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GNDU QUESTION PAPERS 2025
Bachelor of Computer Applicaon (BCA) 6th Semester
(Batch 2023-26) (CBGS)
PAPER-II: COMPUTER NETWORKS
Time Allowed: 3 Hours Maximum Marks: 75
Note: Aempt Five quesons in all, selecng at least One queson from each secon. The
Fih queson may be aempted from any secon. All quesons carry equal marks
SECTION-A
1. What are the basic components of a Network? Discuss dierent types of network in
detail.
2. (a) What are various transmission mediums in computer networks? Discuss.
(b) Dene TCP/IP reference model. How it works? Discuss dierent layers of TCP/IP model
in detail.
SECTION-B
3. What is Mulplexing? What are its dierent types? Discuss in detail.
4. Explain the following:
(a) Pulse Code Modulaon
(b) Hybrid switching
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SECTION-C
5. (a) What is IEEE 802 standard? How many IEEE 802 standards are there? Discuss in
detail.
(b) How Token Bus Network works?
6. How Data Link Layer works? Explain Data Link Layer Design Issues in detail.
SECTION-D
7. What is cryptography in computer networks? What are its types ? Discuss in detail.
8. What is a network service ? What services can a computer network 15 oer ? Discuss in
detail.
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GNDU ANSWER PAPERS 2025
Bachelor of Computer Applicaon (BCA) 6th Semester
(Batch 2023-26) (CBGS)
PAPER-II: COMPUTER NETWORKS
Time Allowed: 3 Hours Maximum Marks: 75
Note: Aempt Five quesons in all, selecng at least One queson from each secon. The
Fih queson may be aempted from any secon. All quesons carry equal marks
SECTION-A
1. What are the basic components of a Network? Discuss dierent types of network in
detail.
Ans: What is a Network?
Imagine you and your friends are connected through WhatsApp. You send messages,
photos, and videos to each other. This connection between devices (phones, computers,
etc.) is what we call a network.
In simple words:
A network is a group of devices connected together to share information and resources.
Basic Components of a Network
Every network, whether small (home Wi-Fi) or large (internet), is made up of some essential
parts. Let’s understand them one by one.
1. Devices (Nodes)
These are the main elements of a network.
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• Computers
• Laptops
• Mobile phones
• Printers
• Servers
Think of them as “participants” in a conversation.
2. Server
A server is a powerful computer that provides services to other devices.
• Stores data
• Manages files
• Controls access
Example: Google Drive stores your files on servers.
3. Clients
Clients are normal devices that request services from servers.
Example: Your phone is a client when you open YouTube.
4. Transmission Media
This is the path through which data travels.
• Wired: cables (like Ethernet)
• Wireless: Wi-Fi, Bluetooth
Like roads connecting cities.
5. Networking Devices
These help connect and manage the network:
• Router – connects different networks (like internet to your home)
• Switch – connects multiple devices in a network
• Modem – converts signals for internet use
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6. Protocols
Protocols are rules for communication.
Example: HTTP, FTP, TCP/IP
Think of them as the “language” devices use to talk.
Diagram: Basic Network Components
[Server]
|
-------------
| | |
[PC] [Laptop] [Mobile]
\ | /
[Router]
|
[Internet]
Types of Networks
Now let’s move to the second part: Types of Networks
Networks are classified based on their size and coverage area.
1. PAN (Personal Area Network)
This is the smallest type of network.
• Covers a very short distance (1–10 meters)
• Used for personal devices
Example:
• Bluetooth connection
• Connecting phone to earbuds
Diagram: PAN
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[Mobile]
|
[Earbuds]
|
[Laptop]
All devices are very close to each other.
2. LAN (Local Area Network)
This is the most common network.
• Covers small areas like:
o Home
o School
o Office
Example:
• Wi-Fi in your home
• Computer lab in college
Features:
• High speed
• Low cost
• Easy to maintain
Diagram: LAN
[Computer] [Laptop]
\ /
[Switch]
|
[Router]
|
[Internet]
3. MAN (Metropolitan Area Network)
This network covers a city or town.
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Example:
• City-wide internet services
• Cable TV network
Features:
• Larger than LAN
• Faster communication across a city
Diagram: MAN
[LAN] ---- [LAN] ---- [LAN]
\ | /
[City Network]
4. WAN (Wide Area Network)
This is the largest type of network.
• Covers countries and continents
Example:
• The Internet
Features:
• Very large coverage
• Connects multiple LANs and MANs
Diagram: WAN
[City] ---- [Country] ---- [Continent]
\ | /
[Internet]
5. WLAN (Wireless Local Area Network)
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This is a type of LAN but without wires.
