Basics of Computer Networks-complete.pptx

COMPUTER NETWORK
• A network is a set of devices (often referred to as nodes) connected by
communication links. A node can be a computer, printer, or any other
device capable of sending and/or receiving data generated by other
nodes on the network
• A computer network is a set of computers connected together to share
resources.
• Software modules in one system are used to communicate with one or
more software modules in the distant System. Such interfaces across a
distance are termed as “peer-to-peer” interfaces;
• Local interfaces are termed as “service” interfaces. The modules on
each end are organized as a sequence of functions called “layers”. The
set of modules organized as layers is also commonly called a “protocol
stack”.

BASIC PARTS OF DATA COMMUNICATION:
• Message: A message is a piece of information that is to be transmitted from one
person to another. It could be a text file, an audio file, a video file, etc.
• Sender: It is simply a device that sends data messages. It can be a computer, mobile,
telephone, laptop, video camera, or workstation, etc.
• Receiver: It is a device that receives messages. It can be a computer, telephone,
mobile, workstation, etc.
• Transmission Medium / Communication Channels: Communication channels are
the medium that connect two or more workstations. Workstations can be connected
by either wired media or wireless media.
• Set of rules (Protocol): When someone sends the data (The sender), it should be
understandable to the receiver also otherwise it is meaningless.

GOALS OF THE NETWORK
• Resource Sharing - Many organization has a substantial number of
computers in operations, which are located apart. Ex. A group of
office workers can share a common printer, fax, modem, scanner,
etc.
• High Reliability - If there are alternate sources of supply, all files
could be replicated on two or more machines. If one of them is not
available, due to hardware failure, the other copies could be used.
• Inter-process Communication - Network users, located
geographically apart, may converse in an interactive session
through the network. In order to permit this, the network must
provide almost error-free communications.
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GOALS OF THE NETWORK
• 4. Flexible access - Files can be accessed from any computer in the
network. The project can be begun on one computer and finished on
another.
• 5. Security- Computer networks must be secure to protect against
unauthorized access, data breaches, and other security threats. This
includes implementing measures such as firewalls, antivirus software, and
encryption to ensure the confidentiality, integrity, and availability of data.
• 6. Performance- Computer networks must provide high performance and
low latency to ensure that applications and services are responsive and
available when needed. This requires optimizing network infrastructure,
bandwidth utilization, and traffic management.
• 7. Scalability- Computer networks must be designed to scale up or down
as needed to accommodate changes in the number of users, devices, and
data traffic. This requires careful planning and management to ensure the
network can meet current and future needs.
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CLASSIFICATION OF
COMPUTER NETWORKS
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BASED ON TRANSMISSION MODE 9
Simplex
Communication is one-way only.
Direction: Unidirectional (sender → receiver)
Example:TV broadcasting,Keyboard to CPU
Half Duplex
Definition: Communication is two-way, but only one direction at a time.
Direction: Bidirectional (but not simultaneously)
Example:Walkie-talkie
Full Duplex
Definition: Communication is two-way and simultaneous.
Direction: Bidirectional (at the same time)

BASED ON TIME IN
TRANSMISSION TYPE
Synchronous Transmission: In synchronous Transmission both the sender and the receiver use the
same time cycle forthe transmission. We send bits one after another without start/stop bits or gaps.
It is the responsibility of the receiver to group the bits. Bit stream is delivered with a fixed delay and
given error rate. Each bit reaches the destination with the same time delay after leaving the source.
Asynchronous Transmission: In Asynchronous Transmission we send one start bit at the beginning
and one stop bit at the end of each byte. There may be a gap between each byte. Bit stream is
divided into packets. Packets are received with varying delays, so packets can arrive out of order.
Some packets are not received correctly.
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BASED ON AUTHENTICATION
Peer to Peer Connection: In peer-to-peer networks, there are no dedicated
servers. All the computers are equal and, therefore, are termed as peers. Normally,
each computer functions as both a client and a server. No one can control the
other computers.
Server Based Connection: Most networks have a dedicated server. A dedicated
server is a computer on a network which functions as a server, and cannot be used
as a client or a workstation. A dedicated server is optimized to service requests
from network clients. A server can control the clients for its services.
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BASED ON RELIABILITY
Connection-oriented This type of communication establishes a
session connection before data can be sent. This method is often
called a "reliable" network service. It can guarantee that data will
arrive in the same order.
Connection less This type of communication does not require a
session connection between sender and receiver for data transfer.
The sender simply starts sending packets to the destination. A
connectionless network provides minimal services.
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BASED ON GEOGRAPHICAL
LOCATION
• LAN
• MAN
• WAN
• PAN
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LAN (LOCAL AREA NETWORK)
Range: Covers a small area like a room, office, building, or
campus.
Purpose: Connect multiple devices within a localized area.
Examples: Computers in a school lab or home Wi-Fi.
Devices Used: Switches, routers, Ethernet cables,Wi-Fi access
point..
Speed: High-speed (100 Mbps – 10 Gbps)
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Advantages:
● High speed (typically 100 Mbps to 10 Gbps).
● Low cost (installation and maintenance).
● Easy to manage due to limited size.
● Secure because it's restricted to a small area.
● Resource sharing (e.g., printers, files).
Disadvantages:
● Limited coverage (within a building or campus).
● Maintenance may require technical expertise.
● Hardware failure (like switch or router) can affect all users.

