OSI Model

International Standards Organization (ISO) is a multinational body dedicated to worldwide

agreement on international standards. An ISO standard that covers all aspects of network
communications is the Open Systems Interconnection model. It was first introduced in the
late 1970s. An open system is a set of protocols that allows any two different systems to
communicate regardless of their underlying architecture. The purpose of the OSI model is to
show how to facilitate communication between different systems without requiring changes
to the logic of the underlying hard ware and software. The OSI model is not a protocol; it is a
model for understanding and designing a network architecture that is flexible, robust, and
interoperable
The OSI model is a layered framework for the design of network systems that allows
communication between all types of computer systems. It consists of seven separate but
related layers, each of which defines a part of the process of moving information across a
network
An open system is a set of protocols that allows any two different systems to communicate
regardless of their underlying architecture. The purpose of the OSI model is to open
communication between different systems without requiring, changes to the logic of the
underlying hardware and software. The OSI is not a protocol, it is a model for understanding
and designing a network architecture.
Layered Architecture Data Link Physical. The OSI model is composed of seven ordered layers:
physical (layer 1), datalink(layer2), network (layer 3), transport (layer 4), session (layer 5),
presentation (layer 6), and application (layer 7). Figure 2.3 shows the layers involved when a
message is sent from device A to device B. As the message travels from A to B, it may pass
through many intermediate nodes. These intermediate nodes usually involve only the first
three layers of the OSI model.
Interfaces between Layers Within a single machine, the passing of the data and network
information down through the layers of the sending machine and back up through the layers
of the receiving machine is made possible by an interface between each pair of adjacent
layers. Each interface defines what information and services a layer must provide for the layer
above it. Layer 3, for example, uses the services provided by layer 2
OSI Peer-to-Peer Processes
The processes on each machine that communicate at a given layer called peer to-peer
processes. Each layer in the sending machine adds its own information to the message it
receives from the layer just above it, and passes the whole package to the layer just below it.
This information added in the form of headers or trailers.
Headers are information structures which identifies the information that follows, such as a
block of bytes in communication.
Trailer is the information which occupies several bytes at the end of the block of the data
being transmitted. They contain error-checking data which is useful for confirming the
accuracy and status of the transmission.

During communication of data the sender appends the header and passes it to the lower
layer while the receiver removes header and passes it to upper layer. Headers are added at
layer 6,5,4,3 & 2 while Trailer is added at layer 2.
In the OSI model, headers and trailers are metadata added to data at different layers for
control, addressing, and error detection, a process called encapsulation. At the Data Link
layer (Layer 2), an Ethernet header contains source/destination MAC addresses and a trailer
contains an error-checking code like Frame Check Sequence (FCS). At the Network layer
(Layer 3), an header adds IP addresses for routing, and at the Transport layer (Layer 4), an
header adds port numbers for application-to-application communication.

Example of Headers and Trailers in the OSI Model

Imagine sending an email from your computer (Layer 7) to a web server:
1. Application Layer (Layer 7):
Your email client uses an application-layer protocol (e.g., SMTP for email) to prepare the
data, which is then passed to the next layer.
2. Transport Layer (Layer 4):
The data is received, and a Transport Layer header (e.g., TCP or UDP) is added to it. This
header includes port numbers to identify the sending and receiving applications and helps
ensure the data arrives reliably.
3. Network Layer (Layer 3):
The data is then passed to the Network Layer, which adds a Network Layer header
containing the source and destination IP addresses. This allows the data to be routed across
different networks.
4. Data Link Layer (Layer 2):
At this layer, the data unit is now called a frame. A Data Link layer header and trailer are
added:
• Header: Contains the physical (MAC) addresses for the next hop on the local
network segment, including the source and destination MAC addresses.
• Trailer: Includes an FCS, a code used for error detection.
5. Physical Layer (Layer 1):
The frame is then converted into electrical signals or light pulses for transmission across the
physical medium.
On the Receiving End
At the receiving computer, the process reverses:
1. The physical layer receives the signals.
2. The Data Link layer processes the trailer to check for errors, and then strips off the
header and trailer to pass the frame up.
3. The Network layer examines its header to determine if the packet is for this host,
strips off its header, and passes the data up.
4. The Transport layer uses its header to identify the correct application and passes the
data up.
5. Finally, the Application layer receives the data in its original format.
Protocols
Protocols are the set of rules that govern data communication. Protocols can be defined as a
communication standard followed by both the parties (Sender and Receiver) in a computer
network to communicate with each other.
1. Syntax: Syntax basically represents the format of the data means in which order data is
presented. It also indicates how to read the data. It simply means the way to represent data.
For Example, let us suppose a data packet has 16 bits, in which the first 4 bits are the sender's
address, the last 4 bits are the receiver's address and the rest is the message. So, this is a
syntax to represent data bits.
2. Semantics: Semantics basically refers to the meaning of each section mentioned in syntax.
It includes control information for coordination and error handling. It also specifies which file
defines which action.
3. Timing: Timing simply means when the data is to be sent and how fast the data can be sent.
For Example, if the Sender sends the data at 100 MBPS and the receiver receives it at 1 MBPS,
then the data gets overflowed at the receiver end.
Encapsulation and Protocol data unit (PDU)
As application data is passed down the protocol stack on its way to be transmitted across the
network media, various protocols add information to it at each level. This is commonly known
as the encapsulation process. The form that a piece of data takes at any layer is called a
protocol data unit (PDU). During encapsulation, each succeeding layer encapsulates the PDU
that it receives from the layer above in accordance with the protocol being used. At each
stage of the process, a PDU has a different name to reflect its new functions.
• Data - The general term for the PDU used at the application layer
• Segment - Transport layer PDU
• Packet - Network layer PDU
• Frame - Data Link layer PDU
• Bits - A PDU used when physically transmitting data over the medium
Functions of the Layers
1. Physical Layer
• Line configuration: How can two or more devices are linked physically? Are transmission
lines to be shared or limited to use between two devices? Is the line available or not?
• Data transmission mode: It does transmission flow one-way or both ways between two
connected devices. simplex, half-duplex, or full-duplex.
• Topology: How network devices arranged? Do they pass data directly to each other or
through an intermediary? And by what paths?
• Signals: What type of signals is useful for transmitting information?
• Encoding: How are bits (0s and 1s) to be represented by available signalling systems? How
data are represented by signals?
• Interface: What information must be shared between two closely linked devices to enable
and facilitate communication? What is the most efficient way to communicate that
information?
• Medium: What is the physical environment for the transmission of data?
• Multiplexing: Using a single physical line to carry data between many devices at the same
time.
2.Data link Layer
• Node-to-node delivery: The data link layer is responsible for node-to-node delivery.
• Physical Addressing: Headers and trailers added at this layer include the physical address
of the most recent node and the next intended node.
• Access control: When two or more devices connected to the same link, the data-link layer
protocols are necessary to determine which device has control over the line at any given time.
• Flow control: To avoid overwhelming the receiver, the data link layer regulates the amount
of data that can be transmitted at one time. It adds identifying numbers to enable the
receiving node to control the ordering of the frames.
• Error handling: Data link layer protocols provide for data recovery, usually by having the
entire frame retransmitted.
3.Network Layer
• Source-to-destination delivery: Moving a packet (best effort) from its point of origin to its
intended destination across multiple network links.
• Logical addressing: Inclusion of the source and destination addresses in the header.
• Routing: Deciding which of multiple paths a packet should take.
• Address translation: Interpreting logical addresses to find their physical equivalents.
4.Transport Layer
• End-to-end message delivery: Overseeing the transmission and arrival of all packets of a
message at the destination point.
• Service-point (port) addressing: Guaranteeing delivery of a message to the appropriate
application on a computer running multiple applications.
• Segmentation and reassembly: Dividing a message into transmittable segments and
marking each segment with a sequence number. These numbers enable the transport layer
to reassemble the message correctly at the destination and to identify and replace packets
lost in transmission.
• Connection control: Deciding whether or not to send all packets by a single path.