Example:
• Wi-Fi network
Features:
• No cables
• Flexible and easy to use
Diagram: WLAN
[Router]
/ | \
[Phone][PC][Laptop]
Easy Comparison Table
Network Type
Coverage Area
Example
PAN
Personal
Bluetooth
LAN
Small area
Home Wi-Fi
MAN
City
Cable network
WAN
Global
Internet
WLAN
Wireless LAN
Wi-Fi
Final Understanding
Let’s wrap everything in a very simple way:
• A network connects devices to share information.
• It has components like:
o Devices
o Server
o Transmission media
o Protocols
• Networks are classified based on size:
o PAN → very small
o LAN → home/office
o MAN → city
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o WAN → worldwide
Real-Life Example
Think of a network like a transportation system:
• Devices = vehicles
• Cables/Wi-Fi = roads
• Router = traffic controller
• Protocols = traffic rules
Without rules and connections, nothing would work smoothly!
2. (a) What are various transmission mediums in computer networks? Discuss.
(b) Dene TCP/IP reference model. How it works? Discuss dierent layers of TCP/IP model
in detail.
Ans: Part (a) Transmission Mediums in Computer Networks
When computers communicate, they need a “path” or “medium” to send data. Think of it
like sending a letter—you need a road, a postman, or an airplane to carry it. In networking,
the transmission medium is that carrier.
Broadly, transmission mediums are divided into guided (wired) and unguided (wireless).
Guided (Wired) Media
Guided media means the data travels through a physical path, like cables.
1. Twisted Pair Cable
• Looks like telephone wires—two copper wires twisted together.
• Cheap and widely used in LANs.
• Types: Unshielded Twisted Pair (UTP) and Shielded Twisted Pair (STP).
• Speeds: Can support up to gigabit Ethernet.
Analogy: Like two friends whispering to each other while covering their mouths to reduce
noise.
2. Coaxial Cable
• A copper core with insulation, shielding, and an outer cover.
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• Used in cable TV and older computer networks.
• Better shielding than twisted pair, so less noise.
Analogy: Like a water pipe with multiple protective layers to keep the flow clean.
3. Optical Fiber
• Uses glass or plastic fibers to transmit data as light.
• Extremely fast, supports long distances.
• Immune to electromagnetic interference.
• Backbone of the internet.
Analogy: Like sending information at the speed of light through a transparent tunnel.
Unguided (Wireless) Media
Unguided media means data travels through air or space without physical cables.
1. Radio Waves
• Used in Wi-Fi, Bluetooth, and mobile phones.
• Can travel long distances but are prone to interference.
2. Microwaves
• Used in satellite communication and point-to-point links.
• Requires line of sight.
3. Infrared
• Used in remote controls and short-range communication.
• Limited to very short distances.
4. Satellite Communication
• Data sent to satellites and then relayed back to Earth.
• Useful for global coverage.
Diagram 1: Transmission Media
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Part (b) TCP/IP Reference Model
Now let’s move to the TCP/IP model, the backbone of how the internet works.
Imagine sending a parcel from one city to another. You don’t just throw it on the road—you
follow steps: packaging, labeling, transporting, delivering. Similarly, when computers send
data, they follow a layered model.
The TCP/IP model has four layers:
1. Application Layer
2. Transport Layer
3. Internet Layer
4. Network Access Layer
1. Application Layer
This is the layer closest to the user. It provides services like email, web browsing, and file
transfer.
• Protocols: HTTP, FTP, SMTP, DNS.
• Example: When you type a website address, HTTP handles the request.
Analogy: Like writing a letter in English—the language both sender and receiver understand.
2. Transport Layer
This layer ensures reliable delivery of data. It breaks data into segments, numbers them, and
reassembles them at the destination.
• Protocols: TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).
• TCP: Reliable, error-checked, ordered delivery.
• UDP: Faster, but no guarantee (used in streaming, gaming).
Analogy: Like a courier service that either guarantees safe delivery (TCP) or just drops
parcels quickly without checking (UDP).
3. Internet Layer
This layer decides the best path for data to travel. It handles addressing and routing.
• Protocol: IP (Internet Protocol).
• IP addresses identify devices uniquely.
• Routers work at this layer to forward packets.
Analogy: Like the postal system deciding which roads to take to deliver your letter.
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4. Network Access Layer
This is the physical layer—how data actually travels through cables or wireless signals.
• Includes Ethernet, Wi-Fi, and other link technologies.
• Deals with frames, MAC addresses, and hardware.
Analogy: Like the delivery truck or airplane that physically carries your parcel.
Diagram 2: TCP/IP Model
How TCP/IP Works Together
Let’s walk through an example: You open your browser and type “www.example.com.”