MAN (METROPOLITAN AREA
NETWORK)
• Range: Covers a city or metropolitan area.
• Purpose: Connect multiple LANs in a city.
• Examples: City-wide Wi-Fi, cable TV networks.
• Devices Used: Routers, fiber optics, microwave links.
• Speed: Moderate to high.
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Advantages:
● Covers more area than LAN (e.g., across city offices).
● Higher bandwidth compared to WAN.
● Good for connecting multiple LANs.
Disadvantages:
● More complex to manage than LAN.
● Higher installation and maintenance cost.
● Vulnerable to external attacks due to larger coverage.

WAN (WIDE AREA NETWORK)
• Range: Spans over large geographic areas (countries, continents).
• Purpose: Connects LANs and MANs together over long distances.
• Examples: The Internet, international banking networks.
• Devices Used: Routers, satellites, leased lines, fiber optics.
• Speed: Varies based on technology and infrastructure.
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Advantages:
● Enables global communication.
● Scalable – supports thousands of devices.
● Ideal for large organizations with branches worldwide.
Disadvantages:
● Expensive to set up and maintain.
● Lower speed compared to LAN/MAN.
● High latency and more security challenges.
● Dependent on third-party providers (e.g., ISPs).

PAN (PERSONAL AREA NETWORK)
• Range: Within a few meters (around 10 meters)
• Purpose: Connect devices around a single person.
• Examples: Bluetooth headset, smartwatch connected to a phone, file
sharing between phone and laptop via Bluetooth.
• Devices Used: Bluetooth, USB, Infrared.
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Advantages:
● Simple setup (Bluetooth, USB, Wi-Fi).
● Low cost – often no additional hardware required.
● Convenient for connecting personal devices (e.g., phone, laptop, smartwatch).
Disadvantages:
● Very limited range.
● Low speed and bandwidth.
● Security concerns (especially with Bluetooth).
● Can be affected by interference from other devices.