• Flow control: Like the data link layer, the transport layer is responsible for flow control.
However, flow control at this layer is performed end to end rather than across a single
link.
• Error control: Like the data link layer, the transport layer is responsible for error control.
However, error control at this layer is performed process-to process rather than across a
single link. The sending transport layer makes sure that the entire message arrives at the
receiving transport layer without error (damage, loss, or duplication). Error correction is
usually achieved through retransmission

5.Session Layer
• Session management: Dividing a session into sub sessions by the introduction of
checkpoints and separating long messages into shorter units called dialog units appropriate
for transmission.
• Synchronization: Deciding in what order to pass the dialog units to the transport layer and
where in the transmission to require confirmation from the receiver. • Dialog control:
Deciding who sends, and when.
• Graceful close: Ensuring that the exchange has been completed appropriately before the
session closes.
• Dialog control. The session layer allows two systems to enter into a dialog. It allows
the communication between two processes to take place in either half duplex (one
way at a time) or full-duplex (two ways at a time) mode.

6.Presentation layer:
• Translation: Changing the format of a message from that used by the sender into one
mutually acceptable for transmission. Then, at the destination, changing that format into the
one understood by the receiver.
• Encryption: Encryption and decryption of data for security purposes.
• Compression: Compressing and decompressing data to make transmission more efficient.
• Security: Validating passwords and log-in codes.
7. Application Layer
• Web Services: Allows accessing web content in the form of text, images and multimedia
using the HTTP protocol
• File access, transfer, and management: Allows a user at a remote computer to access files
in another host (to make changes or read data); to retrieve files from remote computer for
use in the local computer; and to manage or control files in remote computer at that
computer.
• Mail services: Provides the basis for electronic mail forwarding and storage.
• Directory services: Provides distributed database sources and access for global information
about various objects.

TCP/IP Protocol Suite

The TCP/IP protocol stack is the de-facto standard in networking.

• It is an alternative for the OSI 7-layer model, which has never really been implemented in
practice. • TCP/IP is an open standard and is the protocol used over the Internet.
• It can be found in most modern-day operating systems The main differences between
TCP/IP and the OSI 7-layer model are: • Number of layers - TCP/IP defines only 4 or 5 layers.
• TCP/IP protocol model while OSI reference model
• In TCP/IP application layer equal to application, presentation and session layers in OSI
• Functions performed at a given layer - In the OSI model each layer performs specific
functions - In TCP/IP different protocols may be defined within a layer, each performing
different functions. What is common about a set of protocols at the same layer is that they
share the same set of support protocols at the next lower layer.
• Interface between adjacent layers - In the OSI model, a protocol at a given layer may be
substituted by a new one without impacting on adjacent layers. - In TCP/IP the strict use of all
layers is not mandated

Addressing
Four levels of addresses are used in an internet employing the TCP/IP protocols: physical
(link) addresses, logical (IP) addresses, port addresses.
Physical Addresses: The physical address, also known as the link address or mac address, it is
the address used in local network it has a 6-byte (48-bit) physical address that is imprinted on
the network interface card (NIC) example 07:01:02:01:2C:4B A 6- byte (12 hexadecimal digits)
physical address
Win R CMD getmac

Logical Addresses (IP): Logical addresses are necessary for network-to-network

communication A logical address in the Internet is currently a 32-bit/128 address that can
uniquely define a host connected to the Internet. No two publicly addressed and visible hosts
on the Internet can have the same IP address.
192.168.1.4
Win R CMD ipconfig
Port Addresses: It is implemented in transport layer as process to process delivering programs
address

