in which queued packets are sent.

    • FIFO: first come, first served; simple, no differentiation
    • Priority queuing: higher-priority queues are served first (risk of starving low priority)
    • Weighted Fair Queuing (WFQ): each class gets a share of bandwidth by weight

    2. Traffic shaping

    Controls the rate at which traffic enters the network.

    • Leaky bucket: constant output rate
    • Token bucket: allows bursts up to the saved tokens

    3. Admission control

    A router or switch accepts a new flow only if it can meet the flow's QoS request, otherwise it rejects it.

    4. Resource reservation

    Bandwidth, buffer and CPU are reserved in advance for a flow along its path, e.g. with RSVP (Integrated Services model).

    5. Other techniques

    • Over-provisioning: more capacity than needed
    • Buffering at the receiver to reduce jitter (streaming)
    • Packet classification and marking with DiffServ (DSCP) so routers treat classes differently

    Technique	Purpose
    Scheduling	Fair/priority use of bandwidth
    Traffic shaping	Smooth bursts, prevent congestion
    Admission control	Avoid overload
    Resource reservation	Guaranteed service

25. 8(a). How is data transmitted using guided and unguided media? Explain simplex, half-duplex and full-duplex communication.20257m

    Transmission Media

    How is data transmitted using guided and unguided media? Explain simplex, half-duplex and full-duplex communication.

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    Worked Solution

    Data transmission over media

    Guided media

    Signals travel along a physical path (conductor), as electrical signals or light.

    • Twisted pair: electrical signals on copper pairs (LANs, telephone)
    • Coaxial cable: electrical signals, better shielding (cable TV)
    • Optical fiber: data sent as light pulses by total internal reflection; very high bandwidth, immune to EMI

    Unguided media

    Signals travel through free space as electromagnetic waves, with no physical conductor.

    • Radio waves: omnidirectional (radio, TV, Wi-Fi)
    • Microwaves: line-of-sight, unidirectional (cellular, satellite)
    • Infrared: short range (remote controls)

    	Guided	Unguided
    Path	Physical cable	Air / free space
    Security	Higher	Lower (can be intercepted)
    Mobility	Fixed	High
    Interference	Low	Higher

    Transmission modes

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    Mode	Description	Example
    Simplex	Communication in one direction only; the receiver cannot reply	Keyboard to computer, TV broadcast, radio
    Half-duplex	Both directions, but not simultaneously; the link is shared by turns	Walkie-talkie, old Ethernet hub
    Full-duplex	Both directions at the same time (separate paths or frequencies)	Telephone, switched Ethernet

26. 8(b). Compare Unicasting, Broadcasting and Multicasting with examples.20257m

    Network Layer

    Compare Unicasting, Broadcasting and Multicasting with examples.

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    Worked Solution

    Unicasting, Broadcasting and Multicasting

    SVGVector Architecture Diagram (SVG)

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    Basis	Unicast	Broadcast	Multicast
    Communication	One sender to one receiver	One sender to all hosts in the network	One sender to a selected group
    Destination address	Specific host address	Broadcast address (e.g. 255.255.255.255, MAC FF:FF:FF:FF:FF:FF)	Group address (IPv4 class D 224.0.0.0 to 239.255.255.255)
    Bandwidth use	One copy per receiver	One copy, flooded to all	One copy per link, shared by group
    Efficiency	Poor for many receivers	Wastes bandwidth on uninterested hosts	Efficient for group delivery
    Examples	Web browsing, email, FTP	ARP request, DHCP discover	Video conferencing, IPTV, stock quotes
    IPv6	Supported	Not used (replaced by multicast)	Supported

27. 9(a). Write a short note on Public and Private Key Cryptography. > Question 9 asks candidates to write short notes on any two listed topics.20257m

    Cryptography

    Write a short note on Public and Private Key Cryptography.

    Question 9 asks candidates to write short notes on any two listed topics.

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    Worked Solution

    Public and Private Key Cryptography

    Also called asymmetric cryptography: each user has a key pair.

    • Public key: shared openly
    • Private key: kept secret by the owner

    Data encrypted with one key can only be decrypted with the other key of the pair, and the private key cannot practically be derived from the public key.

    SVGVector Architecture Diagram (SVG)

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    Working (confidentiality)

    1. The sender encrypts the message with the receiver's public key.
    2. Only the receiver's private key can decrypt it.

    Digital signature (authentication and integrity)

    The sender signs a hash of the message with the sender's private key; anyone can verify it with the sender's public key.