Topology in Computer Networks
Topology refers to the layout or arrangement of devices (nodes) and how they are connected in a
computer network. It defines the structure of the network — both physical (cables/devices) and
logical (data flow).
Types of Network Topologies:
1. Bus Topology
2. Star Topology
3. Ring Topology
4. Mesh Topology
5. Tree Topology
6. Hybrid Topology
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Bus topology is a network setup in which all devices (nodes) are connected to a single central
cable, called the bus or backbone. Data sent by a device travels along the bus and is received by all
devices, but only the intended recipient processes it.
Advantages of Bus Topology:
1. Easy to install and set up
2. Cost-effective
3. Good for small networks
4. Requires less cable length
Disadvantages of Bus Topology:
5. Single point of failure : If the main cable (bus) fails, the entire network goes down.
6. Difficult to troubleshoot : Identifying the point of failure is not easy.
7. Limited scalability :Not suitable for large networks due to performance drops.
8. Data collisions :Only one device can transmit at a time; collisions occur with multiple devices.
9. Low performance with high traffic : The more devices you add, the slower the network becomes.
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In Star Topology, all devices (nodes) are connected to a central device (usually a
hub or switch). Each node has a dedicated point-to-point link to the central hub.
The central device acts as a controller or communication hub — data sent from
one node goes to the hub and then forwarded to the destination node.
Advantages:
1. Easy to Install and Configure – Each device connects directly to the central hub, making setup simple.
2. Easy to Troubleshoot – If one device or cable fails, the rest of the network remains unaffected.
3. Scalable– New devices can be added without disrupting the network.
4. High Performance – No data collisions because each device has a dedicated link to the hub or switch.
5. Centralized Control – Easy to manage and monitor the network from one point.
Disadvantages:
6. Central Point of Failure – If the hub or switch fails, the entire network goes down.
7. Higher Cost – More cabling and network devices (like switches/hubs) increase the cost.
8. Requires More Cable– Each device needs a separate cable to connect to the hub.
9. Hub/Switch Capacity Limits– The performance depends on the capacity and speed of the central device.
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In Ring Topology, each device (node) is connected to exactly two other devices, forming a
closed loop or ring.
Data travels in one direction (unidirectional) or sometimes both directions (bidirectional),
passing through each node until it reaches the destination.
Advantages:
1. Equal Access to Network– All devices have equal rights to transmit data.
2. No Data Collisions– Data flows in a single direction (or token-based), so collisions are avoided.
3. Efficient for Heavy Traffic– Performs better than bus topology when the network has high traffic.
4. Easier Fault Isolation – In token ring setups, it’s easier to detect where the data is lost.
5. No Central Device Needed – Reduces dependency on a hub or switch (as in star topology).
Disadvantages:
6. Single Point of Failure– If one node or connection fails, the entire network can go down.
7. Difficult to Add or Remove Devices– The network must be temporarily stopped to make changes.
8. Slower Data Transmission– Data has to pass through multiple devices to reach its destination.
9. More Cabling Than Bus– Requires more cable and is slightly more expensive than bus topology.
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Mesh Topology, every device is connected to every other device in the network either directly (fully
connected) or indirectly (partially connected). It provides multiple paths for data to travel between
nodes.
Advantages:
1. Highly Reliable – If one link fails, data can be rerouted through other paths.
2. Robust and Fault-Tolerant– Network continues to function even if multiple connections fail.
3. No Traffic Congestion– Dedicated links between devices prevent data collisions.
4. Easy Fault Detection – Each link is independent, so it's easy to isolate and fix problems.
5. Improved Security– Data can travel through secure, private paths, reducing interception risk.
Disadvantages:
6. High Cost– Requires a large amount of cabling and network hardware.
7. Complex Installation– Setting up and managing all connections is time-consuming and difficult.
8. Difficult to Scale– Adding new devices increases the number of required connections significantly.
9. Maintenance is Complicated – More connections mean more effort to maintain and troubleshoot.

Tree topology (also known as hierarchical topology) is a combination of Star and Bus topologies. Devices are connected in
a hierarchical structure where multiple star networks are connected to a central backbone cable.
It resembles a tree:
● Root (central node) at the top
● Branches (intermediate switches/hubs)
● Leaves (end devices)
Advantages:
1. Scalable – Easy to expand by adding new branches or nodes.
2. Hierarchical Structure – Simplifies network management and troubleshooting.
3. Fault Isolation – A fault in one branch doesn’t affect the entire network.
4. Supports Large Networks – Well-suited for organizations with many departments or layers.
5. Combines Features of Star and Bus– Benefits from centralized management (star) and backbone structure (bus).
Disadvantages:
6. Backbone Dependency– If the central backbone fails, the whole network can go down.
7. High Cabling Cost – Requires more cables and network devices than simpler topologies.
8. Complex Setup and Maintenance – More difficult to configure and manage, especially as it grows.
9. Performance May Degrade with Size – Too many nodes can slow down the backbone if not properly managed.
10. Difficult to Reconfigure– Making changes in the upper hierarchy can disrupt large parts of the network.
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Hybrid Topology is a combination of two or more different network topologies (like star,
bus, ring, mesh, etc.) to form a larger and more complex network structure. It is designed
to leverage the strengths and minimize the weaknesses of the individual topologies it
combines.
Advantages:
1. Highly Flexible – Can be designed to fit specific network requirements by combining topologies.
2. Scalable– Easy to expand the network without disrupting existing components.
3. Reliable– Failure in one section does not affect the whole network.
4. Efficient Performance– Allows the use of the most suitable topology for each part of the network.
5. Easy Fault Isolation– Issues can be isolated to specific segments, making troubleshooting easier.
Disadvantages:
6. High Cost – Complex structure and use of various devices increase cost.
7. Complex Design– Planning, designing, and implementing hybrid networks require expertise.
8. Challenging Maintenance – Managing and maintaining a mixed environment is more difficult.
9. More Hardware Needed – Requires multiple types of devices (hubs, switches, routers, etc.).