1. Application Layer: Browser uses HTTP to request the webpage.
2. Transport Layer: TCP breaks the request into segments, ensures reliability.
3. Internet Layer: IP adds source and destination addresses, decides the route.
4. Network Access Layer: Ethernet/Wi-Fi sends the data physically.
At the other end, the server reverses the process to deliver the webpage back to you.
Comparison: TCP/IP vs OSI Model
The OSI model has 7 layers, while TCP/IP has 4. TCP/IP is more practical and widely used,
while OSI is more theoretical.
OSI Model (7 Layers)
TCP/IP Model (4 Layers)
Application
Application
Presentation
Application
Session
Application
Transport
Transport
Network
Internet
Data Link
Network Access
Physical
Network Access
Everyday Example
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Think of sending a WhatsApp message:
• Application Layer: You type the message.
• Transport Layer: TCP ensures the message is delivered correctly.
• Internet Layer: IP finds the route to your friend’s phone.
• Network Access Layer: Wi-Fi or mobile data carries the message physically.
Conclusion
So, to wrap it up:
• Transmission mediums are the paths data takes—wired (like fiber optics) or wireless
(like radio waves).
• TCP/IP model is the framework that makes internet communication possible, with
four layers handling everything from user applications to physical transmission.
Together, they form the backbone of modern networking. Without transmission mediums,
data couldn’t travel; without TCP/IP, devices wouldn’t know how to talk to each other.
SECTION-B
3. What is Mulplexing? What are its dierent types? Discuss in detail.
Ans: What is Multiplexing?
Imagine you are at a highway toll plaza. Instead of building separate roads for every single
car, all cars use the same road but move in an organized way so everyone reaches their
destination safely.
Multiplexing works in a very similar way in communication systems.
Multiplexing is a technique used to combine multiple signals or data streams into a
single signal so they can travel through a single communication channel (like a cable or
wireless medium).
At the receiving end, the combined signal is separated back into its original signals. This
process is called Demultiplexing.
Why Do We Need Multiplexing?
Without multiplexing:
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• Every signal would need its own separate channel
• This would be expensive and inefficient
With multiplexing:
• Many signals share one channel
• It saves bandwidth, cost, and resources
Basic Working of Multiplexing
Here’s how it works step-by-step:
1. Multiple input signals (from different sources) are collected
2. A Multiplexer (MUX) combines them into one signal
3. The combined signal travels through a single medium
4. A Demultiplexer (DEMUX) separates the signals at the receiver
Diagram: Basic Multiplexing System
Types of Multiplexing
There are mainly three important types of multiplexing:
1. Frequency Division Multiplexing (FDM)
2. Time Division Multiplexing (TDM)
3. Wavelength Division Multiplexing (WDM)
Let’s understand each one in a very simple and relatable way.
Frequency Division Multiplexing (FDM)
Idea:
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Different signals are sent at different frequencies at the same time.
Example:
Think of a radio:
• Different stations (FM 91.1, FM 98.3, etc.)
• All signals travel together but on different frequencies
How it works:
• Each signal is assigned a unique frequency band
• Signals are combined and transmitted simultaneously
• At the receiver, filters separate them
Diagram: Frequency Division Multiplexing
Advantages:
• Continuous transmission
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• No time delay between signals
Disadvantages:
• Requires more bandwidth
• Can suffer from interference
Time Division Multiplexing (TDM)
Idea:
Different signals share the same channel but at different time intervals.
Example:
Imagine a classroom:
• Students speak one by one
• Each gets a time slot
How it works:
• The channel is divided into time slots
• Each signal is transmitted in its assigned time
• This happens very fast, so it feels simultaneous
Diagram: Time Division Multiplexing
Types of TDM:
• Synchronous TDM: Fixed time slots (even if no data)
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• Asynchronous TDM: Time slots assigned only when needed
Advantages:
• Efficient use of bandwidth
• No interference between signals
Disadvantages:
• Time delay may occur
• Needs synchronization
Wavelength Division Multiplexing (WDM)
Idea:
Used in fiber optic communication, where signals are separated by different wavelengths
(colors of light).
Example:
Think of a rainbow :
• Different colors travel together
• Each color represents a different signal
How it works:
• Each signal is assigned a different light wavelength
• All wavelengths travel through the same optical fiber
• At the receiver, they are separated
Diagram: Wavelength Division Multiplexing
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Advantages:
• Very high data capacity
• Ideal for long-distance communication
Disadvantages:
• Expensive technology
• Complex setup
Comparison of Multiplexing Types
Feature
FDM
TDM
WDM
Based on
Frequency
Time
Wavelength
Used in
Radio, TV
Digital communication
Fiber optics
Transmission
Simultaneous
Sequential
Simultaneous
Efficiency
Medium
High
Very High
Real-Life Applications
Multiplexing is used everywhere in daily life:
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• Cable TV
• Radio broadcasting
• Telephone networks
• Internet data transfer
• Fiber optic communication
Conclusion
Multiplexing is a smart and efficient technique that allows multiple signals to travel through
a single communication channel. Instead of using separate paths for each signal, it combines
them in an organized way and then separates them at the destination.