Networking and Internetworks Devices

Types of Network Devices
• Repeater
It extends the length of the signal and allows it to transmit over the same network. In
other words, it simply copies the signal bit by bit and re-generates at its original
strength by operating at the physical layer.
• Network Hub
 A network hub is a multiport repeater that connects multiple wires from different
branches. It is used to transfer important data and communicate among diverse
network hosts.
Hub transfers the data as packets through a computer network. When the data
processing is done from one host to another network hub, it gets transmitted to all
the connected ports.
There are three different types of network hubs:
1. Active Hub- Active network hub is used to clean, increase & transfer the signal using
the network on its own power supply. It works as a wiring center and repeater. These
hubs play a major role in expanding the distance between nodes.
2. Passive Hub- Passive network hub is used to collect wiring from the different power
supplies and nodes of an active hub. These hubs transmit the signals over the network
without advancing or cleaning them.
3. Intelligent Hub- Intelligent network hub is like an active hub. It includes remote
management capabilities and offers flexible data rates to network devices. It also
enables admin access to monitor the traffic on the hub and configure every port in the
hub.
• Bridge
A bridge is a device that joins any two networks or host segments together. Its primary
function in a networking architecture is to store and relay frames among the various
connected segments.
They transfer frames using the MAC or the Media Access Control. It can also prevent
data crossing if the MAC addresses are wrong. Besides, it also links different physical
LANs together to form a bigger logical LAN.
There are two types of Bridges:
1. Transparent Bridges- These are the bridges in which the stations are completely
unaware of whether a bridge is present or absent from the network.
2. Source Routing Bridges- In the source bridges, routing operation is performed, and
the frame specifies the path that needs to be followed.
• Network Switch
Switches play a more important role than hubs. A switch is a multi-port device that
enhances network efficiency. It provides limited routing information about nodes in
the internal network and allows systems to connect.
Network switches can read the hardware address of incoming data packets and
transmit them to the applicable destination. A multilayer switch is a high-performance
device that supports routing protocols like routers.
• Modem
Modems are devices that transform digital signals into the form of analog signals that
are of various frequencies. Then it sends the analog signals to receivers.
Afterward, receiver modems reverse the process and send a digital signal to linked
devices like phones and laptops. Telephone companies and cable operators
sometimes use modems as the end terminals to identify residential and business
customers.
• Gateway
 As the name suggests, the gateway is a passage that interlinks two networks together.
It works as the messenger agent that takes data from one system, interprets it, and
transfers it to another system. Gateways are also called protocol converters, and they
can operate at various network layers.
• Access Point
First, as a regular wired network for wireless devices. Second, like a router for
transferring data between different access points.
The AP has various ports to expand the network’s size, firewall capabilities, and DHCP
service. As a result, we have access points that act as a switch, DHCP server, router,
and firewall.

INTERNETWORKING –
An internetworking is the process or technique of connecting different types of
networks or network segments for making large global network by using routing
technology. Internetwork is used in data communication between networks which is
owned and operated by different entities using a common data communication and
the Internet Routing Protocol.

wo or more networks or network segments connect using devices that operate at layer
3 (Network Layer) of the OSI Basic Reference Model such as a router. Any
interconnection among or between commercial, public, private, and industrial or
governmental networks may also be defined as an internetwork. It allows people and
their computers to communicate across different kinds of networks.

Interconnected networks use the Internet Protocol. There are three variants of
internetworks such as Intranet, Extranet and Internet.
1. Intranet – An intranet is a set of networks, using the Internet Protocol and IP-based
tools such as web browsers and file transfer applications which are under control of a
single administrative entity. The administrative entity closes or contact to all but
specific authorized users. In other words, an intranet is the internal network of an
organization. A large intranet network will have one web server to provide users with
organizational information.
2. Extranet – An extranet is a controlled private network which allows access to vendors,
partners, and suppliers or an authorized set of customers. It is a part of intranet in
which selected people outside the company can use.
3. Internet – Internet is the largest network in the world. It consists of a worldwide
interconnection of academic, governmental, private and public networks which is
based upon the networking technologies of the Internet Protocol Suite.

Types of Network Topology

Seven commonly used network topologies are:
1. Point-to-Point
2. Bus
3. Star
4. Ring
5. Mesh
6. Tree
7. Hybrid
Now, let's discuss these topologies one by one.
1. Point-to-Point Topology
Point-to-point topology is the simplest network configuration, connecting two nodes
directly through a dedicated communication link. This setup resembles a direct line
between two endpoints, allowing for efficient and fast data transfer.
Think of a telephone call between two people. In a point-to-point topology, like that
call, two connected devices communicate directly without interference, sharing the
entire bandwidth for high performance and low latency.
Advantages
● High bandwidth and fast communication speeds.
● Easy to maintain and troubleshoot since only two nodes are involved.
Disadvantages
● Limited to two devices; expanding the network requires additional links.
● If the connection fails, communication between the two nodes is disrupted.
Uses
● Used by businesses to connect two offices directly via a dedicated leased line.
● Used in many VPN connections, where a secure tunnel is established between a
user and a remote server.
● Bluetooth devices such as headphones and smartphones use point-to-point
connections
2. Bus Topology
Imagine a long cable, resembling a bus route, with devices connected along its
length. This is the essence of a bus topology. In a bus topology, all devices share the
same communication channel. Data travels along the cable, and each device checks
if the data is intended for it. If so, it accepts the data; otherwise, it ignores it.
Think of a school bus with seats for students. In a bus topology, devices like
computers and printers are arranged in a line along a single cable, which serves as
their communication pathway, similar to the bus route.

Advantages
● Simple to set up and cost-effective.
● Well-suited for small networks with few devices.
Disadvantages
● Limited scalability; adding more devices can degrade performance.
● A single cable break can disrupt the entire network.
Uses
● Used in simple setups where a few computers are connected to share files or
printers.
● Used in older Ethernet networks using coaxial cables.
● Used in manufacturing environments where sensors and controllers are connected
along a single communication line.
3. Star Topology
In a star topology, each device is connected directly to a central hub or switch. All
communication between devices must go through this central point. It's like a hub-
and-spoke model, with the hub being the focal point for data transmission.