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Topology Example
Bus
Topology
Early Ethernet networks in small offices or labs using a single coaxial cable as a
backbone.
Star
Topology
Modern home Wi-Fi networks where devices connect to a central router or switch.
Ring
Topology
Token Ring networks used in some older IBM LANs and some fiber optic MAN
(Metropolitan Area Networks).
Mesh
Topology
Military communication systems, wireless mesh networks in smart cities, or data
centers.
Tree
Topology
Large corporate networks where departments have their own star networks
connected via a backbone cable.
Hybrid
Topology
University campuses combining star networks in dorms and bus or ring topology
for connecting buildings.

SWITCHING
Switching is the process of transferring data packets from one device to
another in a network, or from one network to another, using specific
devices called switches.
A computer user experiences switching all the time for example,
accessing the Internet from your computer device, whenever a user
requests a webpage to open, the request is processed through switching
of data packets only.
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Introduction to Switch
A switch is a hardware device in a network that connects and helps multiple
devices share a network without their data interfering with each other.
Some data packets come from devices directly connected to the switch, like
computers or VoIP phones. Other packets come from devices connected through
hubs or routers.
The switch knows which devices are connected to it and can send data directly
between them. If the data needs to go to another network, the switch sends it to a
router, which forwards it to the correct destination.
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CIRCUIT SWITCHING
• Circuit Switching is a type of switching, in which a connection is established between the
source and destination before communication. This connection receives the complete
bandwidth of the network until the data is transferred completely.
• The dedicated path/circuit established between the sender and receiver provides a guaranteed data
rate. Data can be transmitted without any delays once the circuit is established.
• The telephone system network is one of the examples of Circuit switching.
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Multiplexing and Demultiplexing 44
Multiplexing and Demultiplexing are two fundamental concepts in data communication, especially in the transport and
network layers. They allow multiple signals or data streams to share a single communication channel and then separate them
correctly at the receiving end.
Multiplexing is the process of combining multiple data streams or signals into one single signal over a shared medium or
channel.
Demultiplexing is the reverse process of multiplexing. It involves separating the combined signal into its original
individual signals at the receiver's end. Telecommunication systems
Internet data transfer
Satellite and radio communication
Video streaming services

FDM and TDM
• FDM – Frequency Division Multiplexing
• The available bandwidth is divided into multiple frequency bands, each used for a
separate communication channel. Each circuit gets its own frequency range,
allowing simultaneous transmission.
• In Circuit Switching:
• Used in analog telephone systems (e.g., early landline networks).
• Each call is assigned a separate frequency band within a larger cable or line.
• Signals are transmitted simultaneously but separated by frequency.
• 🎯 Example: Think of a radio – multiple stations broadcast at the same time, but on
different frequencies. Similarly, many voice calls can travel on the same cable
using FDM.
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• TDM – Time Division Multiplexing
• How it works: The available time on a channel is divided into time
slots.
• Each circuit gets a fixed time slot in a repeating schedule.
• Data is sent in bursts, but the receiver reconstructs it in the right
order.
• 🔁 In Circuit Switching:
• Used in digital networks (e.g., ISDN, early digital telephone
systems).
• Each call is assigned a time slot, and multiple calls share the same
physical line by taking turns.
• 🎯 Example: Think of a classroom where each student gets 5 minutes
to speak. They take turns, but everyone gets a chance.
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Process of Circuit Connection
Circuit Establishment: A dedicated circuit between the source and destination is
constructed via a number of intermediary switching center's. Communication
signals can be requested and received when the sender and receiver communicate
signals over the circuit.
Data Transfer: Data can be transferred between the source and destination once the
circuit has been established. The link between the two parties remains as long as
they communicate.
Circuit Disconnection: Disconnection in the circuit occurs when one of the users
initiates the disconnect. When the disconnection occurs, all intermediary linkages
between the sender and receiver are terminated.
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Time Division
Multiplexing: 48
Feature FDM TDM
Basis Frequency Time
Signal Type Analog Digital
Channel Allocation Simultaneous, in frequency Time slots
Equipment Modulator, filters Synchronization clocks
Frequency Division Multiplexing:
Application for FDM
AM/FM Radio
Satellite
Communication
Telephone systems
Application for TDM
ISDN (Integrated Services Digital
Network)
Railway Announcement Systems
Data Transmission in
Microcontrollers