• FDM uses different frequencies
• TDM uses different time slots
• WDM uses different light wavelengths
In simple words, multiplexing is like sharing a road, but in a planned and organized way so
that everyone reaches safely without confusion.
4. Explain the following:
(a) Pulse Code Modulaon
(b) Hybrid switching
Ans: Part (a) Pulse Code Modulation (PCM)
Imagine you’re at a concert. The singer’s voice is an analog signal—smooth, continuous
waves of sound. But computers don’t understand smooth waves; they understand digital
signals (0s and 1s). So, how do we convert that beautiful analog voice into something a
computer can store or transmit?
That’s where Pulse Code Modulation (PCM) comes in. PCM is a method of converting
analog signals into digital form.
Steps in PCM
PCM works in three main steps: Sampling, Quantization, and Encoding.
1. Sampling
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• The analog signal is measured at regular intervals (samples).
• According to the Nyquist theorem, the sampling rate must be at least twice the
highest frequency of the signal.
• Example: For human voice (up to 4 kHz), we sample at 8 kHz.
Analogy: Imagine taking snapshots of a moving car every second. You don’t capture the
continuous motion, but you get enough pictures to reconstruct the journey.
2. Quantization
• Each sample’s amplitude is rounded to the nearest value within a set of discrete
levels.
• This introduces a small error called quantization error.
Analogy: Like rounding your height to the nearest centimeter. You lose tiny details, but the
overall measurement is close enough.
3. Encoding
• Each quantized value is represented as a binary code (0s and 1s).
• These codes are then transmitted or stored.
Analogy: Like writing down your height in digits (e.g., 170 cm) instead of drawing a picture
of yourself.
Features of PCM
• Accuracy: Provides a faithful digital representation of analog signals.
• Noise Resistance: Digital signals are less affected by noise compared to analog.
• Standardization: Widely used in telephony, CDs, and digital audio.
• Bandwidth Requirement: Needs more bandwidth than analog transmission.
Diagram 1: PCM Process
Analog Signal → Sampling → Quantization → Encoding → Digital
Signal
Everyday Example
When you record your voice on a smartphone, the microphone captures analog sound
waves. PCM converts them into digital signals, which are then stored as audio files. That’s
why you can replay your voice exactly as it sounded.
Part (b) Hybrid Switching
Now let’s move to Hybrid Switching, which is about how data travels through networks.
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Imagine you’re at a busy train station. Some trains run on fixed schedules (like circuit
switching), while others run flexibly depending on demand (like packet switching). Hybrid
switching combines both approaches to get the best of both worlds.
Background: Types of Switching
Before understanding hybrid switching, let’s quickly recall the two main types:
1. Circuit Switching
• A dedicated path is established between sender and receiver.
• Example: Traditional telephone calls.
• Advantage: Guaranteed bandwidth and quality.
• Disadvantage: Wastes resources if the line is idle.
2. Packet Switching
• Data is broken into packets, each sent independently.
• Example: Internet communication.
• Advantage: Efficient use of resources.
• Disadvantage: No guaranteed delivery time (packets may be delayed).
Hybrid Switching
Hybrid switching combines circuit switching and packet switching to balance efficiency and
reliability.
How It Works:
• For continuous, real-time communication (like voice calls), it uses circuit switching to
ensure quality.
• For bursty, non-real-time data (like emails or file transfers), it uses packet switching
to save resources.
• The system dynamically decides which method to use based on the type of traffic.
Features of Hybrid Switching
• Flexibility: Adapts to different types of data.
• Efficiency: Saves bandwidth by using packet switching when possible.
• Quality Assurance: Ensures reliable communication for real-time services.
• Scalability: Suitable for modern networks that handle diverse traffic.
Diagram 2: Hybrid Switching
Voice Call → Circuit Switching Path
File Transfer → Packet Switching Path
Hybrid Switching → Combines both
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Everyday Example
Think of WhatsApp:
• When you make a voice call, the system ensures smooth, continuous communication
(like circuit switching).
• When you send a text or photo, it’s broken into packets and delivered efficiently (like
packet switching). Hybrid switching allows both to coexist seamlessly.
Conclusion
So, to wrap it up:
• Pulse Code Modulation (PCM) is the process of converting analog signals into digital
form using sampling, quantization, and encoding. It’s the backbone of digital audio
and telecommunication.
