OSI
Model
@Kiran Kanwar Rathore
@kiranrathore123
OSI
Open systems interconnection
OSI was developed by Standard
organization ISO in the year of
1984 .
OSI Model defines and is used to
understand how data is
transferred from one computer
to another in a computer
network.
NOTE :
ISO-International Organization for
Standardization .
@Kiran Kanwar Rathore
In order to accomplish a
successful communication
between computers or
different networks ,
7 layers of OSI Model was
introduced by ISO.
Each layer in OSI Model is
a packge of Protocols
@Kiran Kanwar Rathore
@kiranrathore123
7 layers of OSI
Model
Application layer
Presentation layer
Session layer
Transport layer
Receiver Network layer Sender
Datalink layer
Physical layer
@Kiran Kanwar Rathore
@kiranrathore123
Let's understand each layers of OSI
Model separately from Sender's side .
1 Application layer
This is the first layer in the OSI
Model.
It is used by Network application.
Network application means
Computer applications that use
internet like Chrome, Firefox etc.
Application layer provides the
interface through which we can
transfer the data .
@Kiran Kanwar Rathore
@kiranrathore123
Some Application layer
Protocols :
1. TELNET
2. FTP
3. HTTP
4. DMTP
5. NNTP
There are dozens of application
layer protocols that enables
various functions at this layer .
All these protocols collectively
form application layer .
@Kiran Kanwar Rathore
@kiranrathore123
2 Presentation layer
Presentation layer receives data
from application layer.
This data is in the form of
characters and numbers.
Presentation layer convert them
into machine understandable
language that is Binary format .
Before data is transmitted to next
layer , the presentation layer
reduces the number of bits that
are use to represent the original
data .
@Kiran Kanwar Rathore
@kiranrathore123
This bit reduction process is
called data compression . And it
can be lossy or lossless .
Data compression reduces the
amount of space used to store
the original file , and as the size
reduces it can reach the
destination in very less time.
To maintain the integrity of
data before transmission , the
data is encrypted.
Encryption enhances the
security of sensitive data.
@Kiran Kanwar Rathore
@kiranrathore123
At sender side the data is encrypted.
At receiver side data is decrypted .
SSL(Secure Sockets Layer) is used
for Encryption/Decryption.
So , in short Presentation layer
performs three main functions:
1. Data translation
2. Data compression
3. Data Encryption and
Decryption
@Kiran Kanwar Rathore
@kiranrathore123
3 Session layer
Session layer is the third layer of
the OSI model from the Sender
side .
It helps in setting up and
managing connections. Enabling
sending and receiving of data.
Authentication and Authorization
both these functions are
performed by Session layer.
@Kiran Kanwar Rathore
@kiranrathore123
Session layer keep track of file
which are being downloaded.
Session layer helps in session
management.
Our web browser performs all
functions of session layer,
Presentation layer and
Application layer .
Now data is transmitted to the
next layer .
@Kiran Kanwar Rathore
@kiranrathore123
4 Transport layer
Transport layer is the fourth
layer of the OSI Model and it is
after the session layer .
Transport layer controls the
reliability of the communication
through :
1. Segmentation
2. Flow control
3. Error control
@Kiran Kanwar Rathore
@kiranrathore123
Segmentation :
The data received from the
session layer is divided into small
units called Segments .
Flow control :
The transport layer controls the
amount of data being transmitted
at a moment of time.
Error control :
Some data does not arrive at
destination, The transport layer
uses Automatic Repeat request
Scheme to retransmit the lost or
encrypted data .
@Kiran Kanwar Rathore
@kiranrathore123
Transport layer protocols :
TCP : Transmission Control
Protocol.
UDP : User Datagram Protocol.
@Kiran Kanwar Rathore
@kiranrathore123
5 Network layer
Transport layer transfer the
data to network layer .
Network layer is the fifth layer
of OSI model .
It works for the transmission of the
received data from one computer
to another computer located in
different networks.
The main functions of network
layer :
Logical Addressing
Routing
Path Determination
@Kiran Kanwar Rathore
@kiranrathore123
Logical addressing :
IP addressing done in Network
layer is called Logical Addressing .
Routing :
It is a method of moving data
packets from Source to
Destination.
Path
Determination :
Choosing the best possible path
for data delivery from source to
destination is called path
determination as there are many
ways.
@Kiran Kanwar Rathore
@kiranrathore123
6 Data Link layer
Data Link layer receives data
packets from Network layer and this
is the sixth layer of OSI Model .
It performs Two basic functions:
It allows upper layer of OSI model
to access the media using
technique such as framing .
It controls how data is placed and
received from media using
technique media access Control
and error Detection .
@Kiran Kanwar Rathore
@kiranrathore123
The physical addressing of the
data is also done at Data Link
layer.
Data Link layer also perform
functions like Flow control and
Error control .
@Kiran Kanwar Rathore
@kiranrathore123
7 Physical layer
The data is now framed by Data
Link Layer and is transmitted to the
last layer of the OSI Model that is
Physical layer.
The data received by physical layer
are in the sequence of binary
format(1s and 0s) .
Physical layer converts these
binary sequence into signal and
transmit it over the local
media .
@Kiran Kanwar Rathore
@kiranrathore123
The signal generated by physical
layer depends on the type of media
used to connect two devices .
Now from the Receiver side-
Physical layer receives the signals
and convert them into binary format
and pass it to the datalink layer as a
frames.
@Kiran Kanwar Rathore
@kiranrathore123
Frame is further Decapsulated,
as data moves through higher
layers.
When it reaches presentation
layer binary format data is
converted into numbers and
characters.
And finally the data is moved to
the Application layer.
@Kiran Kanwar Rathore
@kiranrathore123
Application layer Protocols
makes the sender's message
Visible in the Receiver's
computer screen .
In this way , OSI Model helps in
data Transfer .
@Kiran Kanwar Rathore
@kiranrathore123
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