OSI Reference Model

Content
1. OSI Reference Model
2. Need of OSI Reference model
3. Architecture
● Application Layer
● Presentation Layer
● Session Layer
● Transport Layer
● Network Layer
● Data Link Layer
● Physical Layer
4. How It works
5. Advantage/Disadvantage

Internetworking Model

Internetworking models are used to make communication between two devices flexible.
Before the invent of these models, the computer communicate with other computer if they are from same manufacturer.

OSI reference Model
It stands for Open system interconnection
Created in late 1970’s and it is used in networking.
It is a set of rules and regulations created by ISO (International Organisation for
standardization) to make flexible communication between two devices.
It is a primary layered architectural model of network.
OSI model helps the vendors to create interoperable network devices and
softwares in the form of protocols. So that different vendor’s devices could
work with each other.

Need of Reference Model
Why we need this OSI reference model?
● To make communication possible between two devices which are
manufactured by different vendors.
● To implement rules and regulations which are used by every device in the
network while communicating.
● Basically,OSI model make communication flexible.

OSI Model Architecture
OSI model is not a physical model, but it’s a set of rules that a manufacturer
keep in mind while designing a device or creating a application that runs on a
network.
It consist of 7 layers
1. Application layer (Layer 7)
2. Presentation layer (Layer 6)
3. Session Layer (Layer 5)
4. Transport Layer (Layer 4)
5. Network Layer (Layer 3)
6. Data Link Layer (Layer 2)
7. Physical Layer (Layer 1)

OSI Model Architecture
Application Layer, Presentation Layer and Session Layer are the higher Layers
which incharges the data transmission between hosts/devices.
Transport Layer, Network Layer, Data Link Layer, Physical Layer Are the lowest
layers which incharges the data transmission in network.

OSI Model Architecture
1. Application Layer
It provide an interface between network application and network
protocols.
Network applications such as web browser, gmail, Dropbox etc and
network protocols which carry request to the destination such as http,
ftp, smtp, pop3 etc
It is a user interface where user actually communicate with the
devices.It is a software present in computers or devices
Example: Composing a mail in Gmail Software.

2. Presentation Layer
As it name suggest, it present data to the upper layer i.e. Application
Layer
It is responsible for data formatting.
It indicate the destination system about the type of file being sent to it
by sender.
Example: .mp3 file, .jpg file, .txt file, .docx file etc

3. Session Layer
It establish, maintain and terminate the connection between two
devices.
Connection may be a simplex, half duplex or full duplex.
It also check wheather the destination system is available or not.
Session Layer perform synchronization.

4. Transport Layer
Transport layer function is to support end to end delivery of packets in
network,.
It perform these functions:
● Segmentation: It is a process of dividing a large user data into smaller
units (packets) for transmission over a data communication network.
● Sequence number: In this process, Sequence number is added to the
smaller unit packets.so that it can be arrange at the receiver side.

Assigning Transmission Protocol: it decide how to transmit the data
over network using TCP or UDP protocol
● Assigning logical port number: It define the port of device where the
packets have to routed such as http=80,ftp=21,smtp=25 etc

OSI Model Architecture

TCP Protocol
It Stands for Transmission control Protocol.
It is connection oriented Protocol Reliable connection
Speed is slow
Example: http, https, ftp, telnet, smtp use TCP protocol

UDP Protocol
It Stands for User datagram Protocol.
It is a Simpler, connectionless Protocol.
Not reliable connection.
Speed is fast.
Example: VoIP, DNS,RIP use UDP

5. Network Layer
The main function of network layer is:
This layer done wrapping of segments to form packets.
It add source and destination IP address or Logical address with packets.
At this layer, Router is used to route the packets in the network.

6. Data Link Layer
The main function of data link layer are:
Here the wrapping of packets is done to form frame.
Wrapping is done by adding MAC address or Physical address to the
packets.
At this Layer,Switch is used to forward packet in the network.

Disadvantages of OSI Reference Model
● OSI model has a complex architecture.
● It is not widely used by all devices in the network.
● For fast setup OSI model require agreement between third parties –
user and service provider.

6. Data link Layer
Further Data link layer has two sublayers:
● LLC(Logical Link Control)- The functions perform by this layer are
error checksum/correction,frame synchronization and also check
network layer protocol i.e IPv4 or IPv6 .
● MAC(Media Access Control)- This layer is used to add source and
destination MAC address or machine address to the frame and it
also convert the data in the form of bits.

7. Physical Layer
This layer provide the physical connectivity between the devices via
transmission medium such as coaxial wire, optical fiber and wireless.
It is used to send the digital bits through transmission medium.
It handle the bit flow transmission.
NIC (network interface card) or Hub are used at this Layer.

How OSI model Works?

Advantages of OSI Reference Model
● It make the communication easier by dividing the process into smaller
and simpler units.
● This model define the function occurring at each layer.
● It allow communication between various Hardware and software
devices from different vendors.
● It simplify teaching and learning process.
● OSI is an idealised networking model.

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