• Hybrid Switching is a smart way of combining circuit switching (for continuous, real-
time communication) and packet switching (for efficient, bursty data transfer). It
ensures both reliability and efficiency in modern networks.
Together, these concepts show how communication systems handle both the conversion of
signals and the transmission of data—making sure our voices, videos, and files travel
smoothly across the digital world.
SECTION-C
5. (a) What is IEEE 802 standard? How many IEEE 802 standards are there? Discuss in
detail.
(b) How Token Bus Network works?
Ans: 5 (a) What is IEEE 802 Standard? How many IEEE 802 standards are there?
Understanding IEEE 802 in Simple Words
Imagine a world where every device (laptop, mobile, printer, Wi-Fi router) speaks its own
language. What would happen?
Chaos! Nothing would connect properly.
To solve this problem, we need common rules so that all devices can communicate
smoothly. These rules are called standards.
This is where IEEE 802 comes in.
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IEEE stands for Institute of Electrical and Electronics Engineers
802 refers to a specific group that works on networking standards
Simple Definition:
IEEE 802 is a set of standards that define how devices communicate over local area
networks (LAN) and metropolitan area networks (MAN).
Why IEEE 802 is Important
Think of IEEE 802 as traffic rules for data communication:
• Ensures devices from different companies can connect
• Maintains proper communication without collisions
• Defines how data is sent and received
• Makes networks faster and more reliable
Basic Concept: OSI Layer Connection
IEEE 802 mainly works on:
• Data Link Layer (Layer 2)
• Part of Physical Layer (Layer 1)
It divides Data Link Layer into two parts:
1. LLC (Logical Link Control)
2. MAC (Media Access Control)
Diagram: IEEE 802 Structure
In simple terms:
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• LLC = Controls flow and error checking
• MAC = Controls how devices access the network
How Many IEEE 802 Standards Are There?
There are many IEEE 802 standards, each designed for a specific type of network.
You don’t need to memorize all, just understand the important ones.
Major IEEE 802 Standards
Standard
Name
Description
802.1
Bridging & Management
Network management
802.2
LLC
Logical link control
802.3
Ethernet
Wired LAN (most common)
802.4
Token Bus
Token passing bus network
802.5
Token Ring
Ring network
802.6
MAN
Metropolitan networks
802.7
Broadband TAG
Advisory group
802.8
Fiber Optic TAG
Fiber communication
802.11
Wi-Fi
Wireless LAN
802.15
Bluetooth
Personal area network
802.16
WiMAX
Wireless broadband
Most Important Standards Explained
802.3 – Ethernet
• Used in LAN cables
• Most common network today
802.11 – Wi-Fi
• Wireless communication
• Used in homes, offices
802.15 – Bluetooth
• Short distance communication
Diagram: Types of IEEE 802 Networks
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Summary of Part (a)
IEEE 802 is a family of networking standards
It ensures smooth communication between devices
There are many standards, but Ethernet (802.3) and Wi-Fi (802.11) are most widely
used
5 (b) How Token Bus Network Works
Now let’s understand Token Bus in a fun and simple way.
Imagine a Bus System
Think of a bus route where passengers speak one by one.
• Only the person holding the ticket can speak
• After speaking, they pass the ticket to the next person
This “ticket” is called a TOKEN
What is Token Bus?
Token Bus is a network where:
• Devices are connected like a bus (straight line)
• But communication happens in a logical ring order
• A special signal called a token controls communication
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Key Idea
Even though physically it looks like a bus, logically it behaves like a ring.
Diagram: Token Bus Structure
Step-by-Step Working of Token Bus
Let’s understand it step-by-step:
Step 1: Token Creation
• A special packet called token is created
• It moves in a predefined order
Step 2: Token Passing
• Token moves from one device to another
• Only the device with token can send data
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Step 3: Data Transmission
• If device has data → it sends it
• If not → it passes the token
Step 4: Token Circulation Continues
• Token keeps rotating
• Every device gets a chance
Diagram: Token Passing Process
Advantages of Token Bus
✔ No collision (only one device transmits)
✔ Fair access (everyone gets turn)
✔ Predictable performance
✔ Good for industrial networks
Disadvantages of Token Bus
✖ Complex setup
✖ Token loss can stop network
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✖ Slower than Ethernet
✖ Not widely used today
Real-Life Example
Imagine students in a classroom:
• Teacher gives a mic (token)
• Only student with mic can speak
• After speaking, mic is passed
No noise, no confusion — same as Token Bus!