    Features

    • No need to share a secret key beforehand, which solves key distribution
    • Supports confidentiality, authentication, integrity and non-repudiation
    • Algorithms: RSA, ElGamal, ECC, Diffie-Hellman (key exchange)

    Symmetric vs asymmetric

    	Symmetric	Asymmetric
    Keys	One shared secret key	Public and private key pair
    Speed	Fast	Slower
    Key distribution	Difficult	Easy
    Examples	DES, AES	RSA, ECC

    In practice, asymmetric cryptography exchanges a session key, and symmetric encryption protects the bulk data (as in TLS).

28. 9(b). Write a short note on Token Ring. > Question 9 asks candidates to write short notes on any two listed topics.20257m

    LAN

    Write a short note on Token Ring.

    Question 9 asks candidates to write short notes on any two listed topics.

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    Token Ring (IEEE 802.5)

    A LAN technology in which stations are connected in a ring, and access to the medium is controlled by a special frame called the token.

    SVGVector Architecture Diagram (SVG)

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    Working

    1. A small free token circulates around the ring in one direction.
    2. A station with data captures the token (changes it to a busy token), attaches its data and the destination address, and sends the frame.
    3. The frame passes each station; the destination copies the data and sets the acknowledge bits.
    4. The frame returns to the sender, which removes it and releases a new free token.

    Features

    • Collision-free, deterministic access (no CSMA contention)
    • Works well under heavy load; each station gets a bounded waiting time (priority levels possible)
    • A monitor station handles lost or duplicate tokens
    • Speeds: 4 and 16 Mbps

    Disadvantages

    • Token loss or a station failure can break the ring (dual rings or wiring concentrators help)
    • Delay at light load while waiting for the token
    • Costlier than Ethernet, which is why Ethernet replaced it
29. 9(c). Write a short note on Sliding Window Protocol. > Question 9 asks candidates to write short notes on any two listed topics.20257m

    Data Link Layer

    Write a short note on Sliding Window Protocol.

    Question 9 asks candidates to write short notes on any two listed topics.

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    Sliding Window Protocol

    A flow and error control technique that lets the sender transmit several frames before waiting for an acknowledgement, so the channel stays busy (unlike Stop-and-Wait). The sender and receiver each keep a window of allowed sequence numbers.

    SVGVector Architecture Diagram (SVG)

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    Working

    1. Frames are numbered with $m$-bit sequence numbers (0 to $2^m - 1$).
    2. The sender window holds the numbers of frames that may be sent or are awaiting ACK; it slides forward when ACKs arrive.
    3. The receiver window holds the numbers of frames it can accept.
    4. ACKs may be cumulative, and can be piggybacked on data frames.

    Variants

    	Go-Back-N ARQ	Selective Repeat ARQ
    Sender window	Up to $2^m - 1$	Up to $2^{m-1}$
    Receiver window	1	Same as sender
    On error	Retransmit the lost frame and all after it	Retransmit only the lost frame
    Out-of-order frames	Discarded	Buffered
    Complexity	Low	High

    Efficiency

    $$\text{Efficiency} = \frac{N}{1 + 2a}$$

    where $N$ is the window size and $a = T_p / T_t$ (capped at 1). A larger window gives better utilization than Stop-and-Wait ($N=1$).

30. 9(d). Write a short note on Multiplexing. > Question 9 asks candidates to write short notes on any two listed topics.20257m

    Multiplexing

    Write a short note on Multiplexing.

    Question 9 asks candidates to write short notes on any two listed topics.

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    Multiplexing

    Multiplexing (MUX) combines several signals into one and sends them over a single shared link; a demultiplexer (DEMUX) at the other end separates them again. It improves bandwidth utilization and reduces cost.

    SVGVector Architecture Diagram (SVG)

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    Types

    Type	Principle	Signal	Medium / Use
    FDM (Frequency Division)	Each channel gets its own frequency band, used simultaneously	Analog	Radio/TV broadcast, cable TV
    TDM (Time Division)	Each channel gets a time slot in turn (synchronous or statistical)	Digital	Telephone networks (T1/E1), GSM
    WDM (Wavelength Division)	Each channel uses a different wavelength (colour) of light	Optical	Optical fiber backbones (DWDM)

    Advantages

    • Better utilization of link bandwidth and lower cabling cost
    • Many users share one expensive medium

    Limitations

    • FDM: needs guard bands, crosstalk and intermodulation noise
    • TDM: needs synchronization, wasted slots in synchronous TDM
    • WDM: costly optical components