Advantages
● Easy to install, manage, and troubleshoot.
● Isolates issues to individual connections; a failure in one device doesn't affect
others.
Disadvantages
● Dependence on the central hub; if it fails, the entire network goes down.
● More cabling is required, making it costlier than a bus topology.
Uses
● Common in corporate environments where each computer connects to a central
switch or hub.
● ATMs and branch systems connect to a central server for secure and reliable
transactions.
4. Ring Topology
In a ring topology, each device is connected to exactly two other devices, forming a
closed loop or ring. Data circulates the ring in one direction. When a device receives
data, it processes it and passes it along to the next device until it reaches its
destination.
Advantages
● Even data distribution, as each device has an equal opportunity to transmit.
● Simple and predictable data path.
Disadvantages
● A break in the ring can disrupt the entire network.
● Adding or removing devices can be complex.
Uses
● Used in city-wide networks to connect multiple buildings or institutions in a loop
for efficient data routing.
● IBM’s Token Ring LANs used ring topology to manage access and avoid collisions.
5. Mesh Topology
Mesh topology is like a web of connections, where each device is connected to every
other device. This creates redundancy and multiple paths for data to travel. Mesh
networks can be either full mesh (every device is connected to every other) or partial
mesh (some devices have fewer connections).
Advantages
● High redundancy; network remains operational even if some connections fail.
● Scalable and adaptable; can handle a large number of devices.
Disadvantages
● Expensive due to the numerous cables and ports required.
● Complex to set up and maintain.
Uses
● Used in battlefield networks for high reliability and redundancy.
● Used in large campuses, cities, or rural areas to provide widespread Wi-Fi
coverage.
6. Tree Topology
A tree topology combines characteristics of star and bus topologies, arranging nodes
in a hierarchical structure that resembles a tree. In this layout, multiple star
networks are connected to a central bus, allowing for a scalable and organized
network design.
Think of a family tree, where each branch represents different family members
connected to a common ancestor. Similarly, in a tree topology, the central node acts
as the trunk, with branches extending to various sub-nodes.
Advantages:
● Scalable and easy to expand by adding new nodes without disrupting the entire
network.
● Facilitates better management and organization of devices.
Disadvantages:
● If the central trunk fails, it can disrupt the entire network.
● More complex to configure and maintain compared to simpler topologies.
Uses
● Schools and universities use tree topology to connect labs, libraries, and
administrative offices in a structured hierarchy.
● ISPs use tree topology to manage regional and local networks branching from a
central hub.
7. Hybrid Topology

A hybrid topology combines two or more different topologies into a single network.
This is often done to harness the strengths of one topology while mitigating its
weaknesses. For example, a network might use a star topology for its core
infrastructure and a bus topology for a smaller, isolated segment.
Advantages
● Flexibility to tailor the network to specific needs.
● Enhanced fault tolerance by combining different topologies.
Disadvantages
● Complexity increases with the number of topologies integrated.
● requires careful planning to ensure smooth operation.
Uses
● Hybrid topology helps manage thousands of servers with a combination of mesh
(for redundancy) and star (for control) structures.
● Critical systems like patient records and diagnostics use star topology for control,
while monitoring devices may use mesh for continuous data flow.
What is the Best Type of Network Topology?

The best type of topology depends on the factors you care about the most. Here are
some factors, and the best type of network topology for them.
1. For low costs: Bus and Star
2. For high reliability: Mesh and Hybrid
3. For high scalability: Tree and Mesh
4. For high performance: Mesh and Star
Types of Network Architectures
Network architecture is the overall design and structure of a network, including its
hardware, software, protocols, and layers. It is different from the network topology,
as a topology only defines the layout and connections of devices, whereas network
architecture defines the broader design and structure of the network.
Some common network architectures include 3-tier (core, distribution, access), 2-tier
(collapsed core), spine-leaf, SOHO, and cloud architectures.
1. Two-tier Network Topology
A two-tier network topology is a flat or collapsed core design. It consists of two
layers i.e., the access layer and the core layer. In organizations where the network is
smaller, scalability and complexity are not much concern generally adopt this type of
architecture. Here is the topology of the two-tier network topology for your
reference.

Scenario: In a small office network, a two-tier topology may consist of access

switches connecting end-user devices (such as computers and printers) in the access
layer. These access switches are then connected to a core switch or router, which
provides connectivity to other networks or the internet.
2. Three-tier Network Topology
Three-tier network topology is a 3-layer architecture in which the network is divided
into.
✓ Access layer
✓ Distribution layer
✓ Core layer
It provides better scalability, flexibility, and network segmentation compared to a
two-tier design. Here is the three-tier network topology diagram for your reference.
Scenario: In an enterprise network, a three-tier topology may consist of access
switches in the access layer connecting end-user devices. These access switches are
then connected to distribution switches in the distribution layer, which provide
connectivity between access switches and aggregate traffic.
The distribution switches are further connected to core switches in the core layer,
which handle high-speed backbone connections and connect to other networks.
3. Spine-Leaf Network Topology
The Spine-leaf network topology, also referred to as leaf-spine or Clos architecture,
is a highly scalable and high-performance design frequently employed in large data
centers or cloud environments. It facilitates low-latency and non-blocking
communication among devices, ensuring efficient and rapid data transmission. Here
is the spine and leaf network topology for your reference.

Scenario: In a data center, a spine-leaf topology may consist of leaf switches in the
access layer connecting servers or storage devices. These leaf switches are then
connected to spine switches in the spine layer, which provide connectivity between
leaf switches and facilitate east-west traffic. This design ensures that any device in
the network can reach any other device with minimal latency.
4. WAN Network Topology
WAN (Wide Area Network) topology refers to the network architecture used to
interconnect geographically dispersed locations or branch offices. Here is the WAN
topology in which branch offices, regional offices, remote offices, and data centers
are connected. There can be thousands of branches which are connected to the
WAN infrastructure.
Scenario: In a multi-site organization, a WAN topology may involve multiple branch
offices connected to a central headquarters. Each branch office typically has its own
local area network (LAN) connected to a router, which establishes a connection to
the WAN.
The WAN network can be implemented using technologies such as leased lines,
MPLS (Multi-Protocol Label Switching), VPN (Virtual Private Network), or SD-WAN
(Software-Defined Wide Area Network).
5. Small Office/Home Office (SOHO) Network Topology
SOHO network topology is generally used in a small office or a home office. Here is
the typical SOHO network topology for your reference.