Token Bus vs Ethernet
Feature
Token Bus
Ethernet
Access Method
Token Passing
CSMA/CD
Collision
No
Possible
Speed
Moderate
Fast
Usage
Rare
Very common
Final Summary
IEEE 802
• A group of networking standards
• Defines how devices communicate
• Includes Ethernet, Wi-Fi, Bluetooth
Token Bus
• Uses token passing method
• No collision
• Works in logical ring over bus topology
6. How Data Link Layer works? Explain Data Link Layer Design Issues in detail.
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Ans: What is the Data Link Layer?
Imagine you’re sending a letter through the postal system. You don’t just drop the letter
into the street—you put it in an envelope, write the address, and make sure it’s delivered
safely. In computer networks, the Data Link Layer plays a similar role.
It sits just above the Physical Layer (which deals with raw signals and wires) and just below
the Network Layer (which deals with routing and addressing). Its job is to ensure that data is
packaged into frames, transmitted across a physical medium, and delivered reliably to the
next node.
How the Data Link Layer Works
The Data Link Layer has several key responsibilities:
1. Framing
o Breaks the raw bit stream into manageable chunks called frames.
o Adds headers and trailers to mark the beginning and end of each frame.
o Analogy: Like putting your letter into an envelope with a clear start and end.
2. Error Detection and Correction
o Ensures that data isn’t corrupted during transmission.
o Uses techniques like checksums or Cyclic Redundancy Check (CRC).
o If errors are detected, the frame can be retransmitted.
o Analogy: Like checking if your letter got smudged or torn, and asking for a
replacement if needed.
3. Flow Control
o Prevents a fast sender from overwhelming a slow receiver.
o Ensures smooth communication between devices.
o Analogy: Like speaking slowly so your friend can take notes without missing
anything.
4. Medium Access Control (MAC)
o Decides who gets to use the communication channel when multiple devices
want to send data.
o Uses protocols like CSMA/CD (Ethernet) or CSMA/CA (Wi-Fi).
o Analogy: Like people taking turns to speak in a conversation.
5. Reliable Delivery (in some cases)
o Ensures frames are delivered in the correct order without duplication.
o Some Data Link protocols provide reliability, while others leave it to higher
layers.
Diagram 1: Position of Data Link Layer
+-------------------+
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| Network Layer |
+-------------------+
| Data Link Layer |
+-------------------+
| Physical Layer |
+-------------------+
Design Issues in the Data Link Layer
Now let’s dive into the design issues—the challenges engineers face when building this
layer. These issues define how the Data Link Layer is structured and how it performs its
tasks.
1. Framing
• Problem: How do we mark the start and end of a frame in a continuous stream of
bits?
• Solutions:
o Character Count: Include the number of characters in the header.
o Byte Stuffing: Use special flag bytes to indicate frame boundaries.
o Bit Stuffing: Insert extra bits to avoid confusion with flag patterns.
Diagram 2: Framing with Flags
[Flag] [Data] [Flag]
2. Error Control
• Problem: How do we detect and correct errors caused by noise or interference?
• Solutions:
o Parity Bits: Simple error detection.
o Checksums: Add up values to detect corruption.
o CRC (Cyclic Redundancy Check): Powerful error detection method.
o ARQ (Automatic Repeat Request): Retransmit frames if errors are detected.
3. Flow Control
• Problem: How do we prevent a sender from overwhelming a receiver?
• Solutions:
o Stop-and-Wait Protocol: Sender waits for acknowledgment before sending
the next frame.
o Sliding Window Protocol: Sender can transmit multiple frames before
waiting for acknowledgment.
Diagram 3: Stop-and-Wait Flow Control
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4. Medium Access Control (MAC)
• Problem: How do multiple devices share the same communication medium without
collisions?
• Solutions:
o Random Access Protocols: Devices transmit when ready (e.g., CSMA/CD in
Ethernet).
o Controlled Access Protocols: Devices take turns (e.g., polling, token passing).
o Channelization Protocols: Divide the channel into separate slots (e.g., TDMA,
FDMA).
Diagram 4: MAC Example
Multiple Devices → Shared Channel → MAC decides who speaks
5. Addressing
• Problem: How do we identify which device a frame is meant for?
• Solutions:
o Use MAC addresses (unique identifiers for each device).
o Include source and destination addresses in the frame header.
6. Reliability
• Problem: Should the Data Link Layer guarantee reliable delivery?
• Solutions:
o Some protocols (like Ethernet) provide best-effort delivery.
o Others (like PPP) include mechanisms for retransmission and reliability.
7. Efficiency
• Problem: How do we minimize overhead while ensuring reliability?
• Solutions:
o Optimize framing and error detection methods.
o Balance between speed and accuracy.
Everyday Example
Let’s imagine sending a WhatsApp message over Wi-Fi:
1. Framing: Your message is broken into frames.
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2. Error Control: Wi-Fi checks if the frames are corrupted.