Advantages of Circuit Switching:
1. Dedicated Connection
Once the circuit is established, the path is exclusive between the sender and receiver.Ensures a constant and reliable
connection.
2. Low Latency During Communication
After setup, data is transferred with minimal delay, since the path is reserved.
3. Predictable Performance
No interference or delays due to other network traffic.Useful for real-time applications (e.g., voice calls).
4. Data Arrives in Order
Since data follows the same path, no reordering is needed at the destination.
Disadvantages of Circuit Switching:
1. Inefficient Use of Resources
The dedicated path remains reserved even if no data is being sent.Leads to wastage of bandwidth.
2. Connection Setup Delay
Time is needed to establish the circuit before communication begins.Not suitable for short or bursty data.
3. Scalability Issues
Hard to manage many users due to the need for separate circuits.Not suitable for data-heavy networks like the internet.
4. Failure Sensitivity
If any part of the circuit fails, the entire communication path is lost.
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PACKET SWITCHING
Packet Switching in computer networks is a method of transferring data to a network in the form of packets.
Data is divided into packets, and each packet may take a different path to reach the destination.
Connectionless (packets routed individually)
In order to transfer the file fast and efficiently over the network and minimize the transmission latency,
the data is broken into small pieces of variable length, called packet.
At the destination, all these small parts (packets) have to be reassembled, belonging to the
same file.
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Packet Switching
Packet Switching uses the Store and Forward technique while switching the packets; while
forwarding the packet each hop first stores that packet then forwards.
• This technique is very beneficial because packets may get discarded at any hop for some reason.
More than one path is possible between a pair of sources and destinations.
• Each packet contains the Source and destination address using which they independently travel
through the network. In other words, packets belonging to the same file may or may not travel
through the same path. If there is congestion at some path, packets are allowed to choose
different paths possible over an existing network.
In packet switching the data is divided into small packets which allow faster movement of data.
Each packet contains two parts that is Header and Payload, the header on each packet contains
information. Below is the diagram of packet switching working.
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Types of Delays in Packet Switching
• Transmission Delay: Time required by the spent station to transmit data to the link.
• Propagation Delay: It is the time of data propagation through the link. It depends on
the distance and the propagation speed of the medium (e.g., fiber optic, copper).
• Queueing Delay: It is the time spent by the packet at the destination's queue. It varies
depending on network congestion and traffic load.
• Processing Delay: Processing time for data at the destination. It includes error
checking and protocol handling.
• End-to-End Delay : Total time it takes for a packet to travel from the source to the
destination. It is the sum of all the above delays: End-to-End Delay= Processing +
Queuing + Transmission + Propagation
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DATAGRAM NETWORK
• It is a connection-less service. There is no need for reservation of resources as there is no dedicated path
for a connection session.
• All packets are free to use any available path. As a result, intermediate routers calculate routes on the go
due to dynamically changing routing tables on routers.
• Since every packet is free to choose any path, all packets must be associated with a header with proper
information about the source and the upper layer data.
• The connection-less property makes data packets reach the destination in any order, which means that they
can potentially be received out of order at the receiver's end.
• Datagram networks are not as reliable as Virtual Circuits.
• The major drawback of Datagram Packet switching is that a packet can only be forwarded if resources
such as the buffer, CPU, and bandwidth are available. Otherwise, the packet will be discarded.
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VIRTUAL NETWORK
• It is connection-oriented, meaning that there is a reservation of resources like buffers, CPU, bandwidth, etc.
for the time in which the newly set VC is going to be used by a data transfer session.
• The first sent packet reserves resources at each server along the path. Subsequent packets will follow the
same path as the first sent packet for the connection time.
• Since all the packets are going to follow the same path, a global header is required. Only the first packet of
the connection requires a global header, the remaining packets generally don’t require global headers.
• Since all packets follow a specific path, packets are received in order at the destination.
• Virtual Circuit Switching ensures that all packets successfully reach the Destination. No packet will be
discarded due to the unavailability of resources.
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Message Switching
• Message switching is a switching mechanism in which a message is sent as a single unit and
routed to intermediary nodes where it is stored and forwarded.
• The message-switching approach does not provide a dedicated path between the sender and
receiver. In message switching, end-users communicate by sending and receiving messages that
include the entire data to be shared.
• Messages are the smallest individual unit. Also, the sender and receiver are not directly
connected. Several intermediate nodes transfer data and ensure that the message reaches its
destination. Message-switched data networks are hence called hop-by-hop systems.
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MESSAGE SWITCHING
🔹 How it Works:
1. The entire message is created at the source.
2. It is sent to the first intermediate node (like a router or switch).
3. The intermediate node stores the entire message temporarily.
4. It then forwards the message to the next node when the path is available.
5. This continues until the message reaches the destination.
This method is known as Store-and-Forward.
Example: Imagine sending a letter by post Email , Fax
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APPLICATIONS OF SWITCHING
Circuit Switched Network
Landline telephone systems
Old PSTN (Public Switched Telephone Network)
Traditional mobile voice calls (2G/3G)