Scenario: In a home office setup, a SOHO topology may involve a single router or
gateway device that connects to the internet service provider (ISP). The end-user
devices, such as laptops, phones, etc. can be connected to the router via wireless or
wired connections, and servers can also be connected to the router through wired
connections.
6. On-premises and Cloud Network Topology
When network architectural designs use both On-premises and cloud networks for
deploying network resources. On-premises means deploying network devices
infrastructure locally and integrated with cloud services to fulfill all the needs of an
organization.
Scenario: In a hybrid cloud environment, on-premises network resources such as
servers, storage, and switches are interconnected with cloud resources through
dedicated connections or secure VPN tunnels. The on-premises network is just like
an extension of the cloud network which utilizes the data exchange between the two
environments.
Types of Cables
There are three types of cables in computer networks: coaxial, twisted pair, and
Fiber optic cables, each with unique characteristics and uses. Let's understand each
of them in detail.
1. Coaxial Cable
A coaxial cable is a networking cable used for transmitting data, video, and voice
signals. It transmits data in the form of electrical signals.
Coaxial cable is built with four main components:
1. Inner Conductor: A central wire, typically made of copper, which carries the
electrical signal.
2. Dielectric Insulator: Surrounds the inner conductor, ensuring signal integrity and
preventing interference.
3. Outer Conductor (Shield): Made of braided copper or metal foil, it protects the
signal from external electromagnetic interference (EMI).
4. Outer Jacket: Provides physical protection and insulation for the cable.

Coaxial cables are mostly used in applications that require high-frequency signal
transmission, like cable television, broadband internet, and radio.
The first coaxial cable was patented by Oliver Heavisine in 1880. They come in two
primary impedance types: 50 Ohm for moderate power environments and 75
Ohm for residential installations and antenna connections.
2. Twisted Pair Cable
A twisted pair cable is a type of network cable made of pairs of insulated copper
wires twisted together. The twisting helps reduce electromagnetic interference from
external sources and crosstalk between adjacent pairs.
These cables are widely used in Ethernet networks and come in various categories
(e.g., Cat5e, Cat6) that determine their speed and performance.
The structure of a twisted pair cable typically includes two main components:
1. Insulated Copper Wires: These are twisted together to minimize interference.
2. Shielding (Optional): Some cables include additional shielding to protect against
electromagnetic interference.
Here is an image of twisted pair cable:

Twisted pair cables were invented by Alexander Graham Bell in 1881 and are mostly
used in telephones, local area networks (LANs), and Ethernet connections.
There are two types of twisted pair cables:
● Unshielded Twisted Pair (UTP): UTP cables consist of multiple twisted pairs of
copper wires, typically up to four pairs, each enclosed in a protective plastic jacket.
The twisting helps reduce electromagnetic interference (EMI) and crosstalk between
pairs
● Shielded Twisted Pair (STP): STP cables include an additional layer of shielding,
such as aluminum foil or copper braid, which provides better protection against
electromagnetic interference, making them suitable for high-speed networks and
environments with significant EMI.
3. Fiber Optic Cables
A fiber optic cable is the fastest data transmission cable as it uses light signals
instead of electrical signals to transfer data. Fiber optic cables were first developed
in the 1970s. They transmit data as light signals through thin glass or plastic fibers,
offering high-speed data transmission over long distances with minimal signal
degradation.
The structure of a fiber optic cable typically includes three main components:
1. Core: The central part of the fiber where light travels, made from glass or plastic.
2. Cladding: Surrounds the core, reflecting light into the core to maintain signal
integrity.
3. Outer Jacket: Provides physical protection to the fiber.
Fiber optic cables are mostly used in high-speed internet connections,
telecommunications, and data centers due to their ability to support high
bandwidths and long-distance transmissions.

There are two main types of fiber optic cables:

● Single-Mode Fiber: Used for long-distance transmissions, it has a smaller core
diameter and supports a single light path.
● Multi-Mode Fiber: Suitable for shorter distances, it has a larger core diameter and
supports multiple light paths, making it more versatile for local area networks.

Categories of Cables
Network cables are categorized based on their function and speed. We have from
Cat1 to Cat8 cables right now. Each category has a different purpose in networking,
and the table below describes the specifications and uses of different categories of
network cables.
Category Bandwidth Data Rate Description & Use

Cat1 750 kHz Voice only Used for old telephone lines.
No internet support.

Cat2 1 MHz Up to 4 Early networks. Very slow by

Mbps today’s standards.

Cat3 16 MHz Up to 10 Used in the 90s for basic

Mbps networking.

Cat4 20 MHz Up to 16 Slightly faster than Cat3, but

Mbps also outdated.

Cat5 100 MHz Up to 100 Good for basic internet use.

Mbps Now mostly outdated.

Cat5e 100 MHz Up to 1 Improved version of Cat5.

Gbps Common in homes.

Cat6 250 MHz Up to 10 Great for gaming, streaming,

Gbps and fast internet.

Cat6a 500 MHz Up to 10 Better shielding and

Gbps performance over longer
distances.

Cat7 600 MHz Up to 40 Used in offices. High speed and

Gbps well shielded.