3. Flow Control: If your phone sends too fast, Wi-Fi slows it down.
4. MAC: Wi-Fi decides which device (your phone, your laptop, or your smart TV) gets to
send data at a given moment.
5. Addressing: Frames include your phone’s MAC address and the router’s MAC
address.
6. Reliability: If a frame is lost, it’s retransmitted.
All of this happens invisibly, in milliseconds, while you just see your message delivered
instantly.
Conclusion
So, to wrap it up:
• The Data Link Layer is like the postal worker of the network—it packages data into
frames, ensures it’s not corrupted, controls the flow, and decides who gets to use
the communication channel.
• The design issues—framing, error control, flow control, medium access, addressing,
reliability, and efficiency—are the challenges engineers solve to make sure
communication is smooth and reliable.
Without the Data Link Layer, the raw bits from the Physical Layer would be chaotic and
unreliable. With it, we get structured, dependable communication that makes modern
networking possible.
SECTION-D
7. What is cryptography in computer networks? What are its types ? Discuss in detail.
Ans: What is Cryptography?
Cryptography is the process of converting readable data (plain text) into an unreadable
format (cipher text) so that only authorized people can read it.
In simple words:
It is a technique used to keep information safe and secure from hackers or unauthorized
users.
For example:
When you send a WhatsApp message, it is encrypted so that only the receiver can read it —
not even WhatsApp itself!
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Basic Idea of Cryptography
Imagine you write a message:
“HELLO”
Now you convert it into something like:
“KHOOR”
This coded message is called cipher text, and the process of converting is called encryption.
To read it again, the receiver uses a method called decryption.
Encryption & Decryption Process
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Steps:
1. Sender writes the original message (Plain Text)
2. Encryption algorithm converts it into Cipher Text
3. Message travels through the network
4. Receiver decrypts it back into Plain Text
Objectives of Cryptography
Cryptography is used to achieve the following goals:
1. Confidentiality – Only authorized users can read the data
2. Integrity – Data should not be altered
3. Authentication – Verify the identity of sender/receiver
4. Non-repudiation – Sender cannot deny sending the message
Types of Cryptography
There are mainly three types of cryptography:
1. Symmetric Key Cryptography
2. Asymmetric Key Cryptography
3. Hash Functions
Let’s understand each one clearly.
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Symmetric Key Cryptography
In this method, the same key is used for both encryption and decryption.
Example:
• Sender and receiver both know a secret key (like a password)
Working of Symmetric Key
Steps:
1. Sender encrypts message using a secret key
2. Message is sent over the network
3. Receiver uses the same key to decrypt
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Advantages:
• Fast and efficient
• Suitable for large data
Disadvantages:
• Key sharing is risky
• If key is stolen → data is compromised
Examples:
• AES (Advanced Encryption Standard)
• DES (Data Encryption Standard)
Asymmetric Key Cryptography
This method uses two keys:
• Public Key (shared with everyone)
• Private Key (kept secret)
Working of Asymmetric Key
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Steps:
1. Sender uses receiver’s public key to encrypt the message
2. Message is sent over the network
3. Receiver uses private key to decrypt
Advantages:
• More secure
• No need to share secret key
Disadvantages:
• Slower than symmetric
• More complex
Examples:
• RSA
• Diffie-Hellman
Hash Functions
Hashing is a one-way process.
• It converts data into a fixed-length value
• Cannot be reversed (no decryption)
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Hashing Process
5
Example:
Input: “HELLO”
Output: “5d41402abc4b2a76b9719d911017c592”
Uses:
• Password storage
• Data integrity checking
• Digital signatures
Limitation:
• Cannot recover original data
Examples:
• MD5
• SHA (Secure Hash Algorithm)
Comparison of Cryptography Types
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Feature
Symmetric Key
Asymmetric Key
Hashing
Keys Used
One
Two
No key
Speed
Fast
Slow
Very fast
Security
Moderate
High
Very high (one-way)
Reversible
Yes
Yes
No
Real-Life Applications of Cryptography
Cryptography is used everywhere in real life:
• Online Banking – protects transactions
• Messaging Apps (WhatsApp) – end-to-end encryption
• E-commerce (Amazon, Flipkart) – secure payments
• Emails – encrypted communication
• Passwords – stored using hashing
Final Conclusion
Cryptography is like a digital lock and key system that protects our data from unauthorized
access. It ensures that our information remains private, secure, and trustworthy while
traveling across networks.
• Symmetric cryptography is fast but needs careful key sharing
• Asymmetric cryptography is more secure but slower
• Hashing is used for verification and security without reversing
Together, these methods form the backbone of modern network security.
8. What is a network service ? What services can a computer network oer ? Discuss in
detail.
Ans: What is a Network Service?