ISDN (Integrated Services Digital Network)
2. Packet Switched Network
a. Datagram Network
Web browsing (HTTP/HTTPS)
Streaming services like YouTube or Netflix
VoIP using UDP (e.g., Skype, WhatsApp calls)
b. Virtual Circuit Network
ATM (Asynchronous Transfer Mode)
3. Message Switched Network
Email services (e.g., Gmail, Outlook)
SMS (when recipient is offline)
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SERVICE LAYERED ARCHITECTURE
Service Layered Architecture means a network is built in
layers, where each layer has a specific job and offers
services to the layer above it.
This helps organize the design and simplifies
communication between different parts of a network or
computer system.
The two main models: OSI Model – 7 layers
TCP/IP Model – 4 layers
Each layer performs specific functions, such as:
Sending data, Routing data, Converting data formats,
Handling applications
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OSI Model
• The OSI (Open Systems Interconnection) Model is a set of rules that explains how different
computer systems communicate over a network.
• OSI Model was developed by the International Organization for Standardization (ISO).
• The OSI Model consists of 7 layers and each layer has specific functions and responsibilities.
• This layered approach makes it easier for different devices and technologies to work together.
• OSI Model provides a clear structure for data transmission and managing network issues.
• The OSI Model is widely used as a reference to understand how network systems function.
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Layers of the OSI Model
• There are 7 layers in the OSI Model and each layer has its specific role in
handling data. All the layers are mentioned below:
• Physical Layer
• Data Link Layer
• Network Layer
• Transport Layer
• Session Layer
• Presentation Layer
• Application Layer
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Layer 1: Physical Layer
• The lowest layer of the OSI reference model is the Physical Layer. It is responsible for the
actual physical connection between the devices.
• The physical layer contains information in the form of bits.
• Physical Layer is responsible for transmitting individual bits from one node to the next.
• When receiving data, this layer will get the signal received and convert it into 0s and 1s and
send them to the Data Link layer, which will put the frame back together.
• Common physical layer devices are Hub, Repeater, Modem, and Cables.
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• Functions of the Physical Layer
• Bit Synchronization: The physical layer provides the synchronization of the bits by providing a
clock. This clock controls both sender and receiver thus providing synchronization at the bit
level.
• Bit Rate Control: The Physical layer also defines the transmission rate i.e. the number of bits
sent per second.
• Physical Topologies: Physical layer specifies how the different, devices/nodes are arranged in a
network i.e. bus topology, star topology, or mesh topology.
• Transmission Mode: Physical layer also defines how the data flows between the two connected
devices. The various transmission modes possible are Simplex, half-duplex and full duplex.
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Layer 2: Data Link Layer
• The data link layer is responsible for the node-to-node delivery of the message.
• The main function of this layer is to make sure data transfer is error-free from one node to
another, over the physical layer.
• When a packet arrives in a network, it is the responsibility of the DLL to transmit it to the Host
using its MAC address.
• Packet in the Data Link layer is referred to as Frame.
• Switches and Bridges are common Data Link Layer devices.
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Layer 3: Network Layer
• The network layer works for the transmission of data from one host to the other located in
different networks.
• It also takes care of packet routing i.e. selection of the shortest path to transmit the packet, from
the number of routes available.
• The sender and receiver's IP address are placed in the header by the network layer.
• Segment in the Network layer is referred to as Packet.
• Network layer is implemented by networking devices such as routers and switches.
• Functions of the Network Layer
• Routing: The network layer protocols determine which route is suitable from source to
destination. This function of the network layer is known as routing.
• Logical Addressing: To identify each device inter-network uniquely, the network layer defines
an addressing scheme. The sender and receiver’s IP addresses are placed in the header by the
network layer. Such an address distinguishes each device uniquely and universally.
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Layer 4: Transport Layer
• The transport layer provides services to the application layer and takes services from the
network layer.
• The data in the transport layer is referred to as Segments. It is responsible for the end-to-end
delivery of the complete message.
• The transport layer also provides the acknowledgment of the successful data transmission and
re-transmits the data if an error is found. Protocols used in Transport Layer are TCP, UDP.
• At the sender's side, the transport layer receives the formatted data from the upper layers,
performs Segmentation, and also implements Flow and error control to ensure proper data
transmission. It also adds Source and Destination port number in its header and forwards the
segmented data to the Network Layer.
• At the Receiver’s side, Transport Layer reads the port number from its header and forwards the
Data which it has received to the respective application. It also performs sequencing and
reassembling of the segmented data.
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• Session Layer in the OSI Model is responsible for the establishment of connections, management of
connections, terminations of sessions between two devices. It also provides authentication and security.
• Functions of the Session Layer
• Session Establishment, Maintenance, and Termination: The layer allows the two processes to
establish, use, and terminate a connection.
• Synchronization: This layer allows a process to add checkpoints that are considered synchronization
points in the data. These synchronization points help to identify the error so that the data is re-
synchronized properly, and ends of the messages are not cut prematurely, and data loss is avoided.
• Dialog Controller: The session layer allows two systems to start communication with each other in
half-duplex or full duplex.
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Layer 5: Session Layer