Cat8 2000 MHz Up to Super fast. Used in data

25/40 centers and servers.
Gbps

How Different Cables Carry Data

1. Single-mode Fiber
Single-mode fiber uses a smaller core size (typically 9/125 microns) and a laser as the
light source. It provides a single transmission path for light signals. A laser-based
transmitter sends light at a single angle, hence the name single-mode.
Single-mode fiber cables are typically color-coded yellow. They are designed for
long-distance communication, offering high bandwidth and low signal attenuation
over extended distances.
Single-mode fiber is used for backbone networks and data centers requiring high-
speed, long-distance connectivity. Here is the picture of sa ingle-mode fiber wherein
one end is LC, and the other end is SC.

2. Multimode Fiber
Multimode fiber has a larger core size (commonly 50/125 or 62.5/125 microns) and
uses either LED or laser light sources. It allows multiple light paths to propagate
simultaneously. Cladding reflects the light into the core as it travels through the
core.

Multimode fiber cables are typically color-coded orange. They are suitable for short
to medium distances and provide lower-cost solutions compared to single-mode
fiber.
Multimode fiber is commonly used in LAN environments, data centers, and campus
networks where high-speed connectivity within a limited distance is required. Here is
the picture of a multi-mode fiber wherein one end is LC, and the other end is SC.
3. Copper
Copper interfaces use electrical signals transmitted through copper wires or twisted-
pair cables. Copper cabling includes various types such as Category 5e (Cat5e),
Category 6 (Cat6), Category 6a (Cat6a), etc.
These cables consist of twisted pairs of copper wires enclosed in a protective jacket.
Copper cabling is commonly used for Ethernet connections within shorter distances,
typically up to 100 meters.
It is widely deployed in office networks, home networks, and other LAN
environments. Here are the pictures of RJ-45 connectors and RJ-45 ports on a Cisco
3560CX switch.
 Circuit switching:
At physical layer
continuous flow
No Headers
Efficiency is low
Delay is less
Total Time=Setup Time+ Transmission Time + Propagation Delay + Tear down Time
Transmission Time=Message/BW
Propagation Delay =Distance /Speed

1. Setup Phase: Before two devices communicate, a connection needs to be established.

When devices A and B want to communicate, A sends a request that includes the
address of B. Consider it as dialing a phone number of a person you want to
communicate with. In this phase, devices make the reservation of resources which
includes bandwidth, time slots, switch buffers, and processing time.
2. Data Transfer Phase: When a connection gets established, two devices can start
communicating.
3. Teardown Phase: Once communication gets complete, the connection terminates.
Hence the link is now free for other devices helping in the proper utilization of
resources.

Packet Switching in computer networks

Works at Data Link Layer and Network Layer
Store and Forward
Pipelining is used
Efficiency is High
Delay Low
Total Time=Transmission Time + Propagation Delay
 Circuit switching was primarily used for voice services and is designed accordingly. It
was not compatible with a variety of digital devices. An efficient communication
channel should be able to control bandwidth and make proper utilization of it, but in
the case of circuit switching, it was not possible.

Packet switching is an improved form of circuit and message switching. It overcomes

the limitation of both.
Data originating at the source gets divided into packets. The maximum packet length
is 512 bytes. It contains user data and some control information. By diving this data
into packets, files can be transmitted fast and efficiently over the network. Once these
packets reach their destination, it gets assembled to form complete data.
Each packet can travel using different paths. Packets get equipped with source and
destination addresses which helps each packet to reach the destination independently
and leads to the reduction of congestion in the network. The Internet is an example
of a packet-switched network that relies on Internet Protocol (IP) for routing packets.
Types of Packets Switching:

1. Datagram: It is also called connectionless packet switching. Each packet gets routed
independently. The routing table stored in the router (MAC table in case of the switch)
helps to determine the outgoing link to reach the destination. Since a packet gets
delivered using a different path, it may reach the destination in a different order, and
hence packets are provided with sequence numbers, which helps them assemble at
the destination. It is the most commonly used switching technique.
Works at Network Layer
2. Virtual Circuit: It is also called connection-oriented packet switching. It combines the
feature of both circuit switching and message switching. Before initiating data
transmission, it establishes a logical path called a virtual circuit. All the packet follows
this same virtual circuit established between the sender and the receiver. To identify
the path, the packet contains a virtual circuit identifier bit. This bit is shorter than the
source and destination address header used in the datagram technique. After
establishing the virtual circuit, data gets transmitted in the form of packets.
Works at Data Link

Message5 Switching:
Message switching is a communication technique in which data gets transmitted in
the form of divided messages and packets. In this case, each message is treated as an
individual entity and transmitted individually from one point to another, which means
each message/packet may or may not pass through the same path. They can travel
with different paths to reach their destination. In the case of message switching, a
dedicated connection between sender and receiver is not required.
NOTE: Message switching is also known as the store and forward method.
 Each node in a communications channel stores the packet, receives, reads the header,
and then forwards. If the next hop node is occupied with a packet of a different device
and is unable to receive the existing packet, the previous node stores the packet until
the next hop node is unoccupied. Generally, messages get transmitted on a first come
first serve basis. If the message has the priority header, it is given priority over another
message. Hence each node requires some storage capacity.

The complete message is transferred from one end to another through nodes. There
is no physical connection or link between sender and receiver.
The message contains the destination address. Each node stores the message and
then forward it to the next node as shown in the below diagram.
In telegraphy, the text message is encoded using the morse code into a sequence of
dots and dashes. Each dot or dash is communicated by transmitting a short and long
pulse of electrical current. The following diagram shows the concept of message
switching in computer networks.
Hop by Hop Delivery

Unicast, broadcast, and multicast are network communication methods that differ
in how data is delivered: Unicast sends data from one source to one specific
destination, Broadcast sends data from one source to all devices on a network, and
Multicast sends data from one source to a specific group of interested devices.

Unicast
• One-to-One: A single device sends data to another single, specific device.
• Addressing: Uses a unique destination address for the single recipient.
 • Use Cases: Email, file transfers, and private communication between two endpoints.
• Characteristics: Efficient for targeted data, generates less network traffic than
broadcasting, and provides secure, private communication.