Think of a computer network like a city. The roads are the cables and wireless signals, the
vehicles are the data packets, and the traffic rules are the protocols. But what makes a city
useful isn’t just the roads—it’s the services: buses, electricity, water supply, postal delivery.
Similarly, in a computer network, network services are the useful functions that the
network provides to applications and users.
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In simple terms: A network service is any functionality provided by the network that
allows computers, applications, or users to communicate, share resources, or perform
tasks.
Why Network Services Matter
Without services, a network would just be a bunch of connected machines with no real
purpose. Services make the network valuable by enabling:
• Communication (emails, messaging, video calls)
• Resource sharing (printers, files, internet access)
• Security (authentication, encryption)
• Management (monitoring, troubleshooting)
Types of Network Services Offered by Computer Networks
Let’s go through the major categories of services in detail.
1. Communication Services
This is the most obvious and important role of networks—allowing people and machines to
talk to each other.
• Email Services (SMTP, IMAP, POP3): Sending and receiving electronic mail.
• Messaging Services: Instant messaging apps like WhatsApp or Slack.
• Voice over IP (VoIP): Making phone calls over the internet (Skype, Zoom).
• Video Conferencing: Real-time video communication.
Analogy: Like the postal service, but faster and digital.
2. File and Resource Sharing
Networks allow multiple users to share files, printers, and other resources.
• File Transfer Protocol (FTP): Moving files between computers.
• Network File Systems (NFS, SMB): Accessing files stored on another computer as if
they were local.
• Printer Sharing: Multiple users can print documents using one printer connected to
the network.
Diagram 1: File Sharing
Code
User A → Network → Shared Folder → User B
3. Internet Services
The internet itself is a giant network offering countless services.
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• Web Services (HTTP/HTTPS): Browsing websites.
• Domain Name System (DNS): Translating human-friendly names (like google.com)
into IP addresses.
• Search Engines: Helping users find information.
• Streaming Services: Delivering audio and video content.
Analogy: Like a huge library where DNS is the catalog system that helps you find the right
book.
4. Security Services
Networks must protect data and users from threats.
• Authentication Services: Verifying user identities (e.g., login systems).
• Encryption Services: Protecting data from eavesdropping.
• Firewalls and Intrusion Detection: Monitoring and controlling traffic.
• VPN (Virtual Private Network): Securely connecting remote users.
Diagram 2: Security Service
Code
User → Authentication → Secure Access → Network
5. Management Services
Networks are complex, so they need tools to monitor and manage them.
• Network Monitoring (SNMP): Checking the health of devices.
• Configuration Services: Managing device settings.
• Troubleshooting Tools: Detecting and fixing problems.
Analogy: Like traffic police managing the flow of vehicles in a city.
6. Directory Services
These services organize and manage information about users and resources.
• LDAP (Lightweight Directory Access Protocol): Stores information about users,
groups, and devices.
• Active Directory (AD): Microsoft’s directory service for managing users and
permissions.
Diagram 3: Directory Service
Directory Database → User Info → Access Control
7. Cloud and Virtualization Services
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Modern networks extend into the cloud.
• Cloud Storage (Google Drive, OneDrive): Storing files online.
• Cloud Computing (AWS, Azure): Running applications on remote servers.
• Virtualization Services: Running multiple virtual machines on one physical machine.
Analogy: Like renting an apartment in a huge building instead of owning the whole building.
8. Specialized Services
Networks also provide niche services depending on the environment.
• E-commerce Services: Online shopping platforms.
• Banking Services: Secure financial transactions.
• Educational Services: Online learning platforms.
• Gaming Services: Multiplayer online games.
Classification of Services
We can broadly classify network services into two categories:
Category
Examples
User-Oriented
Email, Web browsing, Messaging, Gaming
System-Oriented
DNS, Authentication, Monitoring, Security
Everyday Example
Let’s imagine you’re working from home:
1. You log in to your company’s VPN (security service).
2. You access shared files on the company server (file sharing service).
3. You join a Zoom meeting (communication service).
4. You browse the company website (internet service).
5. The IT team monitors the network to ensure smooth performance (management
service).
All of these are network services working together seamlessly.
Diagram 5: How Services Interact
User → Application → Network Service → Resource
Conclusion
So, to sum it up:
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• A network service is any functionality provided by a computer network that enables
communication, resource sharing, security, or management.
• Networks can offer a wide range of services: communication (email, VoIP), file
sharing, internet access, security, management, directory services, cloud services,
and specialized services like gaming or banking.
• These services make networks useful, efficient, and secure—transforming simple
connections into powerful tools for everyday life.
“This paper has been carefully prepared for educaonal purposes. If you noce any
mistakes or have suggesons, feel free to share your feedback.”