Layer 6: Presentation Layer
• The presentation layer is also called the Translation layer. The data from the application layer is
extracted here and manipulated as per the required format to transmit over the network. Protocols
used in the Presentation Layer are TLS/SSL (Transport Layer Security / Secure Sockets Layer).
• Functions of the Presentation Layer
• Translation: For example, ASCII to EBCDIC.
• Encryption/ Decryption: Data encryption translates the data into another form or code. The
encrypted data is known as the ciphertext, and the decrypted data is known as plain text. A key
value is used for encrypting as well as decrypting data.
• Compression: Reduces the number of bits that need to be transmitted on the network.
72

Layer 7: Application Layer
• At the very top of the OSI Reference Model stack of layers, we find the Application layer which is
implemented by the network applications. These applications produce the data to be transferred over the
network. This layer also serves as a window for the application services to access the network and for
displaying the received information to the user. Protocols used in the Application layer are SMTP, FTP,
DNS, etc.
73

74
Layer Name Function (What it does)
7 Application Interface between user and network; handles apps like browsers, email, etc.
6 Presentation Translates data between app and network (e.g., encryption, compression, file formats).
5 Session Manages sessions (start, maintain, end conversations between computers).
4 Transport Ensures reliable data transfer (e.g., error checking, TCP/UDP, flow control).
3 Network Handles routing and IP addressing (e.g., choosing the best path for data).
2 Data Link Manages error detection/correction and MAC addressing within the same network.
1 Physical Deals with the physical connection (cables, signals, voltage, etc.).

● TCP/IP (Transmission Control Protocol/Internet Protocol) is the foundation of the Internet.
● It’s a more practical, simplified model compared to the OSI model.
● Developed by the U.S. Department of Defense.
● Used for end-to-end communication over a network.
● Forms the basis of Internet communication.
● Every device using the internet uses TCP/IP protocols.
● Helps developers and network engineers understand, build, and troubleshoot communication
systems.
Sending Email (User → Server):
● Application Layer: SMTP formats the email.
● Transport Layer: TCP breaks it into segments.
● Internet Layer: IP adds IP addresses.
● Network Access Layer: Ethernet sends signals physically.
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APPLICATION LAYER
• The Application Layer is the top layer of the TCP/IP model and the one closest
to the user.
• This is where all the apps you use like web browsers, email clients, or file
sharing tools connect to the network. It acts like a bridge between your
software (like Chrome, Gmail, or WhatsApp) and the lower layers of the
network that actually send and receive data.
• It supports different protocols like HTTP (for websites), FTP (for file transfers),
SMTP (for emails), and DNS (for finding website addresses).
• It also manages things like data formatting, so both sender and receiver
understand the data, encryption to keep data safe, and session management to
keep track of ongoing connections.
78