Broadcast
• One-to-All: A single device sends data to every other device within a network
segment.
• Addressing: Uses a special broadcast address that all devices in the network segment
are programmed to receive.
• Use Cases: DHCP requests (to find a server) or ARP requests (to resolve an IP address
to a MAC address).
• Characteristics: Simple for reaching all devices, but generates the most network
traffic.

Multicast
• One-to-Many (Group):
A single source sends data to a selected group of multiple recipients that have
expressed interest in receiving the data.
• Addressing:
Uses a special multicast address, and routers and switches can use features
like IGMP snooping to efficiently deliver the traffic only to interested subscribers.
• Use Cases:
Video streaming, live broadcasts, and online gaming where multiple people need to
receive the same information.
• Characteristics:
Efficient for delivering data to multiple users without the high traffic overhead of
unicast.

Types of Framing in Data Link Layer

There are two types of framing in the data link layer. The frame can be of fastened or
variable size. founded on the size, the following are the types of framing in data link
layers in computer networks,
Data Link Layer
Data Link Layer is responsible for reliable point-to-point data transfer over a physical
medium. To implement this data link layer provides three functions:
• Line Discipline:
Line discipline is the functionality used to establish coordination between link
systems. It decides which device sends data and when.
• Flow Control:
Flow control is an essential function that coordinates the amount of data the sender
can send before waiting for acknowledgment from the receiver.
• Error Control:
Error control is functionality used to detect erroneous transmissions in data frames
and retransmit them.
• Framing
Grouping of Bits
Frames are the units of digital transmission, notably in Framing in Computer
networks

Flow Control in the Data Link Layer:

Flow control is a set of procedures that restrict the amount of data a sender should
send before it waits for some acknowledgment from the receiver.
• Flow Control is an essential function of the data link layer.
• It determines the amount of data that a sender can send.
• It makes the sender wait until an acknowledgment is received from the receiver’s
end.
• Methods of Flow Control are Stop-and-wait, and Sliding window.

Purpose of Flow Control

The device on the receiving end has a limited amount of memory (to store incoming
data) and limited speed (to process incoming data). The receiver might get
overwhelmed if the rate at which the sender sends data is faster or the amount of
data sent is more than its capacity.
Buffers are blocks in the memory that store data until it is processed. If the buffer is
overloaded and there is more incoming data, then the receiver will start losing
frames.
The flow control mechanism was devised to avoid this loss and wastage of frames.
Following this mechanism, the receiver, as per its capacity, sends an
acknowledgment to send fewer frames or temporarily halt the transmission until it
can receive again.
Thus, flow control is the method of controlling the rate of transmission of data to a
value that the receiver can handle.
Methods to Control the Flow of Data
Stop-and-wait Protocol
 Stop-and-wait protocol works under the assumption that the communication
channel is noiseless and transmissions are error-free.
Working:
• The sender sends data to the receiver.
• The sender stops and waits for the acknowledgment.
• The receiver receives the data and processes it.
• The receiver sends an acknowledgment for the above data to the sender.
• The sender sends data to the receiver after receiving the acknowledgment of
previously sent data.
• The process is unidirectional and continues until the sender sends the End of
Transmission (EoT) frame.

Sliding Window Protocol

The sliding window protocol is the flow control protocol for noisy channels that
allows the sender to send multiple frames even before acknowledgments are
received. It is called a Sliding window because the sender slides its window upon
receiving the acknowledgments for the sent frames.
Working:
• The sender and receiver have a “window” of frames. A window is a space that
consists of multiple bytes. The size of the window on the receiver side is always 1.
• Each frame is sequentially numbered from 0 to n – 1, where n is the window size at
the sender side.
• The sender sends as many frames as would fit in a window.
• After receiving the desired number of frames, the receiver sends an
acknowledgment. The acknowledgment (ACK) includes the number of the next
expected frame.
Example:

1. The sender sends the frames 0 and 1 from the first window (because the window
size is 2).
2. The receiver after receiving the sent frames, sends an acknowledgment for
frame 2 (as frame 2 is the next expected frame).
3. The sender then sends frames 2 and 3. Since frame 2 is lost on the way, the receiver
sends back a “NAK” signal (a non-acknowledgment) to inform the sender that
frame 2 has been lost. So, the sender retransmits frame 2.
What is Error Control in the Data Link Layer?
Error Control is a combination of both error detection and error correction. It
ensures that the data received at the receiver end is the same as the one sent by the
sender.
Error detection is the process by which the receiver informs the sender about any
erroneous frame (damaged or lost) sent during transmission.
Error correction refers to the retransmission of those frames by the sender.

Purpose of Error Control

Error control is a vital function of the data link layer that detects errors in
transmitted frames and retransmits all the erroneous frames. Error discovery and
amendment deal with data frames damaged or lost in transit and the
acknowledgment frames lost during transmission. The method used in noisy
channels to control these errors is ARQ or Automatic Repeat Request.
 Categories of Error Control
Stop-and-wait ARQ
• In the case of stop-and-wait ARQ after the frame is sent, the sender maintains a
timeout counter.
• If acknowledgment of the frame comes in time, the sender transmits the next frame
in the queue.
• Else, the sender retransmits the frame and starts the timeout counter.
• In case the receiver receives a negative acknowledgment, the sender retransmits the
frame.

Sliding Window ARQ

To deal with the retransmission of lost or damaged frames, a few changes are made
to the sliding window mechanism used in flow control.
Go-Back-N ARQ :
In Go-Back-N ARQ, if the sent frames are suspected or damaged, all the frames are
re-transmitted from the lost packet to the last packet transmitted.
Selective Repeat ARQ:
Selective repeat ARQ/ Selective Reject ARQ is a type of Sliding Window ARQ in
which only the suspected or damaged frames are re-transmitted.
Differences between Flow Control and Error Control

Flow control Error control

Flow control refers to Error control refers to the transmission of

the transmission of data error-free and reliable data frames from
frames from sender to sender to receiver.
receiver.