TRANSPORT LAYER
• The Transport Layer is responsible for making sure that data is sent reliably and in
the correct order between devices.
• It checks that the data you send like a message, file, or video arrives safely and
completely.
• This layer uses two main protocols: TCP and UDP, depending on whether the
communication needs to be reliable or faster.
• TCP is used when data must be correct and complete, like when loading a web page
or downloading a file. It checks for errors, resends missing pieces, and keeps
everything in order.
• On the other hand, UDP (User Datagram Protocol) is faster but doesn’t guarantee
delivery useful for things like live video or online games where speed matters more
than perfect accuracy.
79

INTERNET LAYER
• The Internet Layer is used for finding the best path for data to travel across
different networks so it can reach the right destination.
• It works like a traffic controller, helping data packets move from one network to
another until they reach the correct device.
• This layer uses the Internet Protocol (IP) to give every device a unique IP
address, which helps identify where data should go.
• The main job of this layer is routing deciding the best way for data to travel. It
also takes care of packet forwarding (moving data from one point to
another), fragmentation (breaking large data into smaller parts), and addressing.
80

NETWORK ACCESS LAYER(LINK LAYER)
• The Network Access Layer is the bottom layer of the TCP/IP model. It deals
with the actual physical connection between devices on the same local network
like computers connected by cables or communicating through Wi-Fi.
• This layer makes sure that data can travel over the hardware, such as wires,
switches, or wireless signals.
• It also handles important tasks like using MAC addresses to identify devices,
creating frames (the format used to send data over the physical link), and
checking for basic errors during transmission.
81

WORKING OF TCP/IP MODEL
• When Sending Data (From Sender to Receiver)
• Application Layer: Prepares user data using protocols like HTTP, FTP, or SMTP.
• Transport Layer (TCP/UDP): Breaks data into segments and ensures reliable (TCP) or fast
(UDP) delivery.
• Internet Layer (IP): Adds IP addresses and decides the best route for each packet.
• Link Layer (Network Access Layer): Converts packets into frames and sends them over
the physical network.
• When Receiving Data (At the Destination)
• Link Layer: Receives bits from the network and rebuilds frames to pass to the next layer.
• Internet Layer: Checks the IP address, removes the IP header, and forwards data to the
Transport Layer.
• Transport Layer: Reassembles segments, checks for errors, and ensures data is complete.
• Application Layer: Delivers the final data to the correct application (e.g., displays a web
page in the browser).
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83
Feature OSI Model (7 Layers) TCP/IP Model (4 Layers)
Developed By ISO U.S. Department of
Defense
Approach Theoretical Practical, real-world
Layers 7 4
Usage Reference model Real-world communication
Application
Layer
3 layers (App, Presentation,
Session)
1 combined layer
Layer
No.
Name Corresponding OSI Layers Function
4 Application Layer OSI Layers 7, 6, and 5 Provides network services to applications
3 Transport Layer OSI Layer 4 End-to-end communication, error checking
2 Internet Layer OSI Layer 3 Routing, logical addressing (IP)
1 Network Access Layer OSI Layers 2 and 1 Physical transmission of data

Comparison: OSI and TCP /IP model.
84
Aspect OSI Model TCP/IP Model
Full Name Open Systems Interconnection Model Transmission Control Protocol /
Internet Protocol Model
Number of Layers 7 layers 4 layers
Layers 1. Physical2. Data Link3. Network4. Transport5.
Session6. Presentation7. Application
1. Network Interface (Link)2.
Internet3. Transport4. Application
Layer Functions Defines detailed functions for each layer with strict
boundaries.
Combines some OSI layers, focusing
more on protocols.
Development
Purpose
Conceptual framework to understand and design
networks.
Practical model designed for the
Internet protocols.
Protocol
Dependency
Protocol-independent (theoretical). Protocol-dependent (TCP, IP, UDP,
etc.).

85
Data Units Bits (Physical Layer), Frames (Data Link),
Packets (Network), Segments (Transport)
Packets or datagrams across
layers.
Session and
Presentation
Layers
Separate layers for session management and
data translation/encryption.
These functions handled within the
Application layer.
Popularity/Usage Primarily used as a teaching tool and conceptual
model.
Widely used for real-world Internet
communications.
Flexibility More rigid, layers have strict functions. More flexible and practical for
protocol implementations.

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