Approaches for Flow Approaches for error detection are Checksum,

Control : Feedback- Cyclic Redundancy Check, and Parity Checking.
based Flow Control and Approaches for error correction are Hamming
Rate-based Flow code, Binary Convolution codes, Reed-Solomon
Control. code, and Low-Density Parity-Check codes.

Flow control focuses on

the proper flow of data Error control focuses on the detection and
and data loss correction of errors.
prevention.

Examples of Flow
Control techniques are : Examples of Error Control techniques are :
1. Stop and Wait for 1. Stop and Wait for ARQ,
Protocol, 2. Sliding Window ARQ.
2. Sliding Window
Protocol.
Framing:
• Framing
Grouping of Bits
Frames are the units of digital transmission, notably in Framing in Computer
networks

Frame
• Header: It consists of the frame’s source and destination address. A frame header
holds the destination address, the supply address, and 3 management fields kind, seq,
and ack helping the subsequent purposes:
• kind: This field expresses whether the frame could be an information frame, or it’s
used for management functions like error and flow management or link management,
etc.
• Seq: This holds the sequence variation of the frame for transcription of out–of–
sequence frames and causing acknowledgments by the receiver.
• Ack: This contains the acknowledgment variety of some frames, significantly once
piggybacking is employed.
• Payload: It contains the message to be delivered. It contains the particular message
or data that the sender desires to transmit to the destination machine. It contains the
particular message or data that the sender desires to transmit to the destination
machine
• Trailer: It contains the error detection and correction bits.
• Flag: It contains the points to the starting and the ending of the frame.

Types of Framing in Data Link Layer

There are two types of framing in the data link layer. The frame can be of fastened or
variable size. founded on the size, the following are the types of framing in data link
layers in computer networks,
1. Bit-Oriented Framing
Most protocols use a special 8-bit pattern flag 01111110 as a result of the delimiter to
stipulate the beginning and so the end of the frame. Bit stuffing is completed at the
sender end and bit removal at the receiver end.
If we have a tendency to get a zero(0) after 5 1s. we have a tendency to tend to still
stuff a zero(0). The receiver will remove the zero. Bit stuffing is in addition said as bit
stuffing.
framing methods, the popular ones are the Character Count Method, Flag Bytes with
Byte Stuffing, Starting and Ending Flags, with Bit Stuffing
1. Character Count Method
This method uses a field in the header to specify the number of characters in the
frame. When the data link layer at the destination sees the character count, it knows
how many characters follow and hence where the end of the frame is.
Press enter or click to view image in full size
Character count
2. Flag Bytes with Byte Stuffing
Flag bytes with the byte stuffing method get around the problem of resynchronization
after an error by having each frame start and end with special bytes. In this way, if the
receiver ever loses synchronization, it can just search for the flag byte to find the end
of the current frame. Two consecutive flag bytes indicate the end of one frame and
the start of the next one.
Press enter or click to view image in full size
 3. Starting and Ending Flags, with Bit Stuffing
This is a framing technique that is used to mark the beginning and the end of frames
while ensuring that the frames can contain arbitrary patterns, including the flag
pattern itself, and allowing for the transmission of character codes with arbitrary
numbers of bits per character.
Press enter or click to view image in full size

IP Addressing

Class A
 IP addresses belonging to class A are assigned to the networks that contain a large
number of hosts.
• The network ID is 8 bits long.
8-1 =7 Bits net id available 2 ^7=128 networks
0 n/w and 127 loopback
• The host ID is 24 bits long.
The higher-order bit of the first octet in class A is always set to 0. The remaining 7
bits in the first octet are used to determine network ID. The 24 bits of host ID are
used to determine the host in any network. The default subnet mask for Class A is
255.x.x.x. Therefore, class A has a total of:
• 224 - 2 = 16,777,214 host ID
IP addresses belonging to class A ranges from 0.0.0.0 - 127.255.255.255.

Class B
IP address belonging to class B is assigned to networks that range from medium-
sized to large-sized networks.
• The network ID is 16 bits long.
• The host ID is 16 bits long.
The higher-order bits of the first octet of IP addresses of class B are always set to 10.
The remaining 14 bits are used to determine the network ID. The 16 bits of host ID
are used to determine the host in any network. The default subnet mask for class B
is 255.255.x.x. Class B has a total of:
• 214 = 16384 network address
• 216 - 2 = 65534 host address
IP addresses belonging to class B ranges from 128.0.0.0 – 191.255.255.255.

Class C
IP addresses belonging to class C are assigned to small-sized networks.
• The network ID is 24 bits long.
• The host ID is 8 bits long.
The higher-order bits of the first octet of IP addresses of class C is always set to 110.
The remaining 21 bits are used to determine the network ID. The 8 bits of host ID
are used to determine the host in any network. The default subnet mask for class C
is 255.255.255.x. Class C has a total of:
• 221 = 2097152 network address
• 28 – 2 = 254 host address
IP addresses belonging to class C range from 192.0.0.0 – 223.255.255.255.

Class D
IP address belonging to class D is reserved for multi-casting. The higher-order bits of
the first octet of IP addresses belonging to class D is always set to 1110. The
remaining bits are for the address that interested hosts recognize.
Class D does not possess any subnet mask. IP addresses belonging to class D range
from 224.0.0.0 – 239.255.255.255.

Class E
IP addresses belonging to class E are reserved for experimental and research
purposes. IP addresses of class E range from 240.0.0.0 – 255.255.255.255. This class
doesn’t have any subnet mask. The higher-order bits of the first octet of class E are
always set to 1111.

Range of Special IP Addresses

169.254.0.0 – 169.254.0.16 : Link-local addresses
127.0.0.0 – 127.255.255.255 : Loop-back addresses
0.0.0.0 – 0.0.0.8: used to communicate within the current network.
Structure of Classful Addressing
In the above table No. of networks for class A should be 127. (Network ID with all 0 s
is not